Multi-scale Observations and Modeling of West African Tropical Rainfall Systems: AMMA-Weather

Principal Investigators:

Chris D. Thorncroft SUNY at Albany

Chris Davis NCAR

Robert A. Houze University of Washington

Richard H. Johnson Colorado State University

Steven A. Rutledge Colorado State University

Bradley F. Smull University of Washington

PROJECT OVERVIEW

Executive Summary

AMMA-Weather is designed to improve both fundamental understanding and weather prediction in the area of the West African monsoon through deployment of U.S. surface and upper-air observing systems in July and August 2006. These systems will be closely coordinated with International AMMA. The project will focus on the interactions between African easterly waves (AEWs) and embedded Mesoscale Convective Systems (MCSs) including the key role played by microphysics and how this is impacted by aerosol. The pronounced zonal symmetry, ubiquitous synoptic and mesoscale systems combined with the close proximity of the Saharan aerosol make the WAM an ideal natural laboratory in which to carry out these investigations. The observations will provide an important testbed for improving models used for weather and climate prediction in West Africa and the downstream breeding ground for hurricanes in the tropical Atlantic.

The international AMMA program consists of scientists from more than 20 countries in Africa, Europe, and the US. Owing to the efforts of European countries, a strong infrastructure is being installed providing a unique opportunity for US participation. Support in excess of twenty million Euros has already been secured by Europeans for AMMA including the 2006 field campaign. Significantly for AMMA-Weather, this will include support for the U.S. NOAA-P3 aircraft, enhancements to the rawinsonde network, European aircraft including dropsondes and aerosol measurements as well as ground-based radar and surface-based measurements (including rainfall and surface fluxes).

To address the scale interactions objectives of AMMA-Weather, observations and analysis are required on four scales: synoptic, mesoscale, convective and microscale. We propose to achieve this through a combination of strategically located soundings and specialized radars. We propose to enhance the rawinsonde network to support the analysis of AEWs, the impact of MCSs on the synoptic scale, and the modulations of these effects by the diurnal cycle. The 3D analysis of the kinematic and microphysical structure of the MCSs within the synoptic framework will be afforded by the NCAR S-Pol radar, a C-band Doppler radar (either the NASA/TOGA or MIT radar) and an S-band vertical profiler. In addition, dual-Doppler observations made by a NOAA P-3 turboprop aircraft will be used to extend the dual-Doppler coverage of the ground-based radars and in particular to follow the evolution of vortex structures developing in the stratiform regions of MCSs. These observations will form the basis of a hierarchy of modeling activities ranging from operational modeling, to cloud-system resolving models, to idealized modeling studies.

1. Science Background From a large-scale perspective the West African monsoon (WAM) can be described in terms of the annual march of the ITCZ and its associated regional circulations. On the synoptic and mesoscale, the WAM is comprised of a complex collection of wave patterns, organized weather systems and deep convection (see front page). These include synoptic systems such as African easterly waves (AEWs) (e.g. Reed et al, 1977) and mesoscale convective systems (MCSs) (e.g. Houze and Betts, 1981). AEWs also initiate many of the Atlantic tropical cyclones (TCs) downstream (e.g. Avila and Pasch, 1992) and thus are an important part of the interactions that take place between West Africa and the tropical Atlantic. AMMA-Weather will strive to understand the nature and variability of individual weather systems that comprise the WAM. It will expand upon the earlier studies of this region, in particular, GATE (Houze and Betts, 1981) and COPT81 (Sommeria and Testud 1984, Chauzy et al. 1985, Chong et al. 1987, Roux 1988, Sun and Roux 1988, Chong and Hauser 1989). Those studies were conducted over 20 years ago and therefore did not benefit from today’s advanced observational technologies. They were also carried out with a more limited understanding of tropical dynamics and convection than we now have as a result of TOGA COARE (Godfrey et al. 1998), TRMM (Kummerow et al. 1998), other field campaigns, and advances in analysis of satellite data (Wheeler and Kiladis, 1999) and observational platforms such as portable, polarimetric radars, as well as numerous theoretical studies of tropical wave dynamics and other monsoon systems (Webster 1994, Webster et al. 2001, Thorncroft and Hoskins, 1994a,b, Thorncroft and Blackburn, 1999).

AEJ

90oC 50oC θ θe θe θ

60oC 20oC Figure 1.1 Schematic of N-S vertical cross section along the Greenwich Meridian, highlighting the heat low-AEJ-ITCZ system and SAL (shaded yellow) and meridional variations in atmospheric boundary layer θ and θe. Adapted from Parker et al. (2004a).

A key feature of the West African monsoon system is the marked meridional contrast in surface properties. Meridional variations in albedo and vegetation between the desert to the north and rainforest to the south impact the surface fluxes and the boundary layer θ and θe distributions (e.g. Nicholson et al 1998; Cook 1999). θ peaks around 25˚N in the Sahara while θe peaks further south in the rainy zone around 10˚N (Fig. 1.1). Deep moist convection in the high θe region and dry convection in the high θ region impact the nature of the large-scale meridional circulations and help establish the African easterly jet (AEJ, Thorncroft and Blackburn, 1999). As highlighted in the recent observations made during the JET2000 aircraft campaign (Thorncroft et al, 2003, Parker et al, 2004), the baroclinic region between is characterized by two coherent layers: the humid monsoon layer affected by the land surface on diurnal timescales and above this the Saharan air layer (SAL) characterized by low humidity and high aerosol content (shaded yellow in Fig. 1.1). The JET2000 observations indicate that the boundary layer air in this baroclinic zone is more buoyant than the higher θe region south of this, consistent with the presence of a capping inversion provided by the SAL. The presence of the SAL in the baroclinic zone also highlights the potentially important role of aerosol in influencing the thermodynamics and microphysics in this region. Given these contrasts we expect marked contrasts in the nature of convection and MCSs across the baroclinic region – AMMA provides us with an opportunity to investigate these contrasts. The AEJ-environment supports the growth of MCSs and AEWs and in general both types of system co-exist and dominate the WAM (Fig. 1.2 and front page). It is important that we have a good description and understanding of the AEWs and MCSs, their interactions with each other and the large-scale environment (see Fig. 1.2). A complete description of these interactions must also include a consideration of the nature of the convection and how this is influenced by the both the large-scale environment (Fig.1.1) and microphysical processes including the impact or aerosol. Although these scale interactions are central to the WAM, we lack a good description and understanding of how they operate and combine to produce the observed WAM and its variability. The extent to which GCMs used for weather and climate prediction are able to represent these scale interactions, explicitly or through parameterizations requires investigation. We are hindered in carrying out such investigations due to lack of appropriate multi-scale observations.

SAL AEWs

TC MCSs

Figure 1.2 Key weather systems in the West African and Tropical Atlantic regions. Solid line represents a streamline at the level of the AEJ around 600hPa. Grey shading represents peak rainfall and yellow shading indicates the location of the SAL.

1.1 African easterly waves African easterly waves (AEWs) are synoptic-scale weather systems that characterize the WAM and downstream tropical Atlantic during boreal summer. They have a zonal wavelength of 2000-3000 km and propagate westwards along the mid-tropospheric African easterly jet (AEJ) located around 15oN and 600 hPa (Fig. 1.1). These are important systems in the WAM region due to their association with daily rainfall (e.g. Reed et al, 1977), because they can serve as precursors for tropical cyclones (e.g. Avila and Pasch, 1992) and because they influence the spatial and temporal variability of the dry and aerosol laden Saharan air layer (SAL). Despite their importance, fundamental gaps exist in our knowledge and understanding of the evolution of AEWs as they move across Africa and the Atlantic; especially how the AEW dynamics interacts with MCSs and convection. AEWs are usually viewed as developing via a mixed barotropic-baroclinic instability mechanism in association with perturbations of the meridional gradients of potential vorticity (PV) in the core of the AEJ and meridional gradients of low-level potential temperature (θ) polewards of the AEJ (Burpee, 1972, Thorncroft and Hoskins, 1994a). Consistent with this, analyses indicate that AEWs tend to be characterized by peak amplitudes in two locations; one in the vicinity of the mid-level PV gradients in the rainy zone and one in the vicinity of the low- level θ gradients polewards of this where it rains much less (Reed et al, 1977, Pytharoulis and Thorncroft, 1999). Composite analyses of AEWs (Fig. 1.3, Reed et al, 1977, Burpee, 1972) still dominate our perception of an AEW. While such studies provide useful information about the mean structure of AEWs and supporting evidence for the role played by barotropic-baroclinic instability, by exhibiting characteristic horizontal and vertical tilts, the statistical averaging involved in such studies provides us with an ‘out-of-focus’ view of AEWs. They shed little light on the evolution of AEWs or on the mechanisms by which they interact with other physical phenomena. In particular, we still lack detailed knowledge and understanding of the nature of AEWs and the processes that influence their relationship with MCSs and convection and their impact on downstream TCs.

Figure 1.3 Composite AEW structures from phase III of GATE (after Reed et al, 1977). (a) and (b) are relative vorticity at the surface and 700hPa respectively with a contour interval of 10-5s-1. (c) and (d) show percentage cover by convective cloud and average precipitation rate (mm day-1) respectively. Category 4 is location of 700hPa trough and the “0” latitude is 11oN over land and 12oN over ocean.

While we know that AEWs have a characteristic synoptic scale consistent with the development via Rossby-wave dynamics on the AEJ – evidence suggests that AEWs can also be associated with coherent subsynoptic scale potential vorticity centers embedded within them (e.g. Berry and Thorncroft, 2004). Also, based on automatic tracking of vorticity centers in ECMWF analyses Thorncroft and Hodges (2001) showed that two distinct storm tracks exist over the African continent corresponding well to the location of vorticity maxima presented in the composite AEWs (Fig. 1.4). However, it is important to note that the southern vorticity track is also a favored region for MCSs (Hodges and Thorncroft, 1997), which can produce mesoscale vorticity anomalies. It is unclear how many of the tracked vorticity anomalies are associated with large MCSs and how many are associated with AEWs that develop via Rossby-wave dynamics. It is unclear the extent to which diabatically generated PV impacts the propagation of the AEWs and whether the moist Rossby wave model proposed by Parker and Thorpe (1995) has some relevance here. The nature of the interactions between synoptic scale PV anomalies associated with AEWs and the mesoscale PV anomalies associated with MCSs is at the heart of the scale interaction problem. There is a general need to investigate the AEW structures including how the multiple PV centers embedded in them interact with each other and the synoptic-scale AEW.

Figure 1.4 Average vorticity tracking statistics for June-July-August at 700hPa and 850hPa based on ERA40 using methodology of Thorncroft and Hodges (2001). Units are number density per unit area (approximately 106km2.) Coherent cyclonic centers are tracked within the ITCZ at 700hPa and in the low- level baroclinic zone at 850hPa. (Provided by Kevin Hodges.)

In assessing the interactions between the AEW structures and convection, it is important to consider the nature of the background state and in particular the nature of the AEJ on which the AEWs develop and propagate (see Fig. 1.1). The AEJ is characterized by marked vertical wind shear and strong meridional contrasts in thermodynamic properties. As discussed by Thorncroft and Blackburn (1999), the poleward side of the AEJ is characterized by hot dry surface conditions and a deep well-mixed boundary layer that represents the SAL when still in contact with the surface. The equatorward side of the AEJ is characterized by cool moist surface conditions and an environment influenced by deep precipitating convection. The different thermodynamic profiles and convective regimes in these two regions are intimately linked to the existence of the AEJ. As AEWs develop in this basic state, we expect meridional and vertical advections of these contrasting air masses to result in significant zonal variations in thermodynamic properties and vertical wind shear. We do not currently have a good description of these structures and the consequences for convection. The absence of such knowledge contributes profoundly to our extremely limited understanding of how the AEWs interact with MCSs. GATE showed a close relationship between convective activity and wave phase (Fig. 1.4, Reed et al 1977, Payne and McGarry, 1977) confirmed by more modern analyses of satellite data (e.g. Duvel, 1990, Diehdiou et al, 1999). The network of soundings and ship radars off the coast of Africa showed that the convective dynamics were linked to the wave structure, but the diagnostic studies were too limited to identify specific mechanisms of synoptic-convective interaction. GATE did show that convective systems over the ship array had a substantial mesoscale organization with 40% of the precipitation falling in the form of stratiform precipitation (Houze and Betts 1981). The mesoscale stratiform precipitation alters the profile of heating that forces the synoptic-scale environment (Houze 1982, 1989) by reinforcing heating aloft and reducing heating below the melting level. Mesoscale stratiform regions may also have momentum transports that feed back to the larger-scale environment (as suggested, for example, by Houze et al. 2000 for the TOGA COARE region). Mapes and Houze (1995) demonstrated that the feedback of mesoscale convective systems to the large scale consisted of gravity wave responses to the convective and stratiform regions of the systems. The feedback in any given situation depends on the relative amounts of convective and stratiform precipitation (heating) in the mesoscale convective systems. The type of mesoscale convective system depends in turn on the thermodynamic and wind stratification in the large-scale environment. The nature of the convective elements in the MCSs and therefore the processes by which they transport ice to the upper troposphere (that is then detrained to generate stratiform precipitation) is also affected by the aerosol characteristics within the airmass they form in. The mesoscale motions in the tropical convective systems do not satisfy simple scale-separation assumptions (e.g. Arakawa and Schubert 1974, Yanai et al. 1973, Emanuel 1994). Consequently, understanding of the interaction of AEW dynamics and MCSs depends on detailed knowledge of the structure and dynamics of the MCSs themselves. Our knowledge of these processes is seriously lacking and is at the root of improving models used for weather and climate prediction.

1.2 Mesoscale Convective Systems A mesoscale convective system (MCS) can be defined as a cumulonimbus cloud system that produces a contiguous precipitation area ~100 km or more in at least one direction (adapted from Houze 1993). Since MCSs provide a majority of the rainfall over West Africa (e.g. Mathon et al., 2002; Lebel et al., 2003) it may be argued that the WAM is strongly linked to the statistics of these MCSs and that the variability in the WAM in turn is linked to variability in these statistics. However, as noted above, the physical mechanisms of the linkages between the MCSs and the large-scale are poorly understood. It is important that we improve our understanding of the 2-way interactions between the MCSs and the synoptic environment in which they develop, in particular the AEWs and associated perturbations to the AEJ, low-level monsoon layer and dry, aerosol- laden SAL. In addition, it is important to take account of the role of the spatially varying surface conditions and associated surface fluxes that arise due to changes in vegetation and/or pre- existing rainfall events (e.g. Taylor and Lebel, 1998). An MCS typically contains a fluctuating and evolving pattern of convective scale cells (consisting of updrafts, downdrafts, and heavy local rainfall). In addition the MCS can take on mesoscale organization in the form of circulations and rain areas that are of mesoscale space and time scales (~100 km, ~hours). Both the convective scale and mesoscale aspects of MCSs interact with the larger scale environment flow. The extent to which MCSs can become organized, the processes that lead to this organization, and the consequences of the two-way interactions with the synoptic environment require investigation beyond that carried out in GATE and COPT81. MCSs forming over Africa and moving offshore were studied extensively in GATE (Houze and Betts 1981). However, the GATE data set emphasized ship-based (non-Doppler, non-polarimetric) radars off the coast of Africa. In COPT81 the first Doppler radar data were obtained in West African MCSs (Chong et al. 1987). Much was learned from both GATE and COPT81. However, these early studies lacked airborne radars and polarimetric radars from which a more complete dynamical and microphysical description could have been obtained. In addition there were no lightning measurements, which yield important information on dynamical and microphysical processes (Petersen and Rutledge, 1999). To relate the African MCSs to the AEJ and AEWs, modern observational technology is needed. Polarimetric radar data obtained in west African continental MCSs will provide new insights into the precipitation formation and growth mechanisms, while ground based and airborne Doppler radar will reveal convective and mesoscale patterns of winds, divergence, vorticity, and vertical air velocity in these storms. From these kinematic and microphysical observations, the effects of the MCSs on the large-scale environment, including AEWs, can be determined empirically. The kinematic and microphysical observations to be obtained in AMMA will also form a framework for studying electrical processes and lightning in west African MCSs. This region of the world is well-suited to allow further study of the aerosol hypothesis recently advanced to explain variability in lightning flash rates between high flash rate continental-like convection, and low flash rate maritime-like convection, as recently observed over tropical Brazil (Williams et al., 2002). To the north is a prolific ice nucleus factory from the Sahara. To the south of the AMMA domain is a more CCN-based environment associated with moist conditions. More broadly speaking, AMMA provides a unique opportunity to further study the couplings between lightning and CAPE (as advanced by Williams et al., 1992, Rutledge et al., 1992) vs. the aerosol hypothesis (Williams et al., 2002). To establish the impact on the large-scale flow, there is a need to document the detailed water budget of mesoscale convective systems of the West African monsoon. The water budgets of the convective cloud systems imply the relative amounts of vertical mass transport in the convective and stratiform regions of MCSs and how those transports are distributed with height (Houze et al. 1980). From these distributions, the vertical distributions of heating and moistening of the large- scale environment by MCSs can be inferred, and the atmospheric response to the MCS heating can be determined (Houze 1982, 1989, Mapes and Houze 1995, Schumacher et al. 2003). Gamache and Houze (1983) and Chong and Hauser (1989) estimated water budgets of African MCSs over the ocean and land. However, these single cases based on limited measurements need to be verified and documented in more precise relation to the AEJ and AEWs and in the context of the larger continental water cycle. This will be a major objective of AMMA. These studies to be carried out in AMMA-Weather will respond to a general need in tropical meteorology, which is to represent the large-scale impacts of tropical MCSs in weather and climate prediction models. In this respect, major areas for investigation include how MCSs develop in relation to MCSs, the generation of PV in MCSs, and the momentum, heat and moisture transports by MCSs.

2. Scientific Objectives and Observational requirements 2.1 Improve our understanding of the processes that influence the relationship between African Easterly Waves, MCSs and Convection. A major objective of AMMA is to improve our knowledge and understanding of AEWs and their interactions with MCSs and convection. Composite studies indicate that moist convection is preferentially located in the northerlies ahead of the jet-level trough of the AEW in the rainy zone and in the southerlies behind the jet-level trough polewards of the AEJ. We cannot describe with confidence the processes that explain these observed phase relationships. The relative roles of adiabatic ascent expected in this baroclinic environment (Thorncroft and Hoskins, 1994a), meridional advection of temperature and humidity (Parsons et al, 2000) and surface processes (Taylor and Clark, 2001) are unknown and require investigation through analysis of observations and modeling. In order to address these issues it will be necessary to increase the frequency of rawinsonde observations at key locations to capture the evolution of the thermodynamic profiles diurnally and during the passage of the AEWs. Enhancements to the synoptic network are required to describe the dynamic and thermodynamic structure of AEWs including embedded sub-synoptic scale vortices that develop either in the association with the low-level temperature gradients on the poleward side of the AEJ or in association with moist convection within the rainy zone. The role of diabatically generated PV anomalies on the nature of the AEWs including their propagation characteristics will be investigated. This will require analysis of the PV structures at mesoscale and synoptic scales. A major part of this analysis will include documentation of the SAL structure (a significant negative PV anomaly) and its diurnal and synoptic evolution. This should be supported by observations of the surface conditions and surface energy balance measurements to assess the impacts the surface. Previous projects of convective-synoptic interaction have generally made little progress in understanding the mechanisms of interaction. Simply associating convective features with a particular phase of a passing large-scale wave does not indicate necessarily how the interaction is occurring. A basic premise of AMMA is that observational methods have advanced since GATE and COPT81 to the point that focused measurements with new technology on all of the relevant scales can elucidate the mechanisms of interaction between MCSs and AEWs. For this purpose the airflow in and through the MCSs need to be mapped on both the convective and mesoscale at the same time that AEWs are clearly distinguished on the synoptic scale. This requires air motion sensing on three distinct scales (convective, mesoscale, synoptic). To quantify the vertical mass fluxes in up- and downdrafts, it is important to determine the water budget of the MCSs, subdivided into convective and stratiform components. To determine the water budgets we must understand how the water is being condensed, frozen, sublimated, evaporated, and melted. This requires a microphysical component in the observational program. An important side benefit of the microphysical component will be a better understanding of cloud electrification and the global electrical circuit.

2.2 Quantify the impact of MCSs on the synoptic scale. To establish the impact of MCSs of the West African monsoon on the large-scale circulation through latent heating and momentum transport, we need first to document the water budgets of individual mesoscale convective systems. Water budgets are key to understanding the vertical distribution of the vertical mass transport in these systems. Knowing the vertical distribution of the vertical flux of air in the MCSs is pre-requisite to computing the vertical distribution of heating or momentum transports by MCSs. Heating profiles are significantly different in the convective and stratiform regions of MCSs (Houze 1982). Subdividing the precipitation seen on radar into convective and stratiform components provide the weighting functions for computing the profile of net heating by an MCS. The heating profiles associated with observed MCSs will be used to help understand the mesoscale-to-synoptic heating feedback via potential vorticity analysis. The large-scale flow field is balanced and thus described by the evolution of potential vorticity. Hence, the large-scale response to convection is sensitive to the vertical gradient of the latent heat release within mesoscale convective systems (Haynes and McIntyre, 1987, Mapes and Houze, 1995) and to the horizontal gradient of the momentum flux convergence. Most studies have focused on the vertical distribution of heating in oceanic systems (GATE, TOGA COARE, etc.). TRMM data indicate that the vertical distribution of heating varies significantly geographically (Schumacher and Houze 2003) and that the horizontal gradient of the vertical distribution of MCS heating has powerful influences on the large-scale mean circulation in the tropics (Schumacher et al. 2003). These studies indicate large differences in the response of the large-scale circulation to heating by oceanic and continental convection. The response of the large-scale circulation to heating that occurs in connection with tropical continental mesoscale convective systems as opposed to the much-studied processes over tropical oceans needs to be addressed by intensive field observations. MCSs over South America have been documented by Cifelli et al. (2002) and Silva Dias et al. (2002) as part of the TRMM/LBA campaign. However, the TRMM study of Schumacher and Houze (2003) indicates that, on average, the precipitation over tropical South America contains more frequent occurrence of oceanic-type systems, than over Africa. The corresponding vertical distribution of latent heating over these two continents may be quite different, with resulting differing impacts on the large-scale circulation. The role of momentum transports by mesoscale convective systems in determining the large- scale response to convection is less well understood than in the case of heating. However, the momentum transport processes are different in the convective and stratiform regions (e.g. Yang and Houze 1996). The impact of the stratiform region processes on the momentum transport therefore depends on the actual size of the stratiform region (as illustrated by Moncrieff and Klinker 1997 and Houze et al. 2000). It will be important to determine the sizes of convective and stratiform areas from radar observations. In addition the Doppler radar velocity data will be important in determining the profiles of momentum transport in the convective and stratiform regions. It will furthermore be necessary to document the environmental momentum via rawinsondes and wind profilers to combine with radar measurements of storm structure and kinematics. The water budgets of the mesoscale convective systems are not only the foundation for computing MCS heating and momentum transports. They also are a key component of the hydrological cycle of the overland region of tropical Africa. If prediction of the water cycle is going to be accurate, the exact way in which water is processed in mesoscale convective systems needs to be established. When combined with similar research from the S. America (TRMM- LBA) and Maritime Continent regions (DUNDEE, EMEX, MCTEX), we will realize a more complete picture of the kinematic and thermodynamic processes in tropical continental convection in general.

2.3 Determine the relative roles played by dynamics, microphysics and aerosols on the nature of MCSs and their electrical properties. Previous research on tropical convection is weighted heavily to oceanic convection. AMMA- Weather will help put our understanding of continental tropical convection on par with our current understanding of oceanic tropical convection. Figure 2.1 summarizes several key characteristics of West African convection as seen by the TRMM satellite in comparison to the convection occurring immediately offshore over the tropical Atlantic. The TRMM Precipitation Radar shows the band of maximum rainfall associated with the ITCZ over the eastern Atlantic (Fig. 2.1a). The apparent extension of this band of rain over West Africa is a signature of the West African monsoon, and the character of the rainfall changes immediately over the land. Schumacher and Houze (2003) show that the percentage of this rainfall falling from stratiform regions of MCSs is less over the land (Fig. 2.1b), which indicates that the continental MCSs are much different from the oceanic MCSs observed off the coast of Africa in GATE (Houze and Betts 1981). Nesbitt et al. (2000) further showed that the MCSs over land produced maximum scattering of 85 GHz microwave radiation (Fig. 2.1c), an indication that especially large and/or numerous ice particles were present at upper levels in the convection over land. Consistent with the presence of large and/or numerous ice particles in the upper levels of the continental MCSs is the higher frequency of lightning over land compared to the adjacent ocean (Fig. 2.1d). One of the primary objectives of AMMA-Weather is to obtain observations that will confirm and explain the ubiquitous ice particles aloft in the West African continental MCSs, their lightning, and their tendency to have a lower proportion of stratiform precipitation. Observing and explaining these cloud microphysical characteristics of the West African MCSs will lead to a better understanding of continental tropical MCSs in general. Better understanding of the microphysics will require more precise understanding of the dynamics of the MCS, which will lead to better definition of the vertical distribution of latent heat release in the MCSs, and hence a clearer picture of how the MCSs may act as potential vorticity sources for the large scale circulation.

Figure 2.1 TRMM based MCS climatology over Africa and tropical Atlantic for June-July-August (JJA). (a) Total JJA rainfall from the TRMM Precipitation Radar (PR). (b) Fraction of the JJA rainfall that is stratiform. Panels (a) and (b) were obtained by the methods of Schumacher and Houze (2003) and were provided by Courtney Schumacher. (c) Percentage of MCSs detected by the TRMM PR that had extensive ice scattering in the 85 GHz channel of the TRMM Microwave Sensor (TMI). Panel (c) was obtained by the methods of Nesbitt et al. (2000) and was provided by Steve Nesbitt. (d) Average lightning flash density (flashes per month) for June-July-August derived from the Lightning Image Sensor on TRMM (provided by Walt Petersen).

Convection over central tropical Africa, specifically the Congo, produces the largest lightning flash rates of any region on earth (Christian et al., 2003), associated with intense, isolated convective cells (Nesbitt et al., 2000). Significant flash rates occur in the AMMA region in association with organized convection in the form of squall lines and MCSs (Nesbitt et al., 2000) (Fig. 2.1). The substantial flash rates in the AMMA-Weather raise important scientific questions that AMMA-Weather can address. Whether these flash rates are more related to buoyancy and updraft dynamics (lightning-CAPE mechanism as mentioned in the previous section), or to the microphysics associated with enhanced aerosol concentrations, (or a combination of these processes) can be addressed in AMMA. Similar studies have recently been carried out in Brazil (Williams et al., 2002). These authors suggested an aerosol control on lightning flash rates, with maritime CCN concentrations aiding the formation of warm rain via coalescence. Coalescence in turn acts to reduce ice and water contents in the mixed phase region and therefore the efficiency of non-inductive charging via ice-ice collisions, leading to reduced flash rates. In CCN rich air masses, the coalescence process is less efficient, thus ice contents in the mixed phase region are enhanced as is ice-ice charging and flash rates. Nevertheless, evidence exists in Brazil and elsewhere (Petersen and Rutledge, 2001; Rutledge et al., 1992) that the primary control on flash rates may be CAPE, or, more explicitly, the vertical profile of thermal buoyancy. Recent work also suggests that cloud base height may also exercise a fundamental control on flash rate (Mushtak et al., 2004). Cloud base height has been hypothesized as a coupling to both the aerosol hypothesis and the CAPE mechanism. Lower cloud base heights allow a deeper coalescence zone. Higher cloud base heights are consistent with broader thermals and a more efficient conversion between CAPE and updraft kinetic energy. West Africa, in particular, the AMMA region presents a natural laboratory for furthering our understanding of ice phase microphysics and instability in terms of lightning flash rates. The latitudinal gradients in temperature and moisture in this region (Fig. 1.l) should promote reduced cloud base heights in the center of the monsoon trough and higher base heights to the north where more arid conditions are encountered. Related to this thermodynamic structure is the aerosol variability over the region, peaking to the north with expected higher CCN and large ice nucleus concentrations. A key feature will be to examine the lightning variability along the northern edge of the monsoon trough as the trough makes it seasonal march to the north. It is at this leading edge where we expect the greatest contrast in aerosol and thermodynamic characteristics to occur. Understanding the electrification and lightning will provide a more complete understanding of the West African mesoscale convective systems because electrification is a delicate combination of dynamical and microphysical effects. One of the major goals of AMMA-Weather will be to distinguish the relative roles of dynamics and aerosols in controlling the nature of MCSs, such as their intensity and organization, their proportions of convective and stratiform rain, and their characteristic heating profiles. Key observations required for AMMA-Weather will be sounding data to derive pre-storm instability and shear, Doppler observations to diagnose updraft speeds and storm circulation, and dual-polarimetric and dual-frequency data to assess the role of coalescence, riming, aggregation and supercooled water in ice-particle growth and storm electrification.

2.4 Improve our understanding of the processes that determine the diurnal cycle of precipitation and regional circulations in the West African monsoon Studies of the diurnal variation of precipitation over West Africa by McGarry and Reed (1978) and Dai (2001) using surface observations indicate a precipitation maximum in the evening. McGarry and Reed (1978) found that between 15 and 20 N thunderstorms were most frequent in the late afternoon or early evening, rain amounts were greatest shortly before midnight, convective cloud cover was most extensive shortly after midnight, and light rainfall most common near dawn. They could not explain this diurnal cycle in terms of a nocturnal low- level jet (although one may exist, as described below), and invoked the natural cycle of MCS development as an explanation. They argued that near-uniformity in the time of maximum rainfall over large areas ruled out squall propagation as an explanation for the diurnal cycle, despite the existence of westward-propagating AEWs and MCSs over the region. In contrast to these surface based studies, Yang and Slingo (2001) used satellite observations that indicated a precipitation maximum that ranged between the evening and early morning. It has been hypothesized that the early morning peak in the vicinity of the Greenwich meridian may be associated with the passage of MCSs triggered earlier by upstream orography but the nocturnal jet that develops in association with the diurnal variations in the heat low may also have a role. It is important to understand MCS development in terms of the diurnal cycle. The ability of the MCSs to generate midlevel PV anomalies is connected to the heating profile in stratiform regions (Fritsch et al. 1994). As shown in Figure 2.1, the stratiform rain fraction is less over land than over the ocean. Schumacher and Houze (2003) have suggested that the lower stratiform rain fraction over land is connected with the diurnal cooling of the land at night, which tends to truncate the MCS lifecycle. The MCS development will thus likely produce the largest PV anomalies when a process such as a low-level jet overrides the cooling cycle and maintains an MCS through the night. It is therefore important that AMMA include an evaluation of the diurnal cycle with respect to MCS development and maintenance. Observations and theory show that the heat low circulation and associated moisture transport varies strongly during the day due to the changes in surface heating (e.g. Parker et al., 2004b, Racz and Smith, 1999). As discussed by Parker et al (2004b) the basic diurnal cycle arises through an interplay between the diurnal cycle of the heat low over the Sahara (which drives the circulation) and the diurnal cycle of boundary layer convective turbulence (which tends to suppress the circulation). This coherent diurnal cycle of transport and mixing are key processes that influence the continental water budget. Moist air is advected polewards at low-levels in association with an acceleration towards low pressure. A return flow, characterized by much lower humidity above this exists around 600-700mb following the baroclinic variation of the pressure gradient with height. Theory and models suggest that the peak poleward surge in moisture at low levels occurs in the early morning. However there are almost no available observations to support this hypothesis since operational soundings are usually only launched at close to local noon or occasionally also at midnight. High frequency soundings are required to better document the diurnal evolution of the heat low circulation, establishment of the nocturnal jet and associated moisture transports to support the investigation of the role of these diurnal circulations on the continental water budget and precipitation. These observations should ideally take place during undisturbed periods in order to highlight the coherent diurnal cycle that is unaffected by weather. Further analysis of the diurnal cycle and its relationship with MCSs and AEWs is clearly needed, as well as the relationship of convection to the diurnal evolution of the boundary layer and low-level flow. The proposed AMMA networks will provide far more extensive and precise measurements of the diurnal cycle of precipitation than heretofore possible, as well as the relationship of the diurnally varying precipitation systems to AEWs.

2.5 Summary of Science Objectives The fundamental objective of the AMMA-Weather experiment is to document and analyze key scale interactions associated with tropical land-based precipitating systems. To address this objective requires observations and analysis on four scales: synoptic, mesoscale, convective and microscale. The West African Monsoon system is an ideal natural laboratory to carry out these investigations. Below is a summary of the key science objectives and associated key questions:

(A) Improve our understanding of the processes that influence the relationship between African Easterly Waves, MCSs and convection. • What are the relative roles of AEW dynamics (e.g., PV advection, adiabatic ascent, synoptic advection of temperature and humidity) and surface processes on the development of convection and MCSs in AEWs? • What are the relative roles of adiabatic and diabatic processes on the nature and evolution of the AEW-associated PV and low-level θ structures? • How do the MCS-associated PV structures feedback and influence the AEWs?

(B) Quantify the impact of MCSs on the synoptic scale. • What are the relative contributions of deep and stratiform precipitation to MCSs and how does this compare to other parts of the world? • How does the observed three dimensional heating field associated with MCSs impact the large-scale circulations and PV structures? • What are the relative roles of convective and stratiform regions in MCSs on momentum transport and how does this relate to the large-scale response to MCS convection?

(C) Determine the relative roles played by dynamics, microphysics and aerosols on the nature of MCSs and their electrical properties. • What are relevant microphysical processes leading to heavy convective rainfall in the convective elements of the MCSs? • What are the relative contributions of instability, aerosol concentration and type, and cloud base height in determining lightning flash rates? • What are the dynamical (including the pre-storm environment) and microphysical controls on MCS organization and intensity, and convective and stratiform rain proportions? (D) Improve our understanding of the processes that determine the diurnal cycle of precipitation and regional circulations over West Africa • What are the relative roles of upstream orography and local processes (e.g. nocturnal jet) in determining the diurnal cycle of precipitation? • What are the relevant physical processes that determine the coherent diurnal cycle of the heat low circulation and its impact on the continental water budget? • How are AEWs and MCSs and their interactions influenced by the diurnal cycle?

3 Field Program and Experiment Design 3.1 Overview of the international AMMA field program (A) International Planning African Monsoon Multidisciplinary Analysis (AMMA) is an international project to improve our knowledge and understanding of the West African monsoon (WAM) and its variability on daily-to-interannual timescales (http://medias.obs-mip.fr/amma/index.en.html). AMMA will carry out high resolution, short timescale process studies along with long-term monitoring over several seasons. International discussions have led to the establishment of three interrelated observing periods:

• The Long Term Observing Period (LOP) is concerned with multi-year observations of the WAM and establishment of sustained monitoring.

• The Enhanced Observing Period (EOP) covers 2005-2007 and is designed to serve as a link between the LOP and the SOP (below). A major focus will be on improving the rawinsonde network.

• The Special Observing Period (SOP) will provide a multi-scale and multi-process analysis of one monsoon season. As well as continuous monitoring of the WAM through the season, there will be more intensive observations of shorter duration that are concerned with key processes important for weather and climate. Focused process studies will take place during three key stages of the rainy season of 2006: (SOP-1) Monsoon Onset (15 May-30 June), (SOP-2) Peak Monsoon (1 July – 14 August) and (SOP-3) Late Monsoon (15 August-15 September).

The international AMMA program has been endorsed by GEWEX and CLIVAR within WCRP and will make significant contributions to WWW and IGBP (through IGAC). Scientists from more than 20 countries in Africa, Europe, and the US are involved in AMMA. African scientists have formed a network for sharing of data, for setting up collaborative projects and to assist in the organisation of international observations (AMMANET). An AMMA proposal to the EU Framework 6 program to support a wide series of activities has been successful and has been recommended funding at the 11.7M Euro level. It is expected that this will be supported. In addition to the EU, the main French funding agencies (CNES, CNRS/INSU, IFREMER, IRD, Meteo-France) have declared their support for AMMA: to date 8M Euro is expected to be committed to the project for ground based observations and aircraft. In the UK, a proposal of a coordinated land-atmosphere programme within AMMA, centered on Niamey and involving ground-based and aircraft observations has also been successful. Due to the efforts of European countries a strong infrastructure is being installed that will greatly relieve the demands of AMMA-Weather. A map showing locations of key towns and cities referred to in this document is included in Fig. 3.1 for orientation.

Figure 3.1 Towns and cities in West Africa referred to in the text

(B) Enhanced Multi-year Observations over the Continent (EOP, 2005-2007)) The observing strategy over the continent takes advantage of the strong surface network that has already been established through the CATCH project (see Figs. 3.2, 3.3). AMMA will strengthen the atmospheric observing network along this meridional transect in order to support the analysis of the seasonally evolving WAM and its variability.

Rawinsondes: Enhancements to the rawinsonde network are being coordinated by the International AMMA Sondes Working Group, initiated during 2003 (see http://www.env.leeds.ac.uk/~doug/AMMAsondes/). The working group has identified key rawinsonde stations that will launch a minimum of 1 sounding a day between March and October during 2005 and 2007 and a minimum of 2 soundings a day in 2006 (see Fig. 3.2). This network will be achieved with EU-funding (~2M Euro) and in collaboration with GCOS.

Raingauges: Surface rainfall observations are essential to support the analysis of the WAM on timescales from diurnal-to-interannual. AMMA will take advantage of the regional raingauge network that already exists and in particular the enhanced network established in the CATCH- window (Fig. 3.3). Three mesosites form the focus of the analysis along the climate transect. The Oueme catchment and Niamey square have 45 and 34 raingauges respectively all measuring at 5- minute intervals. The Gourma Malien site was established in the summer of 2003 and currently has 11 raingauges.

Surface Flux Stations: Surface flux and radiation measurements are required to support the analysis of surface water and energy budgets on daily-to-interannual timescales. Surface fluxes and radiation measurements will be provided by the Europeans during the EOP and SOP. Ten automatic surface flux stations will be located between the coast and the Saharan region within the CATCH window.

CATCH region and mesoscale sites

EOP Rawindsondes stations (once daily in 05/07 twice daily in 06)

Other currently reporting Rawinsondes stations

Figure 3.2 Rawinsonde network for the EOP (2005-2007) Niamey Mesosite MalianGourma 18 Mesosite 100 Baleyara

16 Ni 80 ge r R iv er Banizoumbou 14 60 NIAMEY

12 40

20 Say 10 0 -40-200 20406080100120 8 Local coordinates in km; Origin at 2°E;13°N

6 Oueme Mesosite 4 -2 024 0 BIRNI INA2

DJOUGOU KPAYEROUN

PARTAGO 5 OKPARA

PARAKOU KOKOUBOU PENESSOULOU BETEROU TCHETOU

BASSILA 9 1.5 2 2.5 Symbols Figure 3. 3 Surface Observations in the CATCH-window (top Automatic Rain Aquifer River middle). Enhanced observations are indicated at the Niamey

Manual Rain Aquifer River Vegetation Mesosite (top right), Oueme Mesosite (bottom right) and Malien Gourma Mesosite (top left). Radar: In addition to operational weather radars in the region, the French will provide an X-band radar that will be located in the Oueme catchment to support the analysis of the continental water budget. Using the polarimetric capabilities of this system, rainmaps will be generated to a range of approximately 50 km. Single Doppler observations will also be made by this radar, allowing 2-D airflows to be deduced as convective systems pass over the site. These observations will be highly complementary to the observations planned for the SOP.

Lightning : The ZEUS lightning (http://sifnos.engr.uconn.edu) is now operational over Africa. ZEUS consists on 10 VLF receivers (7-15 kHz) for detecting VLF emissions from lightning. ZEUS mainly detects the more energetic cloud-to-ground flashes, with discrimination for negative and positive polarity flashes. Fig. 3.3 shows ZEUS detected lightning over W. Africa for September 2003. AMMA will utilize ZEUS data for lightning detection and determination of CG flash rates in support of the lightning-related objectives of this work. Location accuracy of lightning in the AMMA domain is approximately 10-20 km.

Figure 3.4 Lightning flash densities from ZEUS for September 2003. Peak values are on the order of 1 flash/km2 per month. The band of lightning extending southwestwards from western S. America is an artifact of the processing algorithm.

(C) Special Observing Period (2006) The observing strategy for AMMA-International over the continent will take advantage of the surface observations available through CATCH (Fig. 3.3) and the surface flux stations and enhanced sounding network established in the EOP (Fig. 3.2). The overall AMMA observing strategy over the continent will consist of an enhancement of the surface and atmospheric observations along the north-south climate transect that includes CATCH. This will be supported by aircraft observations and remotely sensed data from radar and satellites. Through such enhancements the CATCH hydrology experiment will be developed into a 3D multi-scale observing system to support the analysis of the ITCZ-AEJ-heat low system, AEWS, embedded convective and mesoscale precipitation systems and water, heat and momentum budgets. The main European effort will be centered on the Oueme mesosite in Benin (see Fig. 3.3) whereas the AMMA-Weather effort described below will be centered on the Niamey mesosite in Niger (see Fig. 3.3). This coordinated effort will allow an unprecedented opportunity to compare and contrast the different MCS structures that characterize the two different environments along the climate transect. Before describing the AMMA-Weather effort Table 3.1 highlights the European funded and proposed facilities that are relevant to the AMMA-Weather proposal.

Table 3.1 Key European resources relevant to AMMA-Weather (SOP2) Facility Funding Source Location Contribution (Status) RONSARD + X-Band EU/France (funded) Djougou in Oueme Analysis of MCSs Radars catchment – Fig. 3.3) including microphysics Bistatic receiver Germany (proposed) Djougou in Oueme Analysis of MCSs catchment – Fig. 3.3 including microphysics SODAR-UHF-VHF EU/France (proposed) Djougou in Oueme Analysis of catchment – Fig. 3.3 boundary layer and stability Rawinsondes (extra 4- UK (funded) Parakou, Cotonou, Synoptic analysis, 6 daily in addition to France (proposed) Minna, Tamale, heat and moisture EOP sondes in Fig. Niamey – Fig. 3.1 budgets 3.2) French Falcon + EU/France (funded) Based in Niamey Analysis of MCSs dropsondes and interactions with environment

French ATR-42 EU/France (funded) Based in Niamey Impact of MCSs on boundary layer, aerosol characteristics BAe146 + dropsondes UK (funded) Based in Niamey Impact of MCSs on environment, aerosol characteristics SODAR wind profiler UK (funded) Niamey Analysis of boundary layer – diurnal cycle

Driftsonde French-NCAR Launched from Analysis of Synoptic collaboration N’Djamena (Chad) Environment (proposed – non-NSF)

3.2 Experiment Design for AMMA-Weather (A) Proposed Location and Timing of AMMA-Weather Field Campaign The AMMA-Weather field campaign will be centered on Niamey (13oN, 2oE, 227m above sea level) and will take place during July and August 2006. Table 3.2 shows that climatologically these are the wettest months. During these months, Niamey is located close to the climatological axis of the AEJ (c.f. Fig 1.1). Consistent with this, Niamey experiences frequent passage of AEWs, MCSs and SAL outbreaks making this an ideal location to observe the interactions between synoptic, mesoscale, convective and micro scales including the impacts of aerosol that are at the heart of this proposal. Niamey is located in the Sahel which experiences marked variability in rainfall on intraseasonal to interannual timescales (e.g. Nicholsen (1980), Lamb (1983), Mathews, 2004). Planning for the proposed field campaign must consider such variability and its impact on the science objectives.

Table 3.2 Monthly mean station rainfall (in mm), based on 1921-1990 (obtained from CPC).

May June July August September

Niamey 35 76 149 189 93

Parakou 131 163 185 209 230

There are several advantages of focusing the AMMA-Weather field campaign in the Niamey region rather than in the Oueme region which is climatologically wetter (see Parakou (9N, 2E) in Table 3.2). Recent analysis of rainfall and MCSs in this region highlights the fact that although seasonal rainfall totals are lower in Niamey than in Oueme the MCSs are more intense (Mathon and Laurent (2001), Mohr (2003)). Figure 3.5 shows MCS tracking statistics based on the automatic tracking method used in Hodges and Thorncroft (1997). This analysis, which highlights the more organized and intense MCSs, places the climatological storm track axis close to Niamey. Based on this analysis the mean number of MCSs that pass through a 4ox4o box centered on Niamey is 12 and in the period between 1983 and 2001 the minimum was 8 and maximum was 19. For a box centered on Parakou in the Oueme catchment the mean is 9, the maximum was 13 and the minimum was 5.

Figure 3.5 MCS track density for July and August for MCSs that traveled more than 500km and lasted more than 0.5 days. Units are number density per month per unit area where the unit area is equivalent

to a 5o radius spherical cap which is equivalent approximately to 106Km2. The data used is ISCCP B3 data gridded and smoothed to a 1o grid with data every 3 hours. Methodology follows that used in Hodges and Thorncroft (1997). (Figure provided by Kevin Hodges).

The nature of the environment and the MCSs clearly changes dramatically over a very short distance (c.f. Fig 1.1) and so it is desirable to develop an observing strategy that takes account of these meridional variations. This will be achieved in international AMMA through providing radar measurements in both the Niamey mesosite (AMMA-weather, US) observing the more intense MCSs and in the Oueme mesosite (European) observing the more ubiquitous rainfall. AEWs influence the whole of the West African region but they tend to have peak amplitudes in the AMMA-Weather region (Fig. 3.2, c.f. Albignat and Reed, 1980) where the baroclinic interactions tend to be strongest (Norquist et al, 1977, Berry and Thorncroft, 2004). AEWs are present throughout the summer season but tend to have the strongest convective signal in the Niamey region during July and August (e.g. Grist, 2002) consistent with the peak in the rainy season there (see table 3.2). The critical link between rainfall and synoptic scale disturbances can be seen by comparing dynamical and convective measures of AEW activity. Figure 3.6 shows the interannual variability of two such measures of AEW-variability for the July-August period: (i) 2- 6 day filtered meridional wind variance (in red) and (ii) TD-filtered OLR variance following Wheeler and Kiladis (1999) (in blue). TD-filtered OLR has been shown to effectively isolate westward propagating OLR associated with easterly waves. From this figure we can see that, although AEWs are present every year, they do exhibit variability on interannual timescales. It is also clear from this figure that variability in the convective signal is generally positively correlated to variability in the wave activity. There are some years however (e.g. 1994) where the relationship is weak, highlighting the need to consider more closely the nature of the interactions between AEWs and convection, at the heart of the AMMA-Weather proposal.

Figure 3.6 Time-series of two measures of AEW activity for July-August and averaged for the area between 10W-10E and 7.5N-15N: (i) 2-6 day filtered meridional wind variance based on NCEP reanalysis (in red) and (ii) TD-filtered OLR (in blue). The TD filtering refers to spatial and temporal filtering of the OLR data following the methodology of Wheeler and Kiladis (1999) where TD refers to the “tropical depression” area of the full spectrum, (data courtesy George Kiladis). Based on these statistics, the minimum number of organized systems we might expect is 9 in the July-August timeframe which is more than enough to carry out all the MCS-specific objectives proposed here. We would also expect to observe additional less-organized systems that would also be worthy of study for comparison. Regarding the potential for weak AEW activity, even in years with weak AEW activity such as 1993 we would expect to observe enough coherent AEWs (based on ECMWF reanalyses, more than 8 coherent AEWs passed through the AMMA-Weather region) to be adequate for our AEW-related objectives. Finally, during periods of weak AEW- activity during the 2 months, we will continue to address key scientific objectives concerning the West African monsoon, specifically, MCS interactions with the AEJ and the nature of the heat low circulations and nocturnal jet.

(B) Major US Facilities for AMMA-Weather Observation of the synoptic, mesoscale, convective and microscales needed to address the science objectives highlighted in section 2 can be achieved with a combination of a well-designed rawinsonde network and ground-based radar measurements. These facilities will be requested for the July to August period in 2006 and are summarized in table 3.3. Although this period includes the beginning of SOP3, for convenience in the text we will refer to this 2-month period as SOP2.

Table 3.3: Major Facilities Proposed for AMMA-Weather Facility Funding Source Location PIs 2 ISSs and sondes NSF Gao/Tahoua Thorncroft/Johnson (750 sondes) Additional EU/UK/GCOS/US Niamey/ Parakou EU/UK PIs Sondes (900) Ouagadougou Thorncroft/Johnson S-Pol and C-Band NSF Niamey Houze/Rutledge/Smull radar S-band vertical NSF Niamey Houze/Rutledge/Smull profiler

In addition to the proposed NSF facilities and the European facilities there are additional US facilities that will benefit the AMMA-Weather field campaign. These are summarized in Table 3.4.

Table 3.4: Other Relevant US Facilities Facility Funding Source Location US PIs (Status) NOAA-P3 NOAA/EU Based in Niamey Jorgensen/Dunion/Marks/Smull (proposed) Mobile ARM ARM (proposed) Niamey Lamb is US-contact Facility Unit Africa Lightning NSF (funded) Bulk of Continent Anagnostou Network and adjacent waters

(C) Soundings During SOP-2 enhancements to the rawinsonde network established for the EOP (Fig. 3.2) are needed to provide measurements of the large-scale thermodynamic and dynamic evolution associated with the passage of AEWs and to support the analysis of the impact of MCSs on the larger-scale. These enhancements will also support analysis of the AEJ and also the heat low structure on the poleward side of the AEJ. The proposed enhancements will be established through a combination of enhancing the frequency of sonde launches at a number of key stations and providing extra sonde launches using supplementary systems at other key locations. The enhanced rawinsonde network for the SOP-2 will be coordinated with the proposed location for ground-based radars in Niamey (US) and the Oueme mesosite (EU) (see (D) below). Two overlapping quadrilaterals of sondes centered on Niamey and Parakou (in Oueme) will be established during SOP-2 (Fig. 3.7) This combination of soundings, together with the international network established in EOP is essential for the analysis of the passage of convective disturbances and associated positive PV anomalies in the rainy region as well as the low-level potential temperature anomalies and negative PV anomalies associated with the SAL on the fringes of the Sahara. Such a network is needed for a complete picture of the AEWs and the AEJ. The poleward soundings will be particularly useful for analysis of the diurnal and synoptic evolution of the heat low and the associated SAL. With these quadrilaterals centered on the surface radar networks, high quality heat and moisture budgets can be obtained. Refinements of the budgets over the radar domains can be achieved using radar-derived rainfall rates and the procedure of constrained variational analysis (Zhang and Lin, 1997). In combination with the radar data, these refined latent heating profiles can be partitioned into convective and stratiform components to assess their relative contributions to the development of midlevel PV anomalies within the AEWs.

GAO NIAMEY OUAGADOUGOU TAHOUA

PARAKOU TAMALE MINNA

COTONOU

The CATCH region and mesoscale sites RadioSoundings on GTS RadioSounding not on GTS

Integrated Sounding Units (US)

Sites with enhanced soundings (Fr, Ge, UK, US)

Figure 3.7 Proposed enhancements to the sounding network during SOP2.

The quadrilateral centered around Niamey (13oN, 2E) will be supported by the US (Fig. 3.7 (red solid lines)). Along with the operational sonde stations at Niamey and Ougadougou (Bukina Faso), additional sonde stations are proposed at Parakou (Benin), Gao (Mali) and Tahoua (Niger). These sites have been selected to minimize low-level blockage of the flow and other local, highly nonlinear effects such as land-sea breezes. These will provide the large-scale environment needed to support the radar observations made with US-radar based in Niamey. This network will be achieved through providing additional sondes at Niamey, Ougadougou and Parakou (although Niamey and Parakou will have European support too) and by installing Integrating Sounding Systems (ISSs) at the remaining 2 sites. During intense observing periods of MCSs, sondes are required 6-8xdaily for the 5 stations in the quadrilateral, otherwise 2-4 sondes per day will be sufficient for the analysis of AEW passages and the AEJ. In addition to the rawinsondes, ISSs will include a 915 MHz profiler providing high resolution profiles of lower tropospheric winds, a Radio Acoustic Sounding System (RASS) providing high time resolution profiles of lower tropospheric virtual temperatures and a standard surface meteorological package. This will support the analysis of the evolving meso- and synoptic-scale structures and stability changes. The proposed quadrilateral centered around Parakou (9oN) will be supported by Europeans (Fig. 3.7 (blue solid lines)). Along with a sonde station at Parakou (Benin), this quadrilateral will use sondes launched from Niamey (Niger), Tamale (Ghana), Minna (Nigeria) and Cotonou (Benin). The Europeans will provide extra sondes for Niamey and Parakou (which has been operated successfully through the German IMPETUS project). They will establish new stations at Cotonou, Tamale and Minna as part of the EOP (see Fig.3.2) – this is being achieved in collaboration with ASECNA (regional aviation authority) and local national Met. Sevices. The combination of the two quadrilaterals in addition to the EOP sounding network will provide us with an unprecedented view of the AEWs and AEJ in this period supporting the analysis of the synoptic scale. Efforts will be made to ensure that all sondes will be broadcast on the GTS enabling them to be assimilated into operational NWP models (we are in communication with EMC at NCEP regarding technical issues related to thus).

(D) Ground-based radar measurements Enhancements to the synoptic rawinsonde network as discussed above will enable a comprehensive analysis of large-scale thermodynamic and dynamic variations, which will provide an analysis of the passage of AEWs. The network will also measure the large-scale environmental impact of MCSs and thus support the analysis of MCS-AEW interactions. The 3D analysis of the kinematic and thermodynamic structure of the MCSs within the synoptic framework will be afforded by coordinated ground-based and airborne radar observations. The ground based radar network will consist of facilities provided by France and the U.S., as shown in Fig. 3.8 The airborne radar systems will be aboard the proposed NOAA P3 aircraft. There will be two centers of ground-based radar operation, Djougou in Oueme in the south and Niamey in the north. The northern radar operation will consist of U.S. radars, specifically the NCAR S-polka radar (S and Ka band system) and a Doppler C-band, (either the MIT C-band or the NASA/TOGA C-band). Doppler, polarimetric radars from France will form the southern radar network (C and X-band systems). It is essential to have two radar nodes because of the strong meridional gradient in the WAM, in which the MCSs to the north have a propensity toward rapidly moving MCS structures (Hodges and Thorncroft, 1997). The northern mesoscale convective systems occur in close proximity to the Saharan aerosol source region, and exhibit greater ice scattering (Nesbitt et al., 2000). The MCSs to the south appear to be less affected by the Saharan aerosol, but they account for more rainfall (see Table 3.2). Understanding the WAM entails understanding this meridional gradient of convective behavior. The north and south dual-Doppler radar pairs will be founded on the basis of the highly successful French experiment COPT81, which documented the kinematics of several west African MCSs by a dual-Doppler radar pair in Ivory Coast (Sommeria and Testud 1984, Chauzy et al. 1985, Chong et al. 1987, Roux 1988, Sun and Roux 1988, Chong and Hauser 1989). The dual-Doppler pairs at Djougou and Niamey will each be similar to the COPT81 dual-Doppler installation. In addition, these dual-Doppler pairs will have polarimetric capabilities, which were not available in COPT81. Polarimetric observations are essential to achieve the goal of understanding the microphysical aspects of the MCSs in the WAM.

Sounding network

Primary Doppler

A/C operations

Extended Doppler aircraft zone Supported by French and UK Aircraft with dropsondes

Fi gure 3.8 Location of dual Doppler operations and area of NOAA P3 aircraft The north and south dual-Doppler radar pairs will be founded on the basis of the highly successful French experiment COPT81, which documented the kinematics of several west African MCSs by a dual-Doppler radar pair in Ivory Coast (Sommeria and Testud 1984, Chauzy et al. 1985, Chong et al. 1987, Roux 1988, Sun and Roux 1988, Chong and Hauser 1989). The dual-Doppler pairs at Djougou and Niamey will each be similar to the COPT81 dual-Doppler installation. In addition, these dual-Doppler pairs will have polarimetric capabilities, which were not available in COPT81. Polarimetric observations are essential to achieve the goal of understanding the microphysical aspects of the MCSs in the WAM. The dual-Doppler networks will be capable of determining accurate rainfall estimates via polarimetric techniques, vertical echo structure, 3D high-resolution internal circulation of MCSs, and microphysical internal structure at both the northern and southern locations. Collection of the data in a consistent way at the two sites will allow comparison and understanding of the meridional differences in convective structures. From the observed structural differences of MCSs observed at the two sites, it will be possible to determine differences in how the MCSs interact with the AEWs in the northern and southern regions. The dynamical and microphysical differences in MCS observed at the two radar sites will indicate how the mass transports, heating, and momentum transports accomplished by MCSs varies from north to south. These differences in turn relate to the way that the MCSs act as subsynoptic PV sources and modify the AEWs in which they are embedded. A strength of the dual-Doppler networks is that they can provide a temporally continuous record of MCS structure throughout the experimental period. However, they only sample the MCSs during their passage over the relatively small area covered by the ground-based radars. The time and space scales of the MCSs are such that the systems are typically not observed for their entire lifetimes over the area of radar coverage. For this reason, it is important to supplement the ground based network with airborne radar measurements, as has been done in previous field experiments such as TOGA COARE and BAMEX. The aircraft will extend the dual Doppler coverage of MCSs by the ground based radars and follow systems into and out of the range of the ground based regions in order to cover the life cycles of the MCSs more completely. The aircraft in AMMA can supplement the coverage of MCSs in the vicinity of either the southern dual-Doppler site (Djougou) or the northern site (Niamey). Further, the NOAA P3 can follow an AEW system as it tracks westward using Dakar as an alternate recovery base if needed. Following an AEW for several days will be essential for documenting the role of mesoscale convective vortices (MCVs) play in generating local vorticity that may later play a role in hurricane genesis, as well as addressing other fundamental questions of MCS evolution on time scales exceeding those required for convective systems to transit a ground-based dual- Doppler array. The domain of the NOAA P-3 turboprop Doppler equipped aircraft is shown in Fig. 3.8. The Djougou site will consist of the French RONSARD C-band Doppler/polarimetric radar, and the French X-band X-port radar. The RONSARD has been used successfully in field projects around the world over the past 20 years, including COPT81 and MAP. The RONSARD has recently been upgraded to include dual-polarization capability. The X-port also has polarimetric capability. The polarimetric capabilities of these two radars provide desired mitigation of attenuation affects at these shorter wavelengths. Differential phase measurements can be used to correct for both specific and differential attenuation. These two radars used together will directly support the hydrological observations in the southern CATCH network. The polarimetric data will also allow aspects of the ice particle characteristics in the upper regions of the MCSs to be inferred, as well as mixing ratios to be diagnosed, and relative fractions of water and ice to be determined. The French dual-Doppler pair will thus document the dynamics, microphysics, and precipitation amounts of the MCSs affecting the southern region. The Niamey site will consist of the U.S./NCAR S-Pol radar combined with a supplementary C-band radar (either the TOGA C-band or MIT C-band Doppler radars could be used in this role). This dual-Doppler pair will document the precipitation, microphysics, and dynamics of the MCSs closer to the heart of the AEJ and the Sahara. These systems are expected to be more frequently rapidly moving squall types of mesoscale systems (Aspliden et al. 1976, Houze and Betts 1981, Hodges and Thorncroft, 1997), and they are expected to have greater electrical activity than those to the south. The measurements from this site can be directly compared to the observations at Djougou to understand their different dynamical and microphysical structures and behaviors. The microphysical characteristics of the MCSs will be inferred from the S-pol polarimetric radar. The methodology of Vivekanandan et al. (1999) and others will be used to infer the microphysical characteristics of the ice and liquid particles throughout the 3D volumes of the observed MCSs. This methodology has been used successfully with S-Pol in a variety of storm types (e.g. Medina and Houze 2003). The Ka-band observations from S-Pol can be used to deduce liquid water contents (to identify regions of mixed phase) by measuring differential attenuation between the Ka band and S-band frequencies. One limitation of scanning ground-based radars is that scans can require 5-10 min to complete, and each scan consists of a sequence of antenna sweeps separated by finite elevation angles. To detect fine-scale structure critical to interpreting the microphysical observations from the S-Pol radar, a vertically pointing radar, such as a NOAA/ETL S-band vertical profiler will be operated within the range of the ground-based scanning radars. The fine-scale reflectivity and vertical radial velocity patterns from the vertically pointing radars, which obtain data every few seconds, will help interpret the microphysical particle-type data inferred from the polarimetric radars. In AMMA-Weather, the S-pol radar will be placed in the observational array to play a primary role, in documenting both the dynamics (via multiple Doppler wind fields) and microphysics (via polarimetric and multi-wavelength measurements) of West African MCSs. S- pol exceeds the capabilities of other available radars in that it is both dual-polarized and dual- wavelength (S band and Ka band), and highly portable. The dual-polarization capability allows for improved rain estimation and identification of hydrometeor type (Vivekanandan et al. 1999). As noted above these polarimetric capabilities at a single wavelength allow for the MCS dynamics and microphysics to be documented simultaneously, as has been done in previous projects such as MAP (e.g. Medina and Houze 2003). The ability to couple the dynamics and microphysics extends the results of AMMA-Weather beyond the dual-Doppler results of COPT81 and allows the patterns of mass transport and heating within MCSs to be diagnosed more precisely. As a result, how the MCSs feedback to the dynamics of AEWs, (e.g. via PV thinking), can be better assessed—indeed this feedback is one of the principle objectives of AMMA-Weather. The combination of S-band polarimetric measurements with the Doppler radial velocity fields will also address mesoscale, convective and microphysical objectives of AMMA- Weather through improved understanding of the dynamical-microphysical interactions by which the MCSs over continental Africa produce systematically large ice particles at upper levels leading to scattering in the upwelling radiation in the 85GHz channel (Nesbitt et al. 2000) and excessive electrification and lightning (Fig. 2.1(d)). The dual wavelength capability of S-Pol will allow the microphysics to be probed in more detail than at one wavelength alone, resulting in key additional information. The ratio of echo intensity at S and Ka bands will indicate the sizes of the ice particles in the MCSs, a crucial feature related to the electrification and the ability of the MCSs to form stratiform regions. These aspects of MCSs are not only of intrinsic interest but are indirect indicators of the spatial distribution of mass transport and latent heating, which are in turn related to the PV production by the MCSs. The differential attenuation at Ka and S band will indicate the path integrated liquid water content and thus the indirectly the mass transport at lower levels of the cloud by the convective updrafts—again related to the pattern of heating in the interior of MCSs. The availability of Ka band echo also allows the rain measuring capability to be enhanced, thus more accurately indicating the net mass transport and latent heat release in MCSs. Complementing the dual-Doppler observations to the east will be the expected operation of the NASA N-pol, S-band polarimetric radar operating at Dakar, Senegal. NASA expects to operate N-pol during at least SOP-3 to aid verification of satellite-derived rainfall data. N-pol will benefit AMMA by providing single-Doppler and polarimetric measurements of systems near the coast, allowing for comparison with systems observed by AMMA over the continental interior and well offshore.

(E) Coordination with Aircraft Ground-based platforms are a valuable source of high quality and temporally-continuous quantitative measurements within convective systems passing through the AMMA experiment region. Data from these platforms allow statistically meaningful measures of key MCS attributes to be accumulated over the coarse of a field campaign. However, because individual systems may exhibit lifecycles extending over many hours during which storms transit large distances, specification of the evolving character of individual systems is sometimes difficult using even the highest quality data available from a ground-based dual-Doppler array. For this reason, available mobile platforms play a complementary role in the experiment design. U.S. PI’s bring a wealth of experience in terms of operating and analyzing data collected by instrumented research aircraft within the context of fixed observational arrays. Real-time communications and analysis tools available in conjunction with sophisticated ground-based systems such as S-Pol may effectively be utilized in conjunction with analogous European resources to guide and coordinate operation of multiple airborne platforms within range of the ground-based dual-Doppler arrays when suitable opportunities exist. Several aircraft that may contribute to meeting AMMA objectives have been identified or are being sought. Platforms particularly key to satisfying objectives identified by U.S. PI’s include:

NOAA P3 Airborne pseudo-dual-Doppler observations will ideally made by a NOAA P-3 turboprop aircraft equipped with a vertically scanning X-band radar. If made available to AMMA via support from NOAA or other sources external to NSF, this aircraft will be used to extend the dual-Doppler coverage of the ground-based radars and in particular to follow the evolution of vortex structures developing in the stratiform regions of MCSs. These observations would help link the ground-based radar observations to the AEW development on the synoptic scale. As a weather-hardened platform capable of flying abeam of intense convective lines and penetrating the more stratiform interior regions of MCSs, the NOAA P3 is ideally suited to collection of a variety of remote and in situ (e.g., microphysical) measurements of importance to AMMA. Envisioned flight patterns will likely be similar to those successfully utilized in other climatological regimes during projects such as TOGA COARE and BAMEX, and will be used to selectively highlight evolving structure of deep convection and, in a few select cases, development of MCV circulations within long-lived systems.

European Aircraft In addition to the ground-based rawinsounding network additional soundings are required that target more precisely the large-scale environment in the vicinity of MCSs to assess the environmental impact of the MCSs and vice versa. Concomitant profiles of thermodynamic conditions and vertical shear are critical not only for understanding the response of convective systems to their environment (viz. through specification of boundary conditions on numerical simulations), but will also prove invaluable for representation of PV distributions and other key attributes associated with up-scale developments including MCVs. These soundings will ideally be provided by the following European aircraft with dropsonde capability:

• French Falcon: This aircraft will release dropsondes from at high altitudes around MCSs to provide the large-scale environment in which the MCSs are embedded. These flights will be coordinated with the NOAA P-3 and the French ATR aircraft carrying out boundary layer measurements pre- and post-storm passage. • French ATR-42: This aircraft will fly ahead of and behind targeted MCSs to assess the impact of the MCS on the boundary layer environment. This will support the analysis of the impact of MCSs on the environment and combined with other observations will allow an investigation of boundary layer recovery. This aircraft is also expected to make important aerosol measurements in the vicinity of MCSs, especially to reveal the aerosol characteristics of air being ingested into the convective systems. • British BAe 146: This aircraft will release dropsondes from an altitude of about 12km along meridional transects before and after the passage of MCSs to support the analysis of the evolution of the surface conditions and boundary layer before and after a rainfall event. This will be coordinated with US and French aircraft.

The following table summarizes flight characteristics of the various aforementioned instrumented aircraft platforms: Table 3.5 Aircraft Capabilities Research Aircraft Endurance (hours) Range (naut. miles) Ceiling (feet)

US NOAA P-3 9.5 3,800 27,000 France French Falcon 5 2,500 42,000 French ATR-42 6 1,800 25,000 UK BAe 146 5+ 2,500 40,000

4. Logistics and Feasibility of Deployment 4.1 Niamey Airport Niamey airport will be the operations center during SOP2. It will provide security and an access to utilities and infrastructure that will be established through the AMMA International Project Office. While the international logistical planning has only just begun to support the various groups that will be based in Niamey, we know that the airport has basic utilities such as power (220V/50Hz), water, phone and internet (although we may require a SAT phone to be installed) and local accommodation. Logistical support for operations out of Niamey will be provided through JOSS supported by the local headquarters of IRD-Niger. IRD (Institut de Recherche pour le Developpement) is a French public science and technology research institute under the joint authority of the French ministries in charge of research and overseas development (http://www.ird.fr/us/).

4.2 Deployment of Soundings AMMA-Weather plans to deploy two ISSs – one in Gao ((16N, 0W), Mali) and one in Tahoua ((15N, 5E), Niger) (see Fig. 3.4). The ISS in Gao will be located at Gao International airport. The ISS in Tahoua will also be located at an airport. We expect the ISSs to be shipped to Cotonou and transported by road. In addition to the ISSs, AMMA-Weather plans to provide enhanced soundings at Niamey, Ouagadougou and Parakou. Niamey and Ouagadogou are both the responsibility of ASECNA, the civil aviation authority that oversees many of the regional sounding stations. Niamey currently launches 2 sondes daily while Ouagadougou launches once daily. C. Thorncroft attended a meeting with ASECNA in Dakar in May 2004, along with D. Parker (co-chair of the International Sondes Working Group) and Jan Polcher (coordinator of the AMMA EU project). ASECNA are happy to collaborate with the AMMA project and may become a full partner of the EU consortium. The extra sondes required at Ouagadougou and Niamey will be the responsibility of AMMA-Weather and will be coordinated with ASECNA. If successful, AMMA-Weather must consider several key issues. Most importantly these include human resources for extra launches, equipment upgrades to use new RS92 sondes and hydrogen production. AMMA-Weather is also interacting with the GCOS representative at NOAA (H. Diamond) on these issues since Niamey is a GCOS station. We expect to share costs for the Niamey soundings with the EU and UK.

4.3 Deployment of Radars Radar facilities requested for the U.S. interests in AMMA include the NCAR S-pol, 10 cm polarimetric Doppler radar (field funding provided by the NSF Deployment Pool), and a C-band radar, such as the MIT Doppler (funding to be requested from the Mesoscale Dynamical Meteorology Program at NSF). Both of these radars are highly portable and well-suited for this deployment. Each radar facility is shipped and operated within seatainers, thus their high portability. These are entirely self-contained operations and require no external facilities or out- buildings. They are easily and rather inexpensively transported on container ships. Any modern seaport has adequate crane facilities for loading and offloading. We expect the S-Pol and C-Band radars to be shipped to Cotonou in Benin and transported by road to Niamey. The roads in Benin to Niamey are excellent. Each radar is well suited to operate on generator power, however, commercial power might be readily available in the Niamey area. Since the radars will be separated by approximately 60 km to afford mesoscale detail in the dual-Doppler observations, we should be able to easily communicate between radar sites via radio. A VHF aircraft radio will also be installed at S-pol for communicating with the Doppler-equipped turboprop aircraft. The PI’s on this request are very familiar with the operation of both of these radars, having used them in projects such as MAP, STEPS2000, TRMM-LBA and NAME. Since Niamey is the site of an international airport, heavy equipment such as a small crane should be readily available for installing the S-pol antenna. S-pol does not require a radome or any extensive concrete preparation work for its installation.

4.4 Site Investigation Trips It is envisaged that there will be a series of site investigation or survey trips carried out by JOSS to evaluate the feasibility of operations and make arrangements for a variety of logistical coordination issues. These include (but are not limited to): diplomatic issues, determination of operations sites for the radars and ISSs, health, safety and security issues, travel, accommodation and communications. The AMMA PI’s recommend that this first site survey take place as soon as possible so that the findings can be used to adjust as necessary, the AMMA deployment plans. In the event of an unfavorable site survey for Niamey, we propose that the S-pol radar and the S-band profiler be deployed in the Oueme catchment, in conjunction with the two French radars to be operated by the French around that locale. In this case, we would not request the additional U.S. C-band radar. The NOAA P-3 would therefore be relied upon to sample the convective systems around Niamey. The deployment of S-pol in Oueme would still be of considerable scientific importance. There we would form a dual-Doppler network between the S-pol and the RONSARD C-band radar to investigate the dynamical structures of MCSs. The French polarimetric X-band would be viewed then as a microphysical probe, used especially to diagnose microphysical processes and precipitation rates in weaker convection and attendant stratiform precipitation. This arrangement would also provide a unique opportunity to compare polarimetric- based precipitation estimates at four radar frequencies, S, C, X and perhaps Ka. We would also be able to explore the differential phase technique for correcting both C and X-band attenuation in tropical rain by comparison to the unattenuated S-band data.

5. Modeling Modeling for AMMA-Weather consists of three fundamental parts, with overlap between each. These are all linked together by the use of special AMMA observations for verification of the representation of African convection on multiple scales.

5.1 NWP (A) Use of Operational Products Forecasts and analyses from operational centers will be critical for several aspects of AMMA and its modeling component, including in-the-field decision making for conducting intensive observation periods, large-scale diagnostic studies and initial and boundary conditions for limited area modeling studies. The enhanced observations available in particular from the SOP will provide an opportunity to evaluate the ability of operational NWP forecasts and analyses to represent the evolution of weather systems over West Africa and the tropical Atlantic, their convective characteristics and interactions with the large-scale environment. Global models employing convective parameterizations will be critical to understanding the key scale interactions. The sounding networks will provide a foundation for validating the parameterization schemes via budget studies and via direct detailed observation of the convection. The ability of these models to represent the key scale interactions between mesoscale vortices and synoptic AEWs will be assessed in a PV framework. The radar and aircraft experiments proposed in Section 3 will observe and monitor the evolution of the mesoscale vortices that develop in mature and MCSs and follow them until they are absorbed into the synoptic scale wave structures.

(B) Case Studies High-resolution regional simulations will be used to investigate the multi-scale aspects of convection with time scales of a few days or less, building on earlier simulations performed with either global models or regional climate models. The advantage is chiefly in the improved large- scale analyses available for model initialization (a point to be verified under part (A) above) and the enhanced representation of topography and land surface characteristics. Figure 5.1 gives an example of computation domains that might typify a regional modeling study, in this case using grid increments of 10 km and 3 km. Simulations ranging from 1-3 days duration are currently easily accommodated on computing architectures such as NCAR’s IBM SP (bluesky), and we anticipate even larger simulations will be possible following AMMA. The model of choice for these simulations will be one of the commonly used mesoscale models, MM5, RAMS or, most likely, the new Weather Research and Forecasting (WRF) model (Michalakes et al. 2001). Because of the versatility of regional models, questions about sensitivity to grid spacing and physical parameterization can be addressed in a straightforward way. Regional simulations will help us formulate further hypotheses regarding dynamics of AEWs and MCSs that can be tested with observations and additional, targeted (perhaps quasi-idealized) simulations. For example it

Figure 5.1. (left) hypothetical regional model domain for on a 10-km mesh; (right) hypothetical domain on a 3-km mesh. will be desirable to carry out adiabatic simulations to investigate the role of diabatic processes and in particular, PV production, on the evolution of the AEW-MCS systems. It has been demonstrated recently that WRF can realistically replicate mesoscale convective system structure when run daily in a real-time mode (Done et al. 2004). Given the prominent dynamical control of topography, easterly waves and the variations of surface characteristics, all of which can be specified with reasonable accuracy, we anticipate similarly successful simulations of AMMA convective systems. Regional models would also be used for experimental data assimilation. Incorporation of AMMA soundings (EU, UK and US) would be straightforward. A three-dimensional variational assimilation package is currently available for WRF, and an ensemble Kalman filter is being tested. New techniques are being developed which will allow assimilation of radar reflectivity and radial velocity to initialize mesoscale convective systems in these models and these will be attempted depending on their maturity.

5.2 Use of Cloud-system Resolving Models (A) Studies of individual MCSs Cloud-system resolving models (CRMs) will be an important tool for studying convection characteristics over the continent and tropical Atlantic and in particular the nature of the MCSs observed during the SOPs. The enhanced observations of the atmosphere and surface conditions will be used to initialize and force CRMs to support the investigation of observed convection regimes and their associated heat, water and momentum budgets. This work will be combined with column model studies and investigation and assessment of convective parameterizations used in GCMs used for weather and climate prediction. These models are essential for understanding how the MCSs produce extensive ice scattering and lightning and what effects if any the Saharan aerosols have on the convection and its ultimate feedback to the larger-scale. They will also be crucial in understanding the differences between MCSs over the sub-Saharan continent and those over the ocean (recall Fig. 3.2.6). CRMs will also be used to perform high-resolution studies of the dynamics of MCSs, including sensitivity to microphysical and radiative parameterization and treatment of turbulence. Grid spacing in the range of 100 m to 2 km will be considered. Particular attention will be paid to upscale growth of MCSs and their effect on the mesoscale. These can be diagnosed through different frameworks, including PV budgets or from the momentum transport perspective.

(B) Convection Regimes Laing and Fritsch (1997) showed that numerous areas of the world share a broadly similar convection phenomenology, featuring initation of mesoscale convective complexes (MCCs) downwind of mountain ranges often in the presence of lower-tropospheric jets that locally enhance poleward transport of water vapor. Carbone et al. (2002) have shown that convection over the U.S. exhibits remarkable spatial and temporal coherence and diurnal regularity. It is believed that the African region exhibits similar coherence of convection. However, the dynamics of this coherence is heavily tied to the behavior of the combined easterly wave – MCS system which may prove more coupled than the relationship between synoptic-scale transients and convection over North America. Furthermore, exploration of coherence in large-domain, cloud-system resolving simulations has just begun (Moncrieff and Liu, 2003; Trier et al. 2004). These early studies have shown considerable improvement of cloud-system resolving models relative to those that parameterize convection. Comparison of the two types of simulations has revealed fundamental shortcomings of parameterizations due to their poor treatment of propagating convection (e.g. Davis et al. 2003). However, problems have been noted including an improper phase locking of convection to the orography and diurnal cycle and large-scale errors, the sources of which are unknown. By examining cloud-system resolving simulations integrated on domains up to 3000 km in length (e.g. Fig. 5.1) and extending for 1-2 weeks, the intrinsic cloud system dynamics can be investigated within models with realistic boundary conditions. Here there is no formal distinction from traditional CRMs and state-of-the-art mesoscale models run at a comparable grid spacing of 2-3 km (e.g. the WRF model, Mickalakes et al 2001). Such simulations, while computationally demanding, are only modest extensions of simulations already performed (Moncrieff and Liu 2003, Trier et al. 2004). The coherence properties of convection will be evaluated using satellite inferences of precipitation (highly calibrated with AMMA special observations) in an extension of observational analyses currently underway (A. Laing, personal communication).

5.3 Idealized studies (A) Global models Because of the multi-scale nature of the WAM-AEW-MCS study, a hierarchy of models is necessary to fully address the problem. Models attempting to simulate details of individual AEWs or MCSs, inititalized from observations directly, form one set of simulations. It is also desirable to consider simple models, wherein heating or surface processes can be systematically varied and the responses noted. Large-scale models run in quasi-idealized mode will allow Rossby and gravity wave responses to localized heat sources, with varying temporal behavior, to be examined. For example, it will be important to extend the work of Schumacher et al (2003) to the West African region by considering the dynamical response to heating over the West African continent; the results from the analysis of MCS heating characteristics obtained from AMMA- weather observations will determine the heating functions in these integrations.

(B) Regional models Analogous to the idealized large-scale models, mesoscale models such as WRF will be run in quasi-idealized mode, prescribing aspects of the large-scale flow and lower boundary condition to more systematically address hypotheses that may come from field observations or simulations of more complicated flows. Lin et al. (2004) have examined idealized flows around the Ethiopian Highlands to deduce topographic mechanisms for mesoscale vortices, for instance. Regional models are adept at predicting terrain induced flows. They have also been demonstrated to produce realistic results when simulating convection in horizontally homogeneous environments (e.g. Davis 2000). By considering various factors (baroclinity, topography, land surface contrasts, etc.) as building blocks of the systems composing the WAM, the idealized framework can systematically evaluate their relative contributions to convection behavior.

6. Project Management 6.1 International Management Structure International project management and coordination is envisaged in two stages corresponding to the planning and the field phase of International AMMA. Three international bodies will be established for the planning phase of AMMA. They are:

• The International Governing Panel (IGP) • The International Scientific Steering Committee (ISSC) • The International Coordination and Implementation Group (ICIG)

The International Governing Panel will consist of representatives of national agencies such as the meteorological services and science funding bodies. The IGP carries the final responsibility for the implementation of the overall Program and for any modifications of scientific objectives and schedules. The International Scientific Steering Committee consists of leading atmospheric scientists and technological experts. The ISSC is responsible for the definition of the scientific objectives of the program and for the scientific planning of the overall program and exists. The International Coordination and Implementation Group is responsible for overseeing the detailed conduct of program activities. This activity will be coordinated through the AMMA International Project Office in Paris (expected to be operational summer 2004). More information regarding international planning may obtained from the international AMMA website at http://www.medias.obs-mip.fr/amma.

6.2 AMMA-Weather Management and Coordination Management of AMMA-Weather will be accomplished through the AMMA-Weather group in collaboration with JOSS. At the present time the AMMA-Weather group is included in table 6.1 together with their affiliation and research area. During the pre-field phase the AMMA- Weather group will decide the scientific priorities of AMMA-Weather and will select and secure the appropriate instruments to meet the scientific objectives. It will coordinate with other US- activities in the Niamey region including the potential deployment of the NOAA-P3 (Jorgensen) and the ARM mobile facility (Slingo et al). Such coordination will be achieved through joint planning/scientific meetings during the pre-field phase and joint scientific meetings in the post- field phase. It is envisioned that the UCAR Joint Office for Science Support (JOSS) in Boulder, Colorado will carry out the many planning and coordination tasks and to interface with the numerous international organizations involved in AMMA. During the field phase, U.S. operations will be fully integrated with the international operations. These field operation details will be described in a future AMMA-Weather Operations Plan document. The AMMA-Weather project will be closely coordinated with the International AMMA program. This is facilitated by the fact that C. Thorncroft is co-chair of the ISSC and is also on the International Sondes Working Group. If AMMA-Weather is successful, other members of AMMA-Weather will likely become members of the ISSC and/or ICIG. AMMA-Weather is also a sub-component of the US-AMMA science plan originally presented to NSF in December 2003 and which can be found at http://www.joss.ucar.edu/amma. AMMA-Weather will continue to ensure strong interactions with the AMMA-climate and AMMA-aerosol communities through interactions and meetings organised through the US-AMMA SSG.

Table 6.1 AMMA-Weather Group Scientist Affiliation Research Area Chris Thorncroft (Coordinator) SUNY at Albany AEW-MCS interactions and associated PV structures; AEJ- Heat low interactions and diurnal cycle Chris Davis NCAR Spatio-temporal coherence of convective systems, upscale growth of MCSs and associated MCVs, CRM simulations of MCSs Robert Houze University of Washington Clouds physics of MCSs, heating and PV development in MCSs; MCS-synoptic interaction Richard Johnson Colorado State University MCSs and coupling with the boundary layer, atmospheric heat and moisture budgets Steven Rutledge Colorado State University Thermodynamical, microphysical and lightning aspects of MCSs Brad Smull University of Washington MCS lifecycles, stratiform region development and related vorticity production, resultant AEW interactions

AMMA-Weather will seek to collaborate with other US-PIs and US-research groups who have joint interests in the AMMA-Weather science proposed in this document. These include PIs and groups who will be bringing instrumentation to the region and other key participants and modelers (See Appendix). In addition there is representation of people involved in scientific outreach and capacity building in the West African region. A key aspect of this is the GLOBE program, a hands-on science and education program focusing on Earth system science. Although headquartered at UCAR in the U.S., GLOBE is a truly international program with more than 100 countries participating (see http://www.GLOBE.gov). Most of the West African countries covered in the AMMA area of activity are already members of GLOBE, including Benin, Senegal, Ghana, Cameroon, Nigeria, Burkina Faso, Cape Verde, Guinea and Mali. As we approach the field phase of AMMA, formal coordination between GLOBE and AMMA will be pursued.

7. Data Management 7.1 Background The development and maintenance of a comprehensive and accurate data archive is a critical step in meeting the scientific objectives of AMMA-Weather. The overall guiding philosophy for the U.S. AMMA data management is to coordinate with the international AMMA effort and make the completed U.S. data set available to the world scientific community as soon as possible following the SOP. The U.S. AMMA data will be available to the scientific community through a number of designated distributed International Data Archive Centers (IDACs) coordinated by the UCAR/Joint Office for Science Support (JOSS), Boulder, Colorado, USA in coordination with the Institut National Polytechnique (INP), Grenoble, France. The JOSS activities fall into three major areas: (1) develop and implement an on-line field catalog to provide in-field support and project summaries/updates for the AMMA-Weather Principal Investigators (PIs) and to ensure optimum data collection/inventories; (2) coordinate with INP for the establishment of a distributed archive system; and (3) provide data access/support of both research and operational data sets for the U.S. AMMA PIs and the world scientific community. JOSS will coordinate with INP and the International AMMA Project Office to make arrangements to ensure that “orphan” data sets (e.g. data from smaller regional and local networks) will be archived and made available through the AMMA archive. JOSS will also represent U.S. AMMA Data Management interests either directly with the International AMMA Project Office or through participation on an AMMA Data Management Committee.

7.2 On-line Field Catalog The JOSS will develop and maintain an On-line Field Catalog that will be functional during and following the AMMA-Weather Field Phase (SOP-2). This catalog will be implemented using a WWW-based browser interface and will be operational at the AMMA Operations Center with a "mirror" site(s) in Boulder, CO (or possibly other location(s) as needed). Data collection information about both operational and research data sets (including metadata and overview documentation) will be entered into the system in near real-time approximately beginning one month before field operations through SOP-2. The catalog will permit distribution of data entry (data collection details, field summary notes, certain operational data etc.), updates of the status of platforms and instrumentation (on a daily basis or more), data browsing (listings, plots) and limited catalog information. Daily summaries will be prepared and contain information regarding operations (aircraft flight times, major instrument systems sampling times, etc.). It is important and desirable for the PIs to contribute graphics (e.g. plots in GIF or Postscript format) and/or data into the Field Catalog for in-field instrumentation comparisons and evaluations. Preliminary data/information may be restricted to the AMMA Operations Center or AMMA PIs only. Public access to status information, mission summaries, and operational and selected research data sets outside of the AMMA Operations Center will be available from the mirrored field catalog site(s) such as Boulder.

7.3 Data Access The AMMA-Weather (in coordination with International AMMA) will take advantage of the capabilities at existing IDACs to implement a distributed data management system. This system will provide “one-stop” single-point access using the World Wide Web for search and order of all AMMA data from IDACs operated by different agencies with the capability to transfer small data sets electronically from the respective IDAC to the user. Access to the data will be provided through a Data Management web page (linked from the AMMA-Weather Home page). This Data Management page will contain general information on the data activities on-going in AMMA- Weather (i.e. documents, reports), links to related programs and projects, and direct data access via the various IDACs.

References Arakawa, A. and Schubert, W.H., Interaction of a cumulus cloud ensemble with the large-scale environment, Part I, J. Atmos. Sci. 31, 674-701 (1974). Aspliden, C.L., Tourre Y. and Sabine, J.B., Some climatological aspects of West African disturbance lines during GATE, Mon. Wea. Rev. 104, 1029-1035 (1976). Avila, L.A. and Pasch, R.J., Atlantic tropical systems of 1991, Mon. Wea. Rev. 120, 2688-2696 (1992). Berry, G. and Thorncroft, C., Case study of an intense African easterly wave, submitted to Mon. Wea. Rev. (2004) Burpee, R.W., The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, 77-90 (1972). Dai, A., Global precipitation and thunderstorm frequencies. Part II: diurnal variations. J. Climate. 14, 1112- 1128, (2001) Carbone, R. E., J. D. Tuttle, D. A. Ahijevych, and S. B. Trier, 2002: Inferences of Predictability Associated with Warm Season Precipitation Episodes. J. Atmos. Sci., 59, 2033-2056. Chauzy, S., Chong, M., Delannoy, A. and Despiau, S., The June 22 tropical squall line observed during the COPT 81 experiment: Electrical signature associated with dynamical structure and precipitation, J. Geophys. Res. 90, 6091-6098 (1985). Chong, M. and Hauser, D., A tropical squall line observed during the COPT 81 experiment in W. Africa. Part II: Water budget, Mon. Wea. Rev. 117, 728-744 (1989). Chong, M., Amayenc, P., Scialom, G. and Testud, J., A tropical squall line observed during the COPT 81 experiment in West Africa. Part I: Kinematic structure inferred from dual-Doppler radar data, Mon. Wea. Rev. 115, 670-694 (1987). Christian, H.J., Blakeslee, R.J., Boccippio, D.J., Boeck, W.L., Buecher, D.E., Driscoll, K.T., Goodman, S.J., Hall, J.M., Koshak, W.J., Mach, D.M. and Stewart, F., Global frequency and distribution of lightning as observed from space by the Optical Transient Detector, J. Geophys. Res., 108, No. D1, 4005, doi:10.1029/2002JD002347, (2003) Cifelli, R., Petersen, W.A., Carey, L.D., Rutledge, S.A. and Silva Dias, M.A.F., Radar observations of the Kinematic, Microphysical and Precipitation Characteristics of Two MCSs in TRMM-LBA, J. Geophys. Res. 29(10), 10.1029/2000JD000264 (2002). Cook, K.H. Generation of the African easterly jet and its role in determining West African precipitation, J. Climate, 12, 1165-1184, (1999) Davis, C. A., K. W. Manning, R. E. Carbone, J. D. Tuttle, and S. B. Trier, 2003: Coherence of warm season continental rainfall in numerical weather prediction models. Mon. Wea. Rev., 131, 2667-2679. Diehdiou, A., Janicot, S., Viltard, A., de Felice, P. and Laurent, H., Easterly wave regimes and associated convection over West Africa and the tropical Atlantic: Results from NCEP/NCAR and ECMWF reanalyses. Climate Dyn. 15, 795-822 (1999). Done, J., C. Davis, and M. Weisman, 2004: The Next Generation of NWP: Explicit Forecasts of Convection Using Weather Research and Forecasting (WRF) Model. Atmos. Sci. Lett., In Press. Duvel, J. P., Convection over tropical Africa and the Atlantic ocean during northern summer. Part II: Modulation by easterly waves, Mon. Wea. Rev. 118, 1955-1868 (1990). Emanuel, K. A., Atmospheric Convection, Oxford University Press, NY, 580pp. (1994). Fritsch, J.M., Murphy, J.D. and Kain, J.S. Warm core vortex amplification over land. J. Atmos. Sci., 51, 1781-1806 (1994) Gamache, J.F., and R.A. Houze, Jr., Water budget of a mesoscale convective system in the tropics, J. Atmos. Sci. 40, 1835-1850 (1983). Godfrey, J.S., Houze, R.A. Jr., Johnson, R.H., Lukas, R., Redelsperger, J.-L., Sumi, A. and Weller, R., COARE: An interim report, J. Geophys. Res. 103, Vol. 7–Special TOGA issue, 14395-14450 (1998). Grist, J.P., Easterly waves over Africa. Part 1: The seasonal cycle and contrasts between wet and dry years, Mon. Wea. Rev., 130, 197-211, (2002) Haynes, P.H. and MacIntyre, M.E., On the evolution of vorticity and potential vorticity in the presence of diabatic heating and frictional or other forces, J. Atmos. Sci. 44, 828-841 (1987). Hodges, K.I., and Thorncroft, C.D., Distribution and Statistics of African Mesoscale Convective Weather Systems Based on the ISCCP Meteosat Imagery, Mon. Wea. Rev. 125(11), 2821–2837 (1997). Houze, R.A. Jr., Cheng, C.-P., Leary, C.A. and Gamache, J.F., Diagnosis of cloud mass and heat fluxes from radar and synoptic data, J. Atmos., Sci, 37, 754-773, (1980) Houze, R.A., Jr., Cloud clusters and large-scale vertical motions in the tropics, J. Meteor. Soc. Japan 60, 396-410 (1982). Houze, R.A., Jr., Observed structure of mesoscale convective systems and implications for large-scale heating, Q.J.R. Meteor. Soc. 115, 424-461 (1989). Houze, R.A. Jr., Cloud Dynamics. Academic Press, San Diego, 573pp (1993) Houze, R.A., Jr., Chen, S.S., Kingsmill, D.E., Serra, Y. and Yuter, S.E., Convection over the Pacific warm pool in relation to the atmospheric Kelvin-Rossby wave, J. Atmos. Sci. 57, 3058-3089 (2000). Houze, R.A., Jr., and Betts, A.K., Convection in GATE, Rev. Geophys. Space Phys. 19, 541-576 (1981). Kummerow, C.W. Barnes, T. Kozu, J. Shiue and J. Simpson, The Tropical Rainfall Measuring Mission (TRMM) sensor package, J. Atmos. Oceanic Technol. 15, 808-816 (1998). Lebel, T., Delclaux, F., Le Barbé, L. and Polcher, J., From GCM Scales to Hydrological Scales: rainfall variability in West Africa, Stochastic Env. Res. and Risk Assessment (2000). Laing, A. G., and J. M. Fritsch, 1997: The global population of mesoscale convective complexes. Quart. J. Royal Meteor. Soc., 123, 389-405. Lamb, P.J., West African water variations between recent contrasting Subsaharan droughts, Tellus A35, 198-212 (1983). Lin, Y.-L. K. E. Robertson, and C. M. Hill, 2004: Origin and propagation of a disturbance associated with an Africa Easterly wave as precursors of Hurricane Alberto (2000). Preprints, 26th Conference on Hurricanes and Tropical Meteorology. 21-22. Mapes, B. E. and Houze, Jr., R.A., Diabatic divergence profiles in western Pacific mesoscale convective systems, J. Atmos. Sci. 52, 1807-1828 (1995). Mathews, A.J., 2004 Intraseasonal climate variability over Africa, J. Clim., 17, 2427-2440, (2004). Mathon, V. and Laurent, H., Life-cycle of Sahelian mesoscale convective cloud systems, Q.J.R. Meteorol.Soc., 127, 377-406 (2001) Mathon, V.H., Laurent, H. and Lebel, T. Mesoscale connective system rainfall in the Sahel. J. Appl. Meteorol., 41, 1081-1092, (2002) McGarry, M.M. and Reed, R.J., Diurnal variation in convective activity and precipitation dduring Phases II and III of GATE, Mon. Wea. Rev., (1977) Medina, S., and Houze, R.A. Jr., Air motions and precipitation growth in Alpine storms, Q.J.R. Meteor. Soc., special MAP issue 129, 345-371 (2003). Michalakes, J., S. Chen, J. Dudhia, L. Hart, J. Klemp, J. Middlecoff, and W. Skamarock, 2001: Developments in Teracomputing: Proceedings of the Ninth ECMWF Workshop on the Use of High Performance Computing in Meteorology. Eds, W. Zwieflhofer and N. Kreitz. World Scientific, Singapore. pp. 269-276. Mohr, K.I., Interannual, monthly, and regional variability in the wet season diurnal cycle of precipitation in Sub-Saharan Africa, J. Clim., in press (2003). Moncrieff, M.W. and Klinker, E. Mesoscale cloud systems in the tropical western Pacific as a process in general circulation models,: A TOGA COARE case study. Quart. J. Roy. Met. Soc., 123, 350-369, (1997) Moncrieff, M. W., and C. Liu, 2003: Sequences of Precipitation and Organized Convection: Dynamics and Parameterization. AMS Symposium on Observing and Understanding the Variability of Water Vapor in Weather and Climate. Paper 3.7. Mushtak,V.C., Williams, E.R. and Boccippio, D.J., Latitudinal variations of cloud base height and lightning in the tropics accepted in Atmospherc research, (2004) Nesbitt, S. W., Zipser, E. J. and Cecil, D. J., A census of precipitation features in the Tropics using TRMM: Radar, ice scattering, and ice observations, J. Clim. 13, 4087–4106 (2000). Nicholson, S. E., The nature of rainfall fluctuations in subtropical West Africa, Mon. Wea. Rev. 108, 473- 487 (1980). Nicholsen, S.E., Tucker, C.J. and Ba., M.B. Desertification, drought, and surface vegetation: An example from West African Sahel. Bull. Amer. Meteor. Soc., 79, 815-829, (1998). Norquist, D.C., Recker, E.E. and Reed, R.J., The energetics of African wave disturbances as observed during phase III of GATE, Mon. Wea. Rev., 105, 334-342, (1977). Parker, D.J. and Thorpe, A.J., Conditional convective heating in a baroclinic atmosphere: A model of convective frontogenesis, 52, 10, 1699-1711, (1995) Parker, D.J, Thorncroft, C. D., Burton, R.R., and Diongue-Niang, A., Analysis of the African Easterly Jet using aircraft observations from the JET2000 experiment, conditionally accepted Q. J. R. Meteorol. Soc. (2004a) Parker, D.J., Diongue, A., Ellis, R., Felton, M., Taylor, C.M., Thorncroft, C.D. and Bessemoulin, P., The diurnal cycle of the West African Monsoon in observations and numerical model analyses, submitted (2004b). Parsons, D.P., Yoneyama, K. and Redelsperger, J-L., The evolution of the tropical western Pacific atmosphere-ocean system following the arrival of a dry intrusion, Q.J.R. Meteorol. Soc. 126, 517-548 (2000). Payne, S.W., and McGarry, M.M. The relationship of satellite inferred convective activityto easterly waves over west Africa and the adjacent oceanduring phase III of GATE. Mon. Weather. Rev., 105, 414-420 (1977) Petersen, W. A. and S. A. Rutledge, On the Relationship Between Cloud-to-Ground Lightning and Convective Rainfall. Journal of Geophysical Research, 103, 14025-14040. (1998) Petersen, W. A., and S. A. Rutledge, Regional Variability in Tropical Convection: Observations from TRMM, Journal of Climate, 14, 3566-3586, (2001). Pytharoulis, I. and Thorncroft, C.D. The low-level structure of African easterly waves in 1995, Mon. Wea. Rev. 127, 2266-2280 (1999). Racz, Z. and Smith, R.K., The dynamics of heat lows, Q.J.R. Meteorol. Soc. 43, 225-252 (1999). Reed, R.J., Norquist, D.C. and Recker, E.E., The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105, 317-333 (1977). Roux, F., The West African squall line observed on 23 June 1981 during COPT 81: Kinematics and thermodynamics of the convective region, J. Atmos. Sci. 45, 406-426 (1988). Rutledge, S.A., Williams, E.R., Keenan, T.D., The Down Under Doppler and Electricity Experiment (DUNDEE): Overview and preliminary results. Bull. Amer. Meteor. Soc., 73, 3-16. (1992) Schumacher, C., and R. A. Houze, Jr., Stratiform rain in the tropics as seen by the TRMM Precipitation Radar, J. Clim. 16, 1739-1756 (2003). Schumacher, C., Houze, Jr., R.A. and Kraucunas, I., The tropical dynamical response to latent heating estimates derived from the TRMM Precipitation Radar, J. Atmos. Sci., conditionally accepted (2003). Silva Dias, M.A.F., Rutlege, S., Kabat, P., Silvia Dias, P.L., Nobre, C., Fisch, G., Doman, A.Z., Zipser, E., Garstang, M., Manzi, A.O., Artaxo, P., Gielow, R. and Gatti, L., Cloud and Rain Processes in a Biosphere-Atmosphere Interaction Context in the Amazon Region, J. Geophys. Res. 29(10), 10.1029/2001JD000335 (2002). Sommeria, G., and Testud, J., COPT81: A field experiment designed for the study of dynamics and electrical activity of deep convection in continental tropical regions. Bull. Amer. Meteor. Soc. 65, 4- 10 (1984) Sun, J., and Roux, F., Thermodynamic structure of the trailing-stratifrom regions of two west African squall lines. Annales Geophysicæ. 6, 659-670 (1988). Taylor, C.M. and Clark, D.B., The diurnal cycle and African easterly waves: A land surface perspective, Q.J.R. Meteorol. Soc. 127, 845-867 (2001). Taylor C.M. and Lebel, T., Observational evidence of persistent convective-scale rainfall patterns, Mon. Weath. Rev. 126, 1597-1607 (1998). Thorncroft, C. D. and Hoskins, B.J., An idealized study of African easterly waves, Part I: A linear view. Q.J.R. Meteor. Soc. 120, 953-982 (1994a). Thorncroft, C.D. and Hoskins, B.J., An idealized study of African easterly waves, Part II: A nonlinear view, Q.J.R. Meteor. Soc. 120, 983-1015 (1994b). Thorncroft, C.D. and Blackburn, M., Maintenance of the African easterly jet, Q.J.R. Meteorol. Soc. 125, 763-786 (1999). Thorncroft, C.D. and Hodges, K.I., African easterly wave variability and its relationship to tropical cyclone activity, J. Clim. 14, 1166-1179 (2001). Thorncroft, C.D., Parker, D.J., Burton, R.R., Diop, M., Ayers, J.H., Barjat, H., Devereau, S., Diongue, A., Dumelow, R., Kindred, D., Price, N.M., Saloum., M. Taylor, C.M. and Tompkins, A.M., The JET2000 Project: Aircraft Observations of the African Easterly Jet and African Easterly Waves, BAMS 2003 84, 337-351 (2003). Trier, S. B., C. A. Davis, R. E. Carbone, and S. Frederick, 2004: WRF simulations of rainfall coherence over the continental United States Preprints, 16th Conference on Numerical Weather Prediction. Seattle, WA. American Meteorological Society. Paper 17.1: Vivekanandan, J., Ellis, S.M., Oye, R., Zrnic, D.S., Ryzhkov, A.V., Straka, J., Cloud microphysics retrieval using S-band dual-polarization radar measurements. Bul. Amer. Met. Soc., 80, 3., 381-388. Webster, P. J., Clark, C., Cherikova, G., Fasullo, J., Han, W., Loschnigg, J. and Sahami, K., The monsoon as a self-regulating coupled ocean–atmosphere system, Meteorology at the Millennium, R. Pearce, Ed., Academic Press, pp. 198–219 (2001). Wheeler, M., and Kiladis, G.N., Convectively coupled equatorial waves: Analysis of clouds and temperature in the wavenumber-frequency domain J. Atmos. Sci. 56, 374-399 (1999). Williams, E.R., and Coauthors, Contrasting convective regimes over the Amazon: Implications for cloud electrification, J. Geophys. Res., LBA Special Issue, 107, D20, 8082, doi:10.1029/2001JD000380, (2002) Yanai, M., Esbensen, S. and Chu, J.H., Determination of bulk properties of tropical cloud clusters from large-scale heat and moisture budgets, J. Atmos. Sci. 30, 611-627 (1973). Yang, G.-Y. and Slingo, J., The diurnal cycle in the Tropics. Mon. Wea. Rev., (2001) Yang, M.-J. and R. A. Houze, Jr Momentum budget of a squall line with trailing stratiform precipitation: Calculations with a high resolution numerical model, J. Atmos. Sci., 53, 3629-3652 (1996) Zhang, M.H. and Lin, J.L., Constrained variational analysis of sounding data based on column-integrated budgets of mass, heat, moisture, and momentum: Approach and application to ARM measurements. J. Atmos. Sci., 54, 1503-1524, (1997)

Appendix: Preliminary list of US Scientists expected to collaborate with AMMA-Weather

Participant Affilliation Research Area Emmanouil Anagnostou University of Connecticut Lightning Network, Analysis of convection Kerry Cook University of Cornell Regional numerical modelling of West Africa Jason Dunion NOAA-HRD Analysis of the SAL-AEW/tropical cyclone relationships Silvia Garzoli NOAA-AOML Ron Brown (Buoys and Drifters), coupled atmosphere-land-ocean processes Greg Holland Aerosonde Aerosonde measurements – AEW- tropical cyclone relationships, African Seedlings Hurricane Experiment (ASHE) Greg Jenkins Howard University N-pol, Rainguages in Senegal Analysis of rainfall and modeling David Jorgensen NOAA NOAA P3, Analysis of MCSs and associated MCVs Everette Joseph Howard University N-Pol. Raingauges in Senegal Analysis of rainfall George Kiladis NOAA-Aeronomy AEW-Convection relationships and variability Peter Lamb University of Oklahoma ARM mobile facility, West African climate variability and impacts Peggy Lamone NCAR GLOBE, outreach Arlene Liang NCAR Satellite climatology of MCSs, rainfall estimation Frank Marks NOAA-HRD Analysis of the SAL-AEW/tropical cyclone relationships Kingtse Mo NOAA-CPC Climate Modeling Karen Mohr SUNY at Albany Numerical modelling of Sahelian MCSs and land-surface interactions Bob Molinari NOAA-AOML Ron Brown (Buoys and Drifters), coupled atmosphere-land-ocean processes Dave Parsons NCAR Driftsonde, Impacts of dry air intrusions on convection, THORPEX Courtney Shumacher Texas, A&M Impact of MCSs on large-scale WAM, comparison with oceanic MCSs Joanne Simpson NASA Aerosonde measurements – AEW- tropical cyclone relationships, ASHE Naomi Surgi NOAA-NCEP NWP for West Africa and tropical Atlantic Wasilla Thiaw NOAA-CPC Weather and climate modeling, capacity building, impact studies Peter Webster Georgia Tech. AEW-tropical cyclone relationships, ASHE Chidong Zhang University of Miami Coupled atmosphere-land-ocean processes