Mesoscale Convective Systems Over Western Equatorial Africa and Their Relationship to Large-Scale Circulation

1272 MONTHLY WEATHER REVIEW VOLUME 137

Mesoscale Convective Systems over Western Equatorial Africa and Their Relationship to Large-Scale Circulation

BRIAN JACKSON,SHARON E. NICHOLSON, AND DOUGLAS KLOTTER Department of Meteorology, The Florida State University, Tallahassee, Florida

(Manuscript received 14 January 2008, in ﬁnal form 9 September 2008)

ABSTRACT

This study examines mesoscale convective systems (MCSs) over western equatorial Africa using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. This region experiences some of the world’s most intense thunderstorms and highest lightning frequency, but has low rainfall relative to other equatorial regions. The analyses of MCS activity include the frequency of occurrence, diurnal and annual cycles, and associated volumetric and convective rainfall. Also evaluated is the lightning activity associated with the MCSs. Emphasis is placed on the diurnal cycle and on the continental-scale motion fields in this region. The diurnal cycle shows a maximum in MCS count around 1500–1800 LT, a morning minimum, and substantial activity during the night; there is little seasonal variation in the diurnal cycle, suggesting stationary influences such as orography. Our analysis shows four maxima in MCS activity, three of which are related to local geography (two orographic and one over Lake Victoria). The fourth coincides with a midtropospheric convergence maximum in the right entrance quadrant of the African easterly jet of the Southern Hemisphere (AEJ-S). This maximum is substantially stronger in the September–November rainy season, when the jet is well developed, than in the March–May rainy season, when the jet is absent. Lightning frequency and flashes per MCS are also greatest during September–November; maxima occur in the right entrance quadrant of the AEJ-S. The lightning maximum is somewhat south of the MCS maximum and coincides with the low-lying areas of central Africa. Overall, the results of this study suggest that large-scale topography plays a critical role in the spatial and diurnal patterns of convection, lightning, and rainfall in this region. More speculative is the role of the AEJ-S, but this preliminary analysis suggests that it does play a role in the anomalous intensity of convection in western equatorial Africa.

1. Introduction world’s most intense thunderstorms and the highest frequency of lightning flashes (Zipser et al. 2006; Tor- The western equatorial sector of Africa is, from a acinta and Zipser 2001; Petersen and Rutledge 2001). meteorological standpoint, one of the world’s most in- Even within the tropics, western equatorial Africa is a teresting, but also the most poorly understood regions. convective anomaly. The only regions with comparable Some of the world’s highest rainfall totals are reported storm intensity, including the United States, Argentina, over Mount Cameroon, on the western edge of the and parts of the Indian subcontinent, are in the midlati- region; mean annual rainfall exceeds 10 m at the sta- tudes (Mohr and Zipser 1996a,b). Despite the intensity tion Debundscha, Cameroon, according to the Office of storms, rainfall in the region is only moderate com- de la Recherche Scientifique et Technique Outre-Mer pared with equatorial regions of the Amazon and In- (ORSTOM 1978). The coastal sector experiences indonesia (Petersen and Rutledge 2001; Zipser et al. terannual fluctuations of rainfall in association with 2006). Western equatorial Africa is also the only tropi- Atlantic warmings that rival those produced by El Nin˜ o cal region with intense convection in all seasons. Storms along the South American desert coast (Nicholson and are anomalously large compared to other tropical re- Entekhabi 1987). This region also experiences the gions, with the mean size of all precipitation features exceeding 500 km2 in some parts of the region (Nesbitt et al. 2006). This region also makes a disproportionately Corresponding author address: Sharon E. Nicholson, Depart- ment of Meteorology, The Florida State University, Tallahassee, large contribution to overshooting convection (i.e., deep FL 32308. convective systems with radar tops above 14 km; Liu E-mail: [email protected] and Zipser 2005).

DOI: 10.1175/2008MWR2525.1

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This knowledge of the storm regime in equatorial and the relationship to sea surface temperature varies Africa was made possible by the availability of a decade greatly within the region. Control on the variability of observations from the Tropical Rainfall Measuring shifts seasonally between the Atlantic, Pacific, and In- Mission (TRMM) satellite. Research based on TRMM dian Oceans, as do the interregional teleconnections. has described the precipitation regime of western equa- Nicholson and Grist (2003) showed that a midlevel east- torial Africa in great detail. In contrast, virtually no erly jet stream, analogous to the African easterly jet of studies of atmospheric dynamics or synoptic situations northern Africa, is present during much of the year. The and features (such as waves) have appeared in the liter- rainbelt lies between the cores of these two midlevel ature. One reason for this is the continued adherence to jets: the African easterly jet of the Southern Hemi- the traditional climatological explanation for Africa’s sphere (AEJ-S) and the African easterly jet of the equatorial rainfall regime. It has generally been under- Northern Hemisphere (AEJ-N; Grist and Nicholson stood to be localized convection enhanced by the twice- 2001). Both jets are best developed around 650 mb and yearly passage of the intertropical convergence zone migrate seasonally with the rainbelt. The AEJ-N is (ITCZ). Consistent with this scenario, in most of the generally within the latitudes of 58–158N; the AEJ-S is region a bimodal seasonal cycle in rainfall prevails (Fig. generally within the latitudes of 58–108S and is evident 1), with higher amounts during the second rainy season mainly from August to November. A recently discerned and minimum rainfall during the low-sun season of the low-level coastal jet along the Atlantic coast of Angola respective hemisphere. and Namibia (Nicholson 2009a) may also be a factor in Other factors contributing to our lack of meteorolog- the region’s meteorology. ical knowledge of western equatorial Africa include the The overall goal of our work is to increase our un- relatively low interannual variability of rainfall and derstanding both of the atmospheric controls on the the difficulty in obtaining meteorological data. Unlike the seasonal cycle and the interannual variability of rainfall drier regions of Africa, this region has suffered no sig- in western equatorial Africa. This study represents one nificant droughts that focused attention on its meteorol- contribution to that understanding. In particular, its ogy. Also, the bulk of the landmass from 58Nto108S and primary goal is to examine the seasonal and diurnal eastward to the Rift Valley highlands lies in the Demo- cycles of convection throughout this region. A second cratic Republic of the Congo (formerly Zaire) or Angola. goal is to examine the anomalous characteristics of con- In both countries, war and economic depression all but vection in this region, seeking an explanation in the closed down the meteorological services for decades. regional atmospheric circulation. We further hope that Fortunately, the availability of information from sat- the work will help us to develop a better understanding ellites such as TRMM has altered our picture of equa- of the high degree of spatial heterogeneity apparent in torial convection. Satellite studies of mesoscale convec- the rainfall regime (Balas et al. 2007). tive systems (MCSs) have dramatically underscored the This article begins with an overview of MCS activity in fallacy of the local convection scenario (e.g., Laing and equatorial Africa and associated rainfall in section 3a. Fritsch 1993a,b, 1997; Mohr and Zipser 1996a,b; Mohr This is followed in sections 3b and 3c by an examination et al. 1999; Nesbitt et al. 2006). MCSs, systems exceed- of the seasonal and diurnal cycles, respectively. Light- ing 2000 km2 in raining area, produce more than 70% of ning is considered in section 3d. The relationship of MCS the rainfall in western equatorial Africa (Nesbitt et al. activity, rainfall, lightning, and the diurnal cycle to to- 2006). Contributions on this order, 50%–90%, are typ- pography and atmospheric circulation is discussed in ical for heavy rain regions of the global tropics, in- section 4. Overall, the results of the study suggest that cluding the Sahel, the south-central United States, and large-scale topography is probably the most important the west coast of Central America (Nesbitt et al. 2006). factor in the convective regime prevailing in western Other recent work relates to the factors governing equatorial Africa and that the AEJ-S may play some role interannual variability and the seasonal cycle of the as well. general atmospheric circulation in this region. Balas et al. (2007) demonstrated remarkable complexity in the 2. Data and methodology spatiotemporal pattern of the interannual variability of a. Satellite, blended, and conventional datasets to rainfall. The region is highly heterogeneous with respect be utilized to interannual variability, especially compared with West Africa, where a single time series provides a first The TRMM satellite, launched in November 1997, approximation of rainfall variations throughout the re- carried the first quantitative spaceborne precipitation gion. In western equatorial Africa, the regions of co- radar. It also carried a suite of complementary passive herent variability are about an order of magnitude smaller sensors, including the Microwave Imager (TMI), the

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FIG. 1. (a) Location of study area and the annual rainfall distribution within four subsectors. For the three southernmost sectors, rainfall distribution is indicated for both the western portion and the eastern portion of the analysis region. (b) Terrain contours and geographic locations mentioned in the text.

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Visible and Infrared Scanner (VIRS), and the Lightning (e.g., Nicholson and Grist 2003; Nicholson 2008; Nicholson Imaging Sensor (LIS; Kummerow et al. 1998, 2000). and Webster 2007) and have verified select results with Collectively these allowed a number of characteristics of West African pibal and rawinsonde reports (Grist and tropical rainfall to be derived on a global basis for the first Nicholson 2001). However, the conclusions based on time. For example, coincident information on the radar the NCEP–NCAR data should be treated cautiously, reflectivity field and passive microwave brightness tem- because there are uncertainties in some variables and perature allowed for a detailed look at the structure of regions. NCEP–NCAR estimates of wind fields are con- precipitating systems (Nesbitt and Zipser 2003). TRMM’s sidered to be relatively reliable, but there are difficulties low-altitude, 358-inclination, sun-synchronous orbit al- with tropical divergent circulations and rainfall (Poccard lowed for coverage from 368Nto368S and for sampling et al. 2000; Kinter et al. 2004). There are also biases throughout the diurnal cycle (Zipser et al. 2006). related to steep orography (Trenberth and Guillemot This study uses the TRMM database compiled at the 1995), such as the mountain ranges over central Africa. University of Utah, described by Nesbitt et al. (2000) Unfortunately, with the paucity of upper-air soundings, and Cecil et al. (2005). The precipitation feature (PF) blended datasets such as NCEP–NCAR are the best level 3 products utilized here include the frequency of available tool for examining atmospheric circulation. We occurrence, diurnal cycle, and volumetric rainfall asso- have designed our analyses in ways that are intended to ciated with three classes of systems. The database also reduce the effect of the two shortcomings noted above. For includes information on stratiform versus convective one, we use primarily what are termed ‘‘A variables,’’ rainfall and lightning. The stratiform–convective dis- those strongly influenced by observational data and, hence, tinction is based on the precipitation radar (PR) algo- the most reliable (Kalnay et al. 1996). These include, for rithm 2A23 (see Biggerstaff and Listemaa 2000; details example, wind and pressure fields. Less reliable are the of the algorithm may be found online at http://trmm.gsfc. B variables, the derivation of which is about equally nasa.gov/2a23.html). Lightning data are taken from the dependent on observations and modeling. The only B LIS on board TRMM (Christian et al. 1992). variable we utilize is omega. Divergence is also a B vari- This database archives information on three types of able. It was calculated offline from NCEP–NCAR winds, PFs: PFs without ice scattering, PFs with ice scattering, because the NCEP–NCAR analysis provides this vari- and MCSs. This study uses only the last category, de- able at only two sigma levels. The impact of surface fined by Nesbitt and Zipser (2003) as a precipitation topography is perhaps more difficult to remove and in feature with ‘‘at least 2000 km2 of contiguous area with much of the analysis region, we cannot rule out the pos- 85-GHz polarization corrected temperature (PCT) # sibility that the NCEP–NCAR results reflect primarily 250 K and 185 km2 # 225 K.’’ Such features are ensured model physics. However, we attempted to reduce the to be large convective systems (Nesbitt and Zipser impact by examining the 925- and 850-mb (hPa) levels, 2003). The spatial resolution of this database is 18318. which are above the surface over most of the analysis It is important to point out that the numbers we de- sector. rive for lightning flashes and MCSs are based on fea- b. Analyses tures detected during the passage of the TRMM satellite. Thus, the numbers are relative because of the low We will examine the characteristics of MCSs over sampling frequency of TRMM and the narrow swath equatorial Africa, including frequency of occurrence, (215 km) of TRMM’s precipitation radar. Near the diurnal cycle and annual cycles, associated volumetric equator the sampling is on the order of 0.5 times day21. and convective rainfall, and interannual variability. Only This results in significant geographical undersampling. land areas are considered. Also evaluated is the lightning The use of several years of data allows us to produce activity associated with the MCSs. This is a proxy for numbers that are comparable geographically and tem- convective intensity (Zipser et al. 2006), in that lightning porally, but the actual number of ‘‘events’’ (flashes or requires both strong updrafts and a mixed phase mi- MCSs) is only qualitative. The terms ‘‘relative number’’ crophysical environment (Toracinta and Zipser 2001). and ‘‘relative count’’ are used to underscore this point. Our analyses will consider the entire continent from A detailed discussion of the sampling issue is found in 208Nto208S, but will focus on four sectors in western Nesbitt and Zipser 2003). equatorial Africa. Shown in Fig. 1, these extend across The National Centers for Environmental Prediction– the continent to 258E and each covers a 58 latitudinal National Center for Atmospheric Research (NCEP– band, collectively spanning the equatorial latitudes NCAR) reanalysis dataset is used in analyses of atmo- from 108Sto108N. spheric circulation (Kalnay et al. 1996). We have used it Of special interest is the diurnal cycle of convective in numerous studies of atmospheric dynamics over Africa activity. Mechanisms of the diurnal cycle in the tropics

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2 21 FIG. 2. Five-year average of (a) relative number of MCSs per year, (b) total volumetric rainfall (km mm h ) from MCSs, (c) volumetric rainfall per MCS, and (d) percentage of convective rainfall. These quantities are averaged over 18318 grid points. include afternoon boundary layer destabilization by radia- TRMM, the narrow swath of the PR leads to a geo- tion (Wallace 1975; Dai et al. 1999; Dai 2001), local ef- graphic undersampling on a daily basis. The satellite’s fects of sea breezes and complex terrain (Oki and Musiake orbital characteristics result in a sampling time that 1994; Yang and Slingo 2001; Yang and Smith 2006), and varies from 0.5 times per day at the equator to nearly 2 the long, nocturnal life cycle of MCSs (Sherwood and times per day at 358 latitude. Also, the TRMM satellite Wahrlich 1999; Nesbitt and Zipser 2003). All of these takes 46 days to return to a given position at a given local factors are present in varying degrees over western time. As a result, local sampling of the complete diurnal equatorial Africa. At the same time, one of the unusual cycle with any degree of conﬁdence requires consider- climatological features of the region is the remarkable able compositing. Lin et al. (2000) suggested that at least complexity in the spatiotemporal pattern of the inter- 3 months of PR data must be combined to adequately annual variability of rainfall (Balas et al. 2007). The sample a 48358 grid at 1-h resolution. Thus, in this region is very heterogeneous with respect to this inter- study we examine the diurnal cycle on a multiyear, annual variability, especially compared with West Africa. multimonth basis to temper sampling errors. Factors controlling the variability vary seasonally and over short distances. A better understanding of the diurnal cycle may help us to unravel and explain the 3. Results complexity of the factors controlling interannual vari- a. Climatology of MCSs and associated rainfall ability in this region. The TRMM instrument that allows for analysis of the Figure 2 shows the 5-yr mean of the relative number diurnal cycle is the PR. Despite the many advantages of of MCSs over western equatorial Africa. The number

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FIG. 3. Five-year average of (a) relative number of MCSs per month and per grid point and (b) seasonal rainfall (mm month21) from MCSs for the four seasons: DJF, MAM, JJA, and SON. These quantities are averaged over 18318 grid points.

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FIG. 4. Hourly amount of rainfall at selected stations along the shore of Lake Victoria during April and September. The extreme values at Nabuyongo Island, in the center of Lake Victoria, are indicative of the enhance of rainfall by the lake (from Ba and Nicholson 1998). generally increases with decreasing latitude and in- A larger maximum extends from roughly 58Nto58S, creases toward the interior of the continent. Relatively and from near the Atlantic coast to the western edge of few occur east of the Rift Valley. Four maxima are the Rift Valley highlands. Its core lies over eastern clearly apparent, three of which are related to local Congo/Zaire, at roughly 18S, 278E. It is not associated geography. Two orographic maxima are centered on with local topography or lakes. Mount Cameroon and the Ethiopian highlands. A third, Figure 2 also shows for tropical Africa the mean cli- locally induced maximum lies over Lake Victoria. matology of volumetric rainfall from MCSs, rainfall per The Lake Victoria maximum is related to the noc- MCS, and the percentage of convective rainfall. The turnal enhancement of convection by a combination of spatial pattern of volumetric rainfall is very similar to lake breezes and mountain–valley breezes (Flohn and that corresponding to the number of MCSs. Both show a Fraedrich 1966; Fraedrich 1972). As a result, rainfall strong maximum in the central equatorial sector, to the over the lake is nearly 50% greater than in the sur- west of Lake Victoria and the Rift Valley highlands. rounding catchment (1791 mm yr21 versus roughly 1300 There is somewhat more spatial detail in the pattern of mm yr21 in the catchment, 800–1200 mm in most of the rainfall per MCS. In general, the maximum is somewhat surrounding region; Yin and Nicholson 1998). farther north and west of the maxima in MCS activity

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FIG. 5. Monthly means of MCS activity: mean relative number of MCSs within the indicated latitudinal sector, volumetric rainfall per MCS (104 mm km2 h21), and percentage of convective rainfall within the two northern equatorial zones and two southern equatorial zones. Data on volumetric rainfall and percentage of convective rainfall are omitted for months with relatively few MCSs. and rainfall. A second maximum in rainfall per MCS is up well in JJA but not in DJF, while the Lake Victoria evident across a latitudinal band near 108N, an area co- maximum is evident in DJF but not in JJA. inciding with the southern track of African easterly The contrast between the September–November waves (AEWs) associated with the AEJ-N (e.g., Thorn- (SON) and March–May (MAM) seasons is more sur- croft and Hodges 2001). In contrast, the percentage of prising because both of these equatorial rainy seasons convective rainfall is quite uniform throughout most of are traditionally explained by the north–south passage of the region and generally on the order of 60%–70%. A the ITCZ. The local maxima over Mount Cameroon and cursory comparison with local topography shows that the Lake Victoria are evident in both seasons, but that over few scattered maxima (where convective rainfall exceeds the Ethiopian highlands is evident only in SON. The 70%) correspond to low-lying regions. Lake Victoria maximum is most pronounced during Figure 3a shows the seasonal patterns of MCS activ- MAM, when the local enhancement of convection by the ity. The extreme seasons [December–February (DJF) lake is at a maximum (Ba and Nicholson 1998). In con- and June–August (JJA)] contrast sharply in terms of the trast, the Mount Cameroon maximum is stronger in SON. spatial location of MCSs. During DJF, MCS activity is Overall, MCS activity is notably stronger in SON than principally south of the equator, while it is mostly north in MAM. This is evident from the difference between of the equator in JJA. This is commensurate with the these two seasons in terms of the size of the area in movement of the sun and the ITCZ from their most which the relative monthly frequency of MCSs is greater extreme positions in the Southern and Northern Hem- than 0.15 and from the large areas during SON in which ispheres. The JJA pattern reﬂects the track of the the relative number of systems is greater than 0.3 AEWs. In DJF the area of MCS activity is considerably month21. More striking is the spatial pattern of activity. broader than during JJA. The two topographic maxima During MAM the central maximum in activity (i.e., that (Mount Cameroon and the Ethiopian highlands) show not associated with local geographic factors) runs roughly

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the number of MCSs are for the most part apparent in the rainfall ﬁelds. The rainbelt is latitudinally most ex- pansive in SON and most restricted in JJA. The two local topographic maxima are most pronounced in JJA and, to a lesser extent, SON. The MCS rainfall maximum over Lake Victoria is evident throughout the year, but the effect is dramatic in MAM. Surface gauges (Fig. 4) likewise show the maximum during this season. b. The seasonal cycle Figure 5 shows the seasonal cycle of MCS activity and various MCS characteristics for the two northern equatorial sectors and the two southern equatorial sectors. The parameters shown include the mean relative number of MCSs, volumetric rainfﬁgall per MCS, and the percentage of convective rainfall. In the northernmost sector (58–108N) the number of MCSs peaks in August, but is relatively high from April to October. The remaining sectors all have a bimodal rather than unimodal distribution. Just north of the equator (08–58N) the number is relatively uniform from March to November, but peak months are April/May and October/November. In the Southern Hemisphere sectors 08–58S and 58–108S, peak months are April and December and March and November/December, respectively. In all four sectors, the number of MCSs is at a minimum during the winter months of the respective hemisphere. The seasonal cycle of MCS count (Fig. 5) closely re- sembles that of rainfall (Fig. 1), although the maxima may differ by a month or two. Both variables show a single peak in the northernmost sector and a double peak elsewhere, with the intensity and length of the winter dry season increasing from the equator south- ward. The seasonal cycle of volumetric rain per MCS is notably different. This variable shows much less seasonal variation. Volumetric rainfall per MCS is relatively constant from May to September in the sector 58– FIG. 6. Maps of the diurnal cycle: (a) mean relative number of 108N. It peaks in April in the sector 08–58N. Maxima MCSs within each 28328 lat–lon grid box, (b) the mean volumetric occur in September and February in the sector 08–58S rainfall per MCS (104 mm km2 h21), and (c) the percentage of convective rainfall at 3-h intervals, starting at 0000–0300 LT. and in September in the sector 58–108S. The percentage of convective rainfall (Fig. 5c) is likewise fairly steady, but shows some tendency for an east and west. During SON the central maximum lies inverse relationship with the number of MCSs (Fig. 5c). along a diagonal from northeast to southwest. A very In both the Northern and Southern Hemisphere sectors, interesting feature, a pronounced local maximum in it varies between roughly 60% and 80%. The contri- MCS activity, appears near the equator and from about bution of convective rainfall falls to roughly 60% during 258 to 288E. Geographical considerations, such as to- months with a large number of MCSs. This is consistent pographic gradients, provide no immediately obvious with the results of Nesbitt et al. (2006), indicating that explanation for this maximum. for the tropics as a whole 48% of the precipitation over Figure 3b shows the seasonal rainfall associated with land is stratiform and 50% of the precipitation associ- MCS activity. Not surprisingly, the patterns evident in ated with MCSs over land is stratiform.

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FIG. 7. The diurnal cycle of convection (a) seasonal averages for the relative number of MCSs in 3-h intervals for northern and southern sectors (see Fig. 1a for location). (b) The annual averages of the relative number of MCSs, the mean volumetric rainfall per MCS (104 mm km2 h21), and the percentage of convective rainfall in 3-h intervals for the northern and southern sectors for the year as a whole. c. The diurnal cycle was generally in the early morning hours. In the more central regions, rainfall associated with MCSs tended to A handful of studies have examined the diurnal cycle peak during the night. of precipitation and convection in the global tropics, Using a higher-resolution TRMM dataset compiled focusing mainly on ocean versus land contrasts (e.g., Lin by Nesbitt and Zipser, we examined the diurnal cycle et al. 2000; Dai 2001; Yang and Slingo 2001; Yang and over western equatorial Africa in greater detail. Figure Smith 2006). A comprehensive study of Nesbitt and 6 presents a spatial view of the diurnal cycle of MCS Zipser (2003) provided a very detailed view, using the activity, volumetric rainfall per MCS and percentage of advanced capabilities of the TRMM satellite. For the convective rain. Because the diurnal cycle does not African continent as a whole, information on the diur- show substantial seasonal variation, these maps are nal cycle is generally limited to what can be gleaned typical for the year as a whole. The maximum in MCS from this global perspective. The exceptions are a study frequency occurs around 1500–1800 LT. This appears to by Duvel (1989), based on Meteosat data, and a recent be coincident with a minimum in volumetric rain per papers by Mohr (2004) and Futyan and Del Genio MCS and a maximum in convective rainfall. Volumetric (2007), based on Meteosat and TRMM. McGarry and rain per MCS peaks in the night and early morning Reed (1978) and Reed and Jaffe (1981) also provide a hours, when convective rainfall is reaching a minimum. detailed look at the diurnal cycle, but these studies were Figure 7, which presents regional averages of the data limited to West Africa. in Fig. 6, conﬁrms this pattern. In all seasons there is a Using Meteosat data with a resolution of 2.58 of lati- morning minimum in MCS count and a late afternoon to tude and longitude, Duvel showed that deep convection early evening maximum (Fig. 7a). In general, there is over equatorial Africa has a maximum around 1800 LT substantial MCS activity during the night. Some con- and a minimum around 0900 LT. This is in general trasts between the seasons are evident. The most obvi- agreement with the results of Nesbitt and Zipser (2003) ous is the minimal MCS count during JJA in the southern and others. However, the latter study, at a resolution of sector and DJF in the northern sector, the dry seasons in 108 of latitude and longitude, showed some zonal vari- the respective hemispheres. The maxima are SON in ation in the timing of rainfall associated with MCSs. In the Northern Hemisphere and MAM in the Southern the western sectors of equatorial Africa, the maximum Hemisphere.

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FIG. 8. (a) Five-year mean relative number of lightning flashes (per 18318 latitude–longitude) for the year as a whole, (b) mean percent MCSs with flashes, and (c) and number of flashes per MCS. These are indicators of the overall intensity of convection. All data are averaged for a 18318 grid box, with a three-point smoothing applied. The maxima are highlighted by showing only areas with greater than 20 lightning flashes per year or 10 lightning flashes per season.

Figure 7b compares the diurnal cycle of MCS activity, mum and a late afternoon/early evening minimum, volumetric rain per MCS and the percentage of con- while MCS count peaks around 1500–2100 LT. This is vective rainfall. Data are averaged for the year as a consistent with the ﬁndings of Nesbitt and Zipser (2003) whole for the northern and southern equatorial sectors. and other studies of tropical rainfall (e.g., Laing and Diurnal cycles are similar in the two sectors. Notably, at Fritsch 1997). The cycle of convective rainfall is similar night the volumetric rainfall per MCS is greater in the to that for MCSs: a maximum in the afternoon/early Southern Hemisphere sectors. evening hours and a nocturnal to early morning mini- The results in both Figs. 6 and 7 underscore the mum, usually between the hours of 0300 and 0900 LT. contrast in the diurnal cycles of MCS count and rainfall Thus, the minimum in MCS count and convective per MCS. The latter has a nocturnal or morning maxi- rainfall is coincident with the maximum in rainfall per

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FIG. 9. Five-year mean relative number of lightning ﬂashes during each 3-month season (total within each 18318 latitude–longitude grid box; three-point smoothing has been applied).

MCS. This presumably reflects the substantial con- evident also in the percent of MCSs with flashes and the tribution of stratiform raintothetotalvolumeofrain number of flashes per MCS. This belt of strong con- associated with MCSs (Nesbitt et al. 2006). The vective activity generally corresponds to the southerly stratiform rain is dominant during the night, when track of AEWs (Thorncroft and Hodges 2001). The the systems expand and reach their maximum areal maxima within this zone (areas with greater than 40 extent. relative counts per year) are located near topographic features: Mount Cameroon, Darfur, and the Jos high- d. Lightning activity lands. Isolated maxima in flash count also appear over Figure 8a shows the 5-yr mean relative number of Lake Victoria and on the northern slopes of the Ethi- lightning flashes, based on data from the TRMM-LIS. A opian highlands. pronounced maximum extends from roughly 58Nto58S Figure 9 shows the relative seasonal counts of MCSs across the continent from the Rift Valley to the Atlantic with lightning flashes. The most notable feature of the coast. Within this sector, the number increases toward distributions is the relative constancy of the equatorial the interior, so that the highest flash frequency is cen- maximum in flash count. Throughout the year there is a tered over the equator and at roughly 208E. This is as- maximum centered around the equator and extending sociated with a maximum in MCS activity (Fig. 2) and a 58 or 108 of latitude on either side. The location of the maximum in the percent of MCSs with flashes and the maximum in MCS count (Fig. 3a) is much more geo- number of flashes per MCS (Figs. 8b,c). A broad belt graphically variable. This suggests more stationary in- with several maxima is centered at roughly 108N. It is fluences on the lightning activity.

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FIG. 10. Diurnal cycle of lightning (relative number of ﬂashes per 18318 grid within each 3-h time span). Data are averaged for all months of the 5-yr study period.

Figure 10 shows the diurnal cycle in the flash count for 4. Discussion the year as a whole. The phase of the cycle does not vary much by season (not shown), suggesting that fixed geo- A number of authors have commented on the para- graphical factors are important determinants. Lightning dox of the Congo/Zaire basin having the most intense is both frequent and widespread between 1200 and 0000 storms but relatively low precipitation compared to other LT. The diurnal maximum occurs between 1500 and equatorial regions. Zipser et al. (2006) explicitly state 1800 LT and the minimum occurs between 0600 and that a fundamental question is ‘‘why...rainfall in 0900 LT. equatorial Africa is less than that in Indonesia and Figure 11a shows the monthly mean relative flash equatorial South America, but why African storms are count in the four equatorial sectors. In the Southern so often more intense.’’ Petersen and Rutledge (2001) Hemisphere there are more flashes in the sector closest confirmed the anomalous intensity of convection in this to the equator during most months. Notable exceptions region. Of 22 regions evaluated, equatorial Africa had are the months of the SON transition season, when flash the highest ice-water content at the 7–9-km level, a frequency is strongly maximized in the zone 58–108S. In prime factor in the production of lightning. Small ef- the Northern Hemisphere more flashes per MCS occur fective diameter ice crystals are also associated with in the zone closer to the equator during most months. climatological maxima in lightning activity (Sherwood However, there is little difference in the two zones et al. 2006). Various studies (e.g., Sherwood 2002a,b; during August–October and flashes are most frequent Ekman et al. 2004) have suggested that atmospheric farther north from May to July. aerosols reduce this diameter; hence, extensive biomass Figure 11b shows the monthly mean relative number burning over equatorial Africa may play some role in of flashes per MCS in these same sectors. In the creating the lightning maximum (Sherwood et al. 2006). Southern Hemisphere the number of flashes per MCS is The cause of the low rainfall, despite the high con- greater in the sector closest to the equator during most vective intensity, is still unclear. However, a few papers months. Again, notable exceptions are the transition- have provided possible thermodynamic and cloud phys- season months SON, when flash frequency per MCS is ics explanations for this paradox. Factors reducing the maximized in the zone 58–108S. In the Northern Hem- efficiency in rainfall production in this region include isphere there is much less difference between the two thermodynamic stability profiles that inhibit convection sectors. A notable exception is the month of March, and, relative to those observed in South America, a when considerably more flashes per MCS occur in the smaller effective droplet radius, less water vapor in the zone closer to the equator. atmospheric column, higher cloud bases, relatively dry

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FIG. 11. Seasonal cycle of flash count (relative flash count per month) within (top) the indicated latitudinal sector and (bottom) average number of flashes per MCS within the sector. Data are shown for the four latitudinal sectors shown in Fig. 1. A three-point smoothing has been applied. air in the lower troposphere, and a relatively larger and vertical motion. These do not directly assess the proportion of nonraining clouds (Petersen and Rut- storm intensity, which is essentially updraft strength. ledge 2001; McCollum et al. 2000; Liu et al. 2007). A However, these factors do produce a mean basic state that study by Geerts and Dejene (2005) also underscores may favor or inhibit the development of strong updrafts. various contrasts between Central Africa and the Am- We also examined atmospheric moisture during the equa- azon, but the results have to be seen as tentative, since torial rainy seasons. Our analyses are considered jointly the authors evaluated the DJF dry season in central with regional geographic factors and our climatological Africa (see Fig. 1a). Their study did show differences analyses of convective activity in an attempt to further in climatological relative humidity, convective available understand the unique attributes of the convective regime potential energy (CAPE), and low-level wind shear that of western equatorial Africa. In view of the uncertainties might contribute to differences in rainfall amount. of the NCEP analysis in the region and the broad nature of Much less is understood about how the regional at- the analysis, we can merely speculate on associations and mospheric circulation may contribute to the relatively suggest potential avenues for further research. low rainfall or anomalous intensity of convection. The a. The lightning maximum over the only relevant study is that of McCollum et al. (2000), Congo/Zaire basin who show weak moisture flux at low levels and conclude that the Rift Valley highlands block transport from the Figure 12 compares a high-resolution terrain map Indian Ocean. It is relevant to point out that the net of the Congo/Zaire basin and the mean number of result of the circulation is that two dry seasons occur lightning flashes. The latter is for the year as a whole over western equatorial Africa, while only one moder- and is based on 5 yr: 1998, 1999, 2000, 2002, and 2003. ately dry season occurs over the Amazon. This is cer- The lightning maximum is clearly coincident with the tainly a contributing factor. regions of lowest elevation, in the center of the basin, Here we evaluate select kinematic aspects of the re- and the number of flashes generally decreases progres- gional circulation: divergence, midtropospheric winds, sively toward the surrounding regions of higher terrain.

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FIG. 12. Average relative number of ﬂashes per year in each 18318 grid box, superimposed upon a faux 3D terrain map of Africa. The maximum is highlighted by showing only areas with a relative ﬂash count greater than 10 per year.

The association between lightning frequency and low- intense convection results from the interaction of the level terrain is also apparent on a smaller scale. The geostrophic flow, slope winds, and boundary layer and maximum associated with the Jos plateau of north- radiative processes (Yang and Smith 2006). ern Nigeria (108N, 88E) lies in the lower regions to The spatial resolution of NCEP–NCAR data is inade- the north of the plateau. An extremely strong but lo- quate to confirm or reject this hypothesis. However, some calized maximum in lightning flashes (Fig. 12), flashes interesting features are apparent that are consistent with per MCS (Fig. 8c), and rainfall per MCS lies to the west the hypothesis. Figure 13 shows the mean winds at 925 and south of the highlands of Darfur in the Sudan (148N, and 850 mb during SON, the season of most intense 248E). convective activity and highest flash count, based on The spatial coincidence of the lightning maximum NCEP–NCAR reanalysis data. During that season the and the low-level terrain suggests that orographic ef- spatial distribution of lightning is similar to that for the fects play a role in the production of this maximum. The year as a whole (Fig. 8), with maximum frequency in seasonal constancy of the location of the lightning maxi- the Congo/Zaire basin and centered around the equator. mum and the seasonally constant diurnal cycle of light- In the sector between 58 and 108S, on average over 50% ning further implicate stationary geographic factors and, of all lightning flashes (Fig. 11) and nearly 40% percent hence, also support the role of topography in creating of all MCSs (Fig. 5) occur during this season. the lightning maximum. Figure 13 shows that the large-scale geostrophic flow Assuming that the terrain plays a role, the mechanism during SON is such that the Congo/Zaire basin lies in at play may be akin to that described by Tripoli (1986) the lee of high terrain to the north, east, and west, as and Tripoli and Cotton (1989a,b) to explain the diurnal required for the mechanism proposed by Tripoli and cycle of convection over the Great Plains. It applies to Cotton (1989a,b). Early in the afternoon (1200 UTC, the case of geostrophic flow perpendicular to the ter- i.e., 1400 LT) ascent prevails over the highlands. At rain. Interaction of this large-scale flow with upslope 1800 UTC, a peak time of convection and lightning, the flow in the afternoon creates intense convection in the mean vertical motion field shows ascent over the high lee but compensatory subsidence farther leeward over terrain at 850 mb and weak subsidence downslope over the plain. The net spatial and temporal development of the plains (Fig. 14).

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FIG. 13. Mean winds at 850 and 925 mb during SON. b. The low rainfall over equatorial Africa equatorial South America are on the order of 40–50 kg m22. Over Africa values exceed 40 kg m22 in only a McCollum et al. (2000) demonstrate the relative dry- very limited area of the western equatorial region. East ness of the atmosphere above equatorial Africa, com- of the highlands, values between 25 and 30 kg m22 pared to other equatorial regions and South America, in prevail. particular. Equatorial Africa is especially dry in the layer McCollum et al. noted the ‘‘aridity’’ east of the high- surface–700 mb, where most of the moisture resides. lands and suggested that this feature and the restriction Typical values of precipitable water in this layer are 25– of moisture transport from the Indian Ocean by the 40 kg m22 along most of the equator, but less than 25 highlands are major factors in the relatively low over equatorial Africa. For the year as a whole, total amounts of rainfall over equatorial Africa. We suggest column water vapor averages 40–50 kg m22 over South that additional aspects of the regional circulation play a America, compared to 30–40 kg m22 over equatorial role, as well. Africa (McCollum et al. 2000). Figure 15 illustrates the For example, several points of evidence suggest contrast with South America for November and February, that the Atlantic is also an important moisture source two months that are common to the rainy seasons in for this region. The spatial distribution of moisture (Fig. the equatorial regions of both continents. Values over 15) and the low-level ﬂow (Fig. 13) (Nicholson 2009a)

21 22 FIG. 14. Mean vertical motion (omega; mb s 3 10 ) at 850 mb at 1200 and 1800 UTC (LT is generally 2–3 h later).

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22 FIG. 15. Mean precipitable water (kg m ) over Africa and South America in February and November. suggest that the primary transport is from the Atlantic. with the surface air and adiabatic heating further re- During the SON season of intense convective activity, duces the relative humidity. rainfall in the western extreme of the region is strongly c. The Darfur region correlated with Atlantic sea surface temperatures, but not with sea surface temperatures in the Indian Ocean An area south and west of the highlands of Darfur, in (Balas et al. 2007). As with flow from the east, the the southern Sudan (108N, 228E), stands out as a bull’s- transport of moisture is blocked at low levels by the high eye in many of our analyses related to convective in- terrain. tensity. Examples are rainfall per MCS (Fig. 2), lightning We speculate that two other situations may further flash count (Fig. 8a), and flashes per MCS (Fig. 8c). contribute to the anomalously low rainfall. One is the Because of the extreme values noted in the analysis, the advection of relatively dry air from North Africa into data for the region were individually checked and the the northern rim of the Congo/Zaire basin (Fig. 13). The maxima over this region were confirmed. other is the mesoscale situation described by Tripoli and The intensity of convection in this region is noteworthy Cotton (1989a,b) to explain convection in the lee of high because several studies (e.g., Hodges and Thorncroft terrain. In areas of subsidence over the plains in the lee 1997; Mekonnen et al. 2006) have suggested that con- (i.e., within the Congo/Zaire basin), dry area is mixed vection over Darfur triggers many of the AEWs that

Unauthenticated | Downloaded 10/07/21 09:25 AM UTC APRIL 2009 J A C K S O N E T A L . 1289 move westward during the summer rainy season. The anomalous intensity of convection in this regions lends support to this idea. d. The intensity of convective activity. The orographic effects described in section 4a could be a factor in the extreme intensity of convective activity over the Congo/Zaire basin. An examination of regional circulation suggests one other possible factor. Although intense convective activity is ubiquitous over the basin, it clearly has a spatial maximum in the east- ernmost region in the latitudes from the equator to 108S and a seasonal maximum during the SON season. Here, we examine that season in further detail, using NCEP– NCAR reanalysis data and rainfall from the African archive of the second author. An examination of all three months shows similar patterns, so that October is used to represent the season. Monthly means are utilized. Although the time scale of individual MCSs is much shorter, we feel the monthly mean provides a reasonable cursory look. For one, convective activity is present during most days in October. Also, an examination of winds on individual days shows that the monthly pattern is clearly evident on individual days. The most striking feature of the October wind field is a midlevel easterly jet stream (the AEJ-S) with a core at 600 mb and 88S (Fig. 16). The core lies west of the highlands of eastern Africa both in the mean and on the individual days when it is well developed. This feature is present throughout the SON rainy season, but com- pletely absent during the MAM rainy season (Nicholson and Grist 2003). Its core speed averages 12 m s21, but speeds can well exceed 15 m s21 on individual days. The AEJ-S appears to be a result of low-level surface temperature gradients (Fig. 17), which are strong following the dry season of the Southern Hemisphere subtropics (i.e., in SON) but extremely weak following the Southern Fig. 16. The AEJ-S: (a) mean wind (m s21) at 600 mb during Hemisphere rainy season (i.e., in MAM). The same October; (b) vertical cross section of mean zonal wind at 208Easa 2 thermal pattern is evident also at 850 mb, so it is un- function of latitude (October; m s 1). likely to be a result of the NCEP–NCAR model physics and is readily confirmed by numerous satellite analyses of surface temperature. Hemisphere jet divergence in the right entrance region, We hypothesize that the presence of the AEJ-S may convergence in the left entrance region, and the reverse enhance convection during the seasons when it is present patterns in the exit region. The converse pattern pre- and speculate that the physical link is a jet streak circu- vails for a Southern Hemisphere jet. lation similar to that known from midlatitude studies. The conditions for the development of such a jet Upper-level jets are characterized by considerable zonal streak are stringent (Keyser and Shapiro 1986), with the asymmetry, with distinct entrance and exit regions and most basic being geostrophy. Two considerations sug- ensuing vertical circulations with distinct areas of con- gest that the flow in the vicinity of the AEJ-S is approxi- vergence and divergence. Uccellini and Johnson (1979) mately geostrophic. One is that the winds are roughly developed a four-quadrant conceptual model of a ‘‘jet parallel to the geopotential height contours (not shown). streak’’ (core region of maximum velocity) to describe The other is the Rossby number (Ug/Lf, where Ug is the this asymmetry. The model prescribes for a Northern geostrophic speed, L is a typical length scale of the jet,

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FIG. 17. Mean temperature (8C) at 925 mb for April and October, based on NCEP–NCAR data. and f is the Coriolis parameter at 108S). A calculation Our interpretation is that the presence of the AEJ-S with typical speeds and length scale give a value of 0.3, produces the midlevel convergence prevailing over the indicating that quasigeostrophic theory, including jet Congo/Zaire basin and, hence, plays a role in the streak concepts, can be appropriately utilized in evalu- anomalous intensity of convection in this region. None- ating the AEJ-S (Cunningham and Keyser 2000). theless, the question arises as to whether the jet itself Figure 18 shows the mean divergence during October may be a product of the latent heat release associated at 600 and 200 mb, calculated offline from NCEP– with intense convection. Thorncroft and Blackburn NCAR winds, and the mean vertical motion at 600 mb. (1999) suggested that latent heat release is a factor in A four-quadrant checkerboard pattern in divergence is the development of the AEJ-N over North Africa. evident at 600 mb. The axis of the AEJ-S (Fig. 16) The reality of the situation for equatorial Africa separates the right and left quadrants and a large area of cannot be determined from the cursory analysis per- convergence corresponds to the right entrance region of formed here. However, there are several points of the jet. A core of vertical motion on the order of 0.3–0.5 supporting evidence for our conjecture that the AEJ-S mb s21 3 1022 is coincident with this area of conver- may enhance convection, rather than be a result of it. gence. Divergence at 200 mb overrides it. The jet’s axis One is the absence of the convective maximum west of closely follows the edge of the divergence maximum at Lakes Victoria and Tanganyika during the MAM rainy 200 mb and the maximum in convective intensity during season (Fig. 3). Although total rainfall (Fig. 19) and, SON. Thus, the kinematic characteristics of the motion hence, latent heat release in the equatorial latitudes is field are consistent with a jet streak circulation. comparable during the MAM and SON seasons, the Requirements for both intense lightning and strong AEJ-S does not develop during the MAM season convection include strong updrafts (Petersen and Rutledge (Nicholson and Grist 2003). Second, the development of 2001; Toracinta and Zipser 2001). The aforementioned the jet during SON and its absence during MAM are convergence maximum coincides with the lightning consistent with the gradients of surface heating during maximum around 258–308E (Figs. 11 and 12). Although these seasons (Fig. 17). The strong surface gradient convection commences near the surface, its intensifica- during SON results from the prolonged dry season over tion and deepening would be enhanced by the combi- southern Africa. Finally, the convection is equatorward nation of midlevel convergence and strong outflow in the of the jet. During SON, thermal wind considerations upper troposphere. Notably, a similar situation prevails indicate that latent heat release at midlevels would tend during the rainy season over West Africa (Nicholson and to reduce rather than enhance easterly winds. Above Webster 2007; Nicholson 2008, 2009b). Within the core 600 mb, the temperature gradient is reversed because of of the rainbelt, convergence associated with the AEJ-N convective heating equatorward of the jet, resulting in prevails at midlevels, divergence associated with the the location of the core at 600 mb. tropical easterly jet prevails at 200 mb, and a core of The AEJ-S might also contribute to the convective strong vertical motion lies in between. intensity in other ways. Zipser et al. (2006) identify

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commonalities of the other regions with large numbers of huge MCSs, the central United States and southeast South America. Both regions are characterized by strong low-level wind shear, a low-level jet that brings in very moist air, highlands that trigger disturbances that lift the low-level air and release convective instability. Two of these features, highlands and low-level wind shear, are present over the Congo/Zaire basin. Zipser et al. (2006) further note the possible importance of midlevel jets, such as the AEJ-N, in producing the requisite shear. Mohr and Thorncroft (2006) conﬁrm the link between intense convection over West Africa and the AEJ-N, implicating shear as the important factor. The AEJ-S likewise markedly enhances the vertical and horizontal shear in western equatorial Af- rica.

5. Summary and conclusions To further our understanding of the atmospheric dynamics regulating precipitation in western equatorial Africa, this study examined the seasonal and diurnal cycles of convection throughout this region. This study also examined potential factors in the anomalous intensity of convection in this region and the anomalously low rainfall. The most salient results of our study are 1) the seasonally invariant diurnal cycles of convective activity and lightning, 2) the seasonally varying spatial distribution of MCSs, 3) the relatively seasonally invariant location of the lightning maximum, 4) the development of maximum convective intensity during the second rainy season (SON), 5) the afternoon maximum in MCS occurrence and the percentage of convective rainfall, and 6) the morning maximum in total rainfall from MCSs. The results of this study suggest that large-scale topography is a critical factor in the spatial and diurnal patterns of convection, lightning, and rainfall in western equatorial Africa. The interaction of topographic effects and regional circulation may play some role in the relatively low rainfall, but do not fully explain this feature. Our results further suggest that the AEJ-S may play some role in the anomalous intensity of convection in this region. The complexity of these factors provides at least a partial explanation for the complexity and spatial heterogeneity of the rainfall regime, as noted by Balas et al. (2007). FIG. 18. Mean divergence at 600 and 200 mb and mean vertical In the western equatorial latitudes of Africa there are 21 22 motion (omega; mb s 3 10 ) at 600 mb during October. The four maxima in MCS activity, three of which are related thin crossed lines represent the axes of the AEJ-S. to geographical factors. Pronounced maxima in the frequency of occurrence of MCSs and the volumetric rainfall associated with them correspond to the high

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FIG. 19. Mean rainfall (mm) during MAM and SON (from Balas et al. 2007). terrain of Mount Cameroon and the surrounding high- REFERENCES lands, the Ethiopian highlands, and Lake Victoria. The Ba, M. B., and S. E. Nicholson, 1998: Analysis of convective ac- fourth maximum, near 08,258–288E, shows no associa- tivity and its relationship to the rainfall over the Rift Valley tion with geographical factors. It coincides with the right lakes of East Africa during 1983–90 using the Meteosat in- entrance region of the AEJ-S. From this study alone, it frared channel. J. Appl. Meteor., 37, 1250–1264. is impossible to determine if the jet is a factor in or a Balas, N., S. E. Nicholson, and D. Klotter, 2007: The relationship result of the intense convection, or is merely coinci- of rainfall variability in West Central Africa to sea-surface temperature fluctuations. Int. J. Climatol., 27, 1335–1349. dentally collocated with the convective maximum. Biggerstaff, M. I., and S. A. Listemaa, 2000: An improved scheme However, several observations suggest that the AEJ-S for convective/stratiform echo classification using radar re- may contribute to the intensity of convection. flectivity. J. Appl. Meteor., 39, 2129–2150. The conclusions we draw here are tenuous, as the Cecil, D. A., S. J. Goodman, D. J. Boccippio, E. J. Zipser, and S. study is limited by the limitations of the NCEP–NCAR W. Nesbitt, 2005: Three years of TRMM precipitation fea- reanalysis dataset and by use of monthly means in the tures. Part I: Radar, radiometric, and lightning characteristics. Mon. Wea. Rev., 133, 543–566. analysis. Nevertheless, clear temporal and spatial asso- Christian, H. J., R. J. Blakeslee, and S. J. Goodman, 1992: Light- ciations have been demonstrated between convective ning Imaging Sensor (LIS) for the Earth Observing System. activity and regional topography, and circumstantial NASA Tech. Memo. 4350, MSFC, Huntsville, AL, 36 pp. evidence is presented for an association between the Cunningham, P., and D. Keyser, 2000: Analytical and numerical anomalous intensity of convection and the AEJ-S. Fur- modeling of jet streaks: Barotropic dynamics. Quart. J. Roy. ther research should concentrate on determining the Meteor. Soc., 126, 3187–3217. Dai, A., 2001: Global precipitation and thunderstorm frequencies. mesoscale motions in the region and their relationship Part II: Diurnal variations. J. Climate, 14, 1112–1128. to terrain and convection and on evaluating possible ——, F. Giorgi, and K. E. Trenberth, 1999: Observed and model- links between the AEJ-S and convection on synoptic simulated diurnal cycles of precipitation over the contiguous time scales. United States. J. Geophys. Res., 104, 6377–6402. Duvel, J.-P., 1989: Convection over tropical Africa and the At- Acknowledgments. This study was supported by NSF lantic Ocean during northern summer. Part I: Interannual and diurnal variations. Mon. Wea. Rev., 117, 2782–2799. Grant ATM-0004479. Much of the work in this article is Ekman, A. M. L., C. Wang, J. Wilson, and J. Strom, 2004: Explicit taken from the M.S. thesis of the first author. We ex- simulations of aerosol physics in a cloud-resolving model: A press our thanks to Steve Nesbitt, Ed Zipser, Chuntao sensitivity study based on an observed convective cloud. At- Liu, and others who carried out the tedious job of mos. Chem. Phys., 4, 773–791. compiling the precipitation feature database and mak- Flohn, H., and K. Fraedrich, 1966: Tagesperiodische Zirkulation ing it available to others. Without this data base, our und Niederschlagsverteilung am Victoria-See (Ostafrika) (The daily periodic circulation and distribution of rainfall over study could not have been carried out. We also thank Lake Victoria). Meteor. Rundsch., 19, 157–165. Ed Zipser and two anonymous reviewers for their Fraedrich, K., 1972: A simple climatological model of the dy- thorough reading of the manuscript and valuable sug- namics and energetics of the nocturnal circulation at Lake gestions for improvement. Victoria. Quart. J. Roy. Meteor. Soc., 98, 322–335.

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