Simulation of a Serial Upstream-Propagating Mesoscale Convective System Event Over Southeastern South America Using Composite Initial Conditions

2144 MONTHLY WEATHER REVIEW VOLUME 137

Simulation of a Serial Upstream-Propagating Mesoscale Convective System Event over Southeastern South America Using Composite Initial Conditions

VAGNER ANABOR Laborato´rio de Fı´sica da Atmosfera, Departamento de Fı´sica, Universidade Federal de Santa Maria, Santa Maria, Brazil

DAVID J. STENSRUD NOAA/National Severe Storms Laboratory, Norman, Oklahoma

OSVALDO L. L. DE MORAES Laborato´rio de Fı´sica da Atmosfera, Departamento de Fı´sica, Universidade Federal de Santa Maria, Santa Maria, Brazil

(Manuscript received 10 April 2008, in ﬁnal form 29 November 2008)

ABSTRACT

Serial upstream-propagating mesoscale convective system (MCS) events over southeastern South America are important contributors to the local hydrologic cycle as they can provide roughly half of the total monthly summer precipitation. However, the mechanisms of upstream propagation for these events have not been explored. To remedy this situation, a numerical simulation of the composite environmental conditions from 10 observed serial MCS events is conducted. Results indicate that the 3-day simulation from the composite yields a reasonable evolution of the large-scale environment and produces a large region of organized convection in the warm sector over an extended period as seen in observations. Upstream propagation of the convective region is produced and is tied initially to the development and evolution of untrapped internal gravity waves. However, as convective downdrafts develop and begin to merge and form a surface cold pool in the simulation, the cold pool and its interaction with the environmental low-level ﬂow also begins to play a role in convective evolution. While the internal gravity waves and cold pool interact over a several hour period to control the convective development, the cold pool eventually dominates and determines the propagation of the convective region by the end of the simulation. This upstream propagation of a South American convective region resembles the southward burst convective events described over the United States and highlights the complex interactions and feedbacks that challenge accurate forecasts of convective system evolution.

1. Introduction Fritsch 1997). Over South America, a high incidence of MCCs occurs in southeastern South America (SESA) Mesoscale convective complexes (MCCs) are char- during the warm season (Velasco and Fritsch 1987). acterized by both their long lifetimes (.6 h) and the Zipser et al. (2006) further suggest that SESA has the large quasi-circular cloud shields they produce when most intense thunderstorms in South America, making observed at infrared wavelengths by satellite (2328C this area an important region for the study of deep contiguous area .104 km2; Maddox 1980). These com- convection. plexes commonly occur over land in the lee of major It is well known that North American MCCs develop mountain ranges and in association with low-level jets within conditionally unstable environments in asso- (LLJs; Stensrud 1996a), and make significant contribu- ciation with strong low-level warm advection, a LLJ, a tions to local and global hydrologic budgets (Laing and weak midlevel short-wave trough, and a quasi-stationary frontal boundary (Maddox 1983). Over SESA, the Andes Mountains play a large role in producing the mean low- Corresponding author address: Vagner Anabor, Laborato´ rio de Fı´sica da Atmosfera, Departamento de Fı´sica, Universidade level northerly flow that is observed throughout much of Federal de Santa Maria, 97119.900 Santa Maria, RS, Brazil. the year (Byerle and Paegle 2002; Campetella and Vera E-mail: [email protected] 2002). Often embedded in this northerly flow is a LLJ

DOI: 10.1175/2008MWR2617.1

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(Marengo et al. 2004). This South American low-level of the event. The MCSs develop on the anticyclonic jet (SALLJ) is characterized by a poleward penetration side of the entrance region to an upper-level jet as also of the northerly wind, promoting moisture and heat seen in South American MCS environments by Salio transport from the Amazon region into SESA (Saulo et al. (2007). Unfortunately, the use of the low-resolution et al. 2004). Numerous studies show that the SALLJ is NCEP–NCAR reanalysis data to document the envi- important to the hydrologic cycle in the La Plata basin ronments for the upstream-propagating MCS events in of SESA (James and Anderson 1984; Rasmusson and Anabor et al. (2008) does not allow for the investigation Mo 1996; Marengo et al. 2004). Owing to this favorable of the physical mechanisms that produce the upstream mean low-level flow and advection pattern, it is not propagation of the convective region. surprising that MCCs are a common occurrence in this The upstream propagation of convective regions over region. the United States is shown to be a fairly regular occur- A more common type of organized convective region, rence by Porter et al. (1955). An examination of a single containing a contiguous precipitation region of at least long-lived upstream-propagating convective event by 100 km in horizontal extent, is the mesoscale convective Stensrud and Fritsch (1993, 1994) shows that these system (MCS) of which the larger MCC is a subset. southward burst systems propagate upstream with re- Over SESA, Nicolini et al. (2002) find a high correlation spect to the flow at all levels of the atmosphere as also between the presence of MCSs and the SALLJ, with documented over South America by Anabor et al. 81% of the 27 precipitating MCSs investigated occur- (2008). This upstream propagation occurs via a combi- ring during SALLJ events. Salio et al. (2007) further nation of cold pool propagation and internal gravity find that 81% of the spring MCSs and 67% of the waves that move ahead of the convective line and initiate summer MCSs develop during SALLJ events. Using new convective development. Thus, while the individual regional modeling experiments, Saulo et al. (2007) ex- MCSs move downstream, the convective region as a plore the linkages between LLJs and organized con- whole propagates upstream (Stensrud and Fritsch 1993). vection. Their results show that the LLJ generates The importance of MCSs in SESA is emphasized by moisture convergence and warm advection, thereby noting that the rainfall from MCSs contributes nearly providing a favorable environment for the triggering of 90% of the total rainfall in the La Plata basin (Nesbitt deep convection. The subsequent latent heat release et al. 2006). This result strongly suggests that these from the convection reinforces the convergence, help- systems are very important to both the local climate and ing to extend the life of the MCS. Using satellite, radar, human populations. Berbery and Barros (2002) indicate radiosonde, and surface observations in combination that ;80% of the total precipitation in the La Plata with selected regional modeling experiments over the river basin occurs in the austral warm season (October– southwest Amazon basin, Silva Dias et al. (2002) show April) when the main concentration of MCSs is ob- that only a few deep and intense tropical convective served over SESA (Velasco and Fritsch 1987; Laing and cells are necessary to explain the overall convective line Fritsch 2000; Zipser et al. 2006). The SESA region also formation and that the production of multiple convec- produces about 70% of the total combined gross na- tive lines may be related to discrete cell propagation and tional product of Brazil, Uruguay, Argentina, Paraguay, their coupling with upper-atmosphere circulations. and Bolivia, and houses about half of their combined Examining an important subset of all MCSs over population. Thus, the SESA region has a large impact SESA, Anabor et al. (2008) document 10 long-lived on energy production, water resources, agriculture, and serial MCS events (lifetimes .18 h) over South America livestock in South America (Vera et al. 2006), suggest- using satellite data and National Centers for Environ- ing that the accurate prediction of MCS activity in this mental Prediction–National Center for Atmospheric region would be very beneficial to a number of users of Research (NCEP–NCAR) reanalysis data. They show weather information. that a series of individual MCSs in the prefrontal region The aim of this study is to investigate the relationship develop upstream relative to the flow at all levels of the between the SALLJ and the upstream-propagating troposphere and move away from the frontal boundary convective region as documented by Anabor et al. to produce these long-lived events. These serial MCSs (2008) and to analyze the physical mechanisms for up- form within an environment of strong low-level warm stream propagation. Section 2 contains a description of advection and strong moisture advection from the Am- the numerical model, while the methods used to create azon region southward, a situation very reminiscent of the initial and boundary conditions are discussed in the environments of North American MCCs (Maddox section 3. Results are presented in section 4 followed by 1983). A strong surface anticyclone off the Brazilian an examination of the mechanisms of upstream propa- coast influences the low-level flow during the early stages gation. A final discussion is found in section 6.

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the most reasonable evolution of the convective activity in comparison with the available observations.

3. Composite initial and boundary conditions Inspired by Coniglio and Stensrud (2001), realistic horizontally nonhomogeneous initial and boundary conditions are created using a simple composite approach on the 10 serial upstream-propagating MCS events documented by Anabor et al. (2008). This approach is selected to capture the features that are common to the environments of these serial MCS events, while removing features that are unique to individual cases. Thus, this approach focuses attention on the key, repeatable features that can most easily be used in developing forecast tech- niques. The composite approach also allows us to main- tain a more idealized modeling perspective where the FIG. 1. Model domain used in this study. Topography is shaded focus is on the physical processes producing upstream as indicated from sea level to 1000 m above sea level. The Andean propagation instead of the ability of the model to rep- Mountains (Andes) and La Plata Basin (LPB) areas are indicated. licate all observed aspects of a particular event. Since Nicolini et al. (2002) indicate that using NCEP–NCAR reanalysis lacks the horizontal resolution needed for 2. Model description the realistic simulation of SALLJ convective events, The nonhydrostatic Advanced Research version of the we choose to use the 1.0831.08 NCEP Global Final Weather Research and Forecasting Model (ARW-WRF) (FNL) analyses (see online at http://dss.ucar.edu/datasets/ is the model selected for use in this study (Skamarock ds083.2/) available every 6 h, as the dataset for con- et al. 2005). The ARW-WRF uses a terrain-following structing the composite. vertical coordinate and has a variety of physical process Anabor et al. (2008) use the Geostationary Operational parameterization schemes available. The physics options Environmental Satellite-12 (GOES-12) satellite images used in this study are the Lin et al. (1983) microphysics to track the geometric center of 10 serial upstream- scheme, the Kain and Fritsch (1993) convective scheme, propagating MCS events throughout their lifetimes. Since the Mlawer et al. (1997) rapid radiative transfer model we are most concerned with the physical mechanisms of for longwave and the Dudhia (1989) model for short- upstream propagation, the location of the first storms wave radiation, the Dudhia (1996) five-layer soil model, position is used to center the spatial location of the and the Yonsei University (YSU) planetary boundary composite (Table 1). The FNL analyses are interpolated layer (PBL) scheme (Noh et al. 2003) combined with to a 6083608 grid with 18 resolution centered on the first a Monin–Obukov-based similarity theory surface layer storm position for each of the 10 serial MCS events. To approach. produce the composite initial condition, the resulting 10 The three-dimensional model grid used in this study three-dimensional grids are simply averaged. As shown in contains 500 3 500 3 32 grid points using 10-km hori- Table 1, half of the serial MCS events start in the morning zontal grid spacing and encloses an area from 108–608S hours and half start in the evening hours. Owing to the to 858–358W (Fig. 1). The model top is at 100 hPa, no importance of the Andes Mountains to the development damping layer is used, and the vertical grid of 32 layers of the low-level flow patterns, the data are composited is stretched with more layers near the ground surface. with respect to the diurnal cycle instead of the time rel- Some sensitivity tests to ascertain the importance of ative to storm initiation or MCS life cycle stage. While these model choices were performed, with the results this is not a perfect solution, results indicate that the indicating that some scheme combinations do not pro- simulation correctly reproduces the evolution of the duce convective activity ahead of the frontal boundary large-scale environment over the 3-day period, suggesting as observed. The lack of prefrontal convective activity that the composite technique is successful in allowing for (the focus of this study) is considered a significant flaw an accurate evolution of the large-scale features impor- in any simulation and so these options were not used. tant to these serial MCS events in the model. Thus, the results reported below depend on the physics Since convection in SESA typically develops in the options selected. The physics options selected provide morning hours (Velasco and Fritsch 1987), the initial

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TABLE 1. The 10 upstream-propagating serial MCS events that make up the model composite are listed by the date, time (UTC), and location (latitude–longitude) of the ﬁrst storms along with the nearest FNL analysis time. The mean values for all storms also are indicated.

First storm Date Time UTC FNL analysis time Lat (8) Lon (8) 1 9 Sep 2005 0245 0000 UTC 9 Sep 2005 231 256 2 29 Sep 2005 1139 1200 UTC 29 Sep 2005 230 260 3 3 Oct 2005 0640 0600 UTC 3 Oct 2005 230 258 4 4 Nov 2005 1539 1200 UTC 4 Nov 2005 227 261 5 22 Nov 2005 0939 1200 UTC 22 Nov 2005 234 266 6 3 Dec 2005 0240 0000 UTC 3 Dec 2005 240 265 7 11 Jan 2006 2209 1800 UTC 11 Jan 2006 237 261 8 14 Jan 2006 1339 1200 UTC 14 Jan 2006 235 256 9 24 Apr 2006 2345 1800 UTC 24 Apr 2006 233 260 10 14 Aug 2006 0110 0000 UTC 14 Aug 2006 232 257 Mean 1058 233 260 condition is chosen in the local midafternoon at 1800 weak LLJ (;10 m s21 maximum wind speed) is ob- UTC. Thus, the FNL analyses from the 1800 UTC time served along the eastern slopes of the Andes near 228S prior to the time of first storms for each of the 10 cases (Fig. 3b). Warm advection at 850 hPa occurs down- are interpolated to the 6083608 grid and used for the stream and to the south of the LLJ region with magni- composite initial condition (see Table 1). Boundary tudes near 2.88C (12 h)21 (not shown). In contrast, the conditions are created in a similar manner using FNL vertically integrated moisture flux is a maximum in the analyses from each subsequent 6-h time period from LLJ region, reaching values in excess of 200 kg m21 s21 each serial MCS event. Composite boundary conditions (Fig. 4b). At upper levels the flow is westerly with a are provided out to 72 h, allowing for a full 3 days to be trough suggested to the west of the Andes (Figs. 5a,b). simulated. While the composite initial condition at 1800 A thermal low forms in the lee of the Andes Moun- UTC may have some imbalances because of the com- tains between 248 and 328S at 0000 UTC on day 2 (Fig. 2c) positing procedure, the model adjusts to its own balance that closely resembles the northern Argentine low within a few hours. The resulting evolution of the large- (NAL; Lichtenstein 1980; Seluchi et al. 2003). The NAL scale pattern is constrained by the composite boundary has a distinct diurnal pressure cycle with lower pressures conditions. By convention, the simulation starts at 1800 near 2100 UTC and higher pressures near 1200 UTC UTC on day 0 and extends through 1800 UTC on day 3. (Seluchi et al. 2003). Simulated sea level pressures in the The calendar day used to define the amount of solar NAL region decrease over 3 hPa from near 1009 hPa at radiation is chosen to be 3 December, in the middle of 1200 UTC on day 1 to 1006 hPa at 0000 UTC on day 2 the dates of the observed serial MCS events (Table 1). (Figs. 2b,c), following the diurnal cycle of observed The resulting simulations, shown in detail in the next NALs. The development of this low helps to expand the section, reproduce the large-scale features seen in Anabor region of the LLJ southward (Fig. 3c), yielding vertically et al. (2008) in association with the serial upstream- integrated moisture fluxes exceeding 400 kg m21 s21 propagating MCS events. Thus, it appears that the com- along the slopes of the Andes (Fig. 4c). The strongest posite analysis method produces a realistic representation region of warm advection at 850 hPa is again located of the large-scale environment for these cases. to the south of the LLJ region with magnitudes near 2.28C(12h)21 (not shown). At upper levels, the winds at 250 hPa have accelerated south of 328Stospeedsabove 4. Results 30 m s21 (Fig. 5c). By 1200 UTC on day 2, the NAL is slightly weaker, Synoptic setting and convective evolution likely owing to its thermal nature, and a cold front is The dominant low-level feature during day 1 is the now present over southern SESA (Fig. 2d) in associa- South Atlantic subtropical high as it strongly influences tion with a cyclone that is crossing the south Andes and the synoptic-scale circulation. This high helps to induce the arrival of an upper-level shortwave with wind speeds the northeasterly flow, seen both at the surface and at in excess of 45 m s21 (Fig. 5d). This synoptic pattern is a 850 hPa (Figs. 2a,b and 3a,b) in the northern portion common feature during MCS development over SESA of SESA, that advects the warm and moist air from (Salio et al. 2007). The LLJ as represented at 850 hPa the Amazon region to the south (Paegle 2000; Nicolini has expanded in size during the past 12 h and now has and Saulo 2000; Salio et al. 2007; Anabor et al. 2008). A wind speeds of over 20 m s21 in the lee of the Andes

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FIG. 2. Sea level pressure (hPa, solid lines) and 900-hPa wind vectors at (a) 0000 UTC on day 1, (b) 1200 UTC on day 1, (c) 0000 UTC on day 2, (d) 1200 UTC on day 2, (e) 0000 UTC on day 3, and (f) 1200 UTC on day 3. The ﬁlled circle indicates the position of the convective activity centroid starting at 1200 UTC on day 2.

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FIG. 3. As in Fig. 2, but for 850-hPa geopotential height (solid lines) and including wind barbs. Isolines of geopotential height every 30 m. Shading denotes wind speed (see key at bottom).

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21 21 FIG. 4. Vertical integrated moisture ﬂux (kg m s , vectors and shading) and 850-hPa wind magnitude (solid black lines). Filled circle as in Fig. 2. The key at the bottom indicates integrated moisture ﬂux values.

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FIG. 5. As in Fig. 2, but for 250-hPa geopential heights (m), winds, and wind speed shaded (see key at bottom). Filled circle as in Fig. 2. Isolines of geopotential height every 50 m.

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FIG. 6. Accumulated precipitation (mm) over 3-h periods beginning 0900 UTC on day 2–1800 UTC on day 3. Amounts are indicated by key at bottom.

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Mountains near 208S (Fig. 3d). Wind speeds above 10 m s21 are seen as far south as 368S across an east–west band nearly 300 km wide. As expected, the wind speeds in this simulated LLJ are stronger than in the composite analyses of Anabor et al. (2008). Silva Dias et al. (2001) show that the structure of simulated LLJs is dependent on the horizontal grid spacing, with results indicating that the LLJ becomes more confined to the slopes of the Andes as grid spacing is decreased. The strongest 850-hPa warm advection continues to occur downstream of the LLJ region with magnitudes near 2.28C(12h)21 (not shown). Vertically integrated moisture fluxes are now above 500 kg m21 s21 along the slopes of the Andes in the LLJ core with values above 100 kg m21 s21 located as far as 368S (Fig. 4d). Convection in the model simulation starts just after 1200 UTC on day 1 across the 278–338S latitude band and stretching from the Atlantic coast to the eastern slopes of the Andes Mountains (not shown). While this timing of convective activity agrees with the mean time of first storms as observed from satellite data (Table 1), FIG. 7. Total accumulated precipitation (mm) over the 3-day model simulation. Amounts in excess of 80 mm are seen in some the simulated convection in this broad area remains un- locations. organized and largely dissipates by 0000 UTC on day 2. The first organized region of convection develops 3 h later along the cold front near 368S, 668W at 0300 UTC ward (Fig. 2e). As discussed in the next section, this on day 2, while the first organized region of convection organized convective activity propagates upstream rel- in the warm sector ahead of the cold front initiates ;6h ative to the flow at all levels of the troposphere. The later near 308S, 588W (Fig. 2d) over SESA between 0600 cyclone continues to cross the Andes and induces a low- and 0900 UTC on day 2 (Figs. 6a,b). This position is still level, ageostrophic southerly flow that produces the in reasonable agreement with the mean location of the equatorward advection of cold air as described by first storms at 338S, 608W as observed via satellite for the Garreaud and Wallace (1998) and enhances conver- 10 observed events (Table 1). However, since only sat- gence ahead of it. Wind speeds at 850 hPa are northerly ellite observations are available, it is impossible to as- and above 10 m s21 across the warm sector, with the certain when the observed storms begin to organize and strongest winds confined to the higher terrain (Fig. 3e). determine if this model timing of convective evolution The vertically integrated moisture fluxes resemble those is correct. Yet the general evolution of the convective reported by Salio et al. (2007) in typical SESA MCSs, region agrees with the available satellite observations, with the LLJ channeling moisture toward the MCS region yielding some confidence in the results. with values above 500 kg m21 s21 across a broad area The simulated convective region occurs within the from 208 to 308S (Fig. 4e). The convective region (Fig. 6f) zone of strong warm advection and also is in the exit remains within the exit region of the LLJ (Fig. 3e) and is region of the LLJ (Fig. 2d). The large-scale composite at slowly encroached upon by the cold front to the south. the time of first storms from Anabor et al. (2008) shows The moisture transport reaches its maximum at 0000 a jet streak to the south of the convective region, sug- UTC on day 3 (Fig. 4e) in agreement with the results of gesting that this feature may play a role in developing an Salio et al. (2007). At upper levels, the convection is on environment favorable for convective initiation. Simi- the anticyclonic side of the entrance region to an upper- larly, the 250-hPa jet streak in the model simulation level jet (Fig. 5e) in agreement with the results of Salio arrives to the south of SESA between 0000 and 1200 et al. (2007) and Anabor et al. (2008). The evolution of UTC on day 2 (Fig. 5d) near the time of the first storms. the upper-level features, including the deepening of the Thus, the large-scale environment in the convective trough during the past 12 h and the increased winds initiation region has both low-level and upper-level extending northward toward the convective region, may forcing for upward motion. be due in part to the upscale feedbacks of the convective Over the next 12 h, the convection becomes organized region. Maddox et al. (1981), Wolf and Johnson (1995), within the warm sector as the cold front pushes north- and Stensrud (1996b) all demonstrate that a long-lived

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21 FIG. 8. Sea level pressure (hPa, solid lines) and 850-hPa vertical velocity (m s , shaded, see key at bottom) every hour from 0800 to 1300 UTC on day 2. Isolines of sea level pressure are every 0.15 hPa. (f) The location of the cross section depicted in Fig. 9 is shown.

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21 FIG. 9. Sea level pressure (hPa, solid lines) and 850-hPa vertical velocity (m s , shaded, see key at bottom) every hour from 1500 to 1800 UTC on day 2. Isolines of sea level pressure every 1 hPa. The hatched semitransparent region outlines accumulated .5 mm precipitation over 1-h periods.

MCS can significantly alter the upper-troposphere en- of the convective area at 250 hPa, a divergent flow pat- vironmental conditions, producing a jet streak poleward tern is clearly evident, again highlighting the potential and upstream of the MCS region. feedbacks of the convection on the large-scale environ- The organized convective region reaches its mature ment. Over the next few hours, the surface mesohigh stage by 1200 UTC on day 3, with a cold surface meso- expands in size and appears to control the evolution of high clearly seen underneath it (Fig. 2f). The cold front the simulated convection. However, the convective re- is still located over 200 km to the south of the warm gion still moves upstream relative to the flow at all levels sector convection with an intensifying cyclone now pre- of the troposphere (Fig. 6m). Total rainfall from this sent over the Atlantic Ocean. The LLJ at 850 hPa re- simulation is above 80 mm in some locations (Fig. 7), mains strong, with the strongest winds starting to move close to half a typical mean monthly rainfall for many off the elevated terrain (Fig. 3f). However, warm ad- locations in this region (Berbery and Barros 2002). The vection into the convective region is greatly reduced precipitation amount and spatial distribution is consis- from previous times and the vertically integrated mois- tent with the MCSs rainfall patterns during SALLJ ture fluxes also have decreased (Fig. 4f). The upper-level events (Nicolini et al. 2002; Saulo et al. 2007; Salio et al. trough is advancing across the Andes, with the con- 2007). The mechanisms of upstream propagation that vective region remaining on the anticyclonic side of the helped produce these large rainfall totals are now entrance region to the jet (Fig. 5f). Over and downstream explored.

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FIG. 10. Vertical cross section of potential temperature (shaded every 5 K and black lines) and vertical velocity (m s21) isolines every 0.2 m s21 along the line depicted in Fig. 8f. Negative values of vertical velocity are dashed.

5. Mechanisms of upstream propagation increase during and immediately after this transition in convective line orientation (Figs. 6f–h). After the tran- During the first 36 h of the model simulation, only sition occurs, the convection is organized as a convective unorganized convection associated with the diurnal cy- line that moves in a more northerly direction through cle is produced. However, close inspection of the the end of the simulation (Figs. 6i–m). Since the winds 3-hourly accumulated precipitation fields on days 2 and 3 throughout the troposphere are still from the north and after convection starts to become organized shows several west, with the possible exception of the winds in the interesting features (Fig. 6). As mentioned in the previous upper troposphere by 1200 UTC on day 3 that likely are section, convection develops along the cold front by influenced by the long-lived convective activity (Fig. 5f), 0300 UTC on day 1, with a few small areas of convection the convective region is still considered to be propagating in the warm sector to the north. However, 6 h later an upstream through the end of the simulation. arc-shaped region of convection is seen in the warm The initial arc-shaped convective region simulated sector between 308 and 328Salong588W (Fig. 6b). This between 1200 and 2100 UTC on day 2 (Figs. 6b–e) is arc-shaped convective line moves west-northwestward very suggestive of a wavelike feature. Indeed, an ex- and expands outward over the next 6 h until it stretches amination of the 850-hPa vertical motion and sea level over 600 km from 288 to 348S along 608Wat1800UTC pressure fields at 0800 UTC on day 2 indicates two arc- on day 2 (Fig. 6d). The extent of this region of rainfall is shaped regions of upward motion preceded by a surface large enough to be considered an MCS. With the tropo- pressure trough and followed by an arc-shaped region of spheric winds from the northwest at all levels (Figs. 2–5), downward motion preceded by a pressure ridge (Fig. 8a). this west-northwestward movement of the convective Tracking these regions back in time suggest that they region is in an upstream direction. initiate from a small region of convection that developed Over the next 9 h the western edge of the convective to the east in a region of low-level upslope flow over the region continues to move slowly west-northwestward previous hour. These upward–downward motion cou- while the convection transitions from north–south- plets move west-northwestward at ;9ms21 and expand oriented convective lines at 1800 UTC on day 2 (Fig. 6d) outward in size over the next 5 h. The values of upward to an northeast–southwest-oriented convective line at motion reach 0.4 m s21 in some locations along the arc- 0300 UTC on day 3 (Fig. 6g). The largest rainfall amounts shaped line. A second packet of two upward–downward

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FIG. 11. Model grid point sounding from point A in Fig. 8. motion couplets appears to the east of the ﬁrst packet at move toward the northwest (Fig. 9), as indicated by the 1200 UTC on day 2 (Fig. 8d) and is also associated with upward motion leading (i.e., located to the west of) the the development of a small region of convection in an hourly rainfall totals. Thus, the simulated upstream- area of low-level upslope ﬂow. This couplet also ex- propagating gravity waves are acting to trigger the pands outward with time and moves west-northwest- model convective parameterization. The convective ward at the same speed as the earlier wavelike features. scheme is deactivated in the region of descending mo- The relationship between the sea level pressure per- tion associated with the gravity wave, although the turbations and upward motion, with the zone of upward timing of the scheme deactivation may be a function of motion occurring between the pressure trough and the convective scheme adjustment time scale. A west– pressure ridge (Fig. 8), is consistent with an internal east cross section through the waves shows that the gravity wave (Eom 1975; Gossard and Hooke 1975). potential temperature perturbations tilt downstream in Closer inspection reveals that the convection and the the vertical (Fig. 10), indicating that these waves are waves propagate in phase one another (Fig. 9). Upward untrapped (Gossard and Hooke 1975). The simple two- motion associated with the gravity waves occurs prior to layer gravity wave model of Eom (1975) is used to fur- the development of rainfall in the model as the waves ther examine the wave characteristics. This analytic

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21 FIG. 12. Sea level pressure (hPa) isolines every 1- and 850-hPa vertical motion (m s , see key at bottom) at 0300 UTC on day 3. Locations of mesoscale troughs (T) and ridges (R) are indicated by the dashed and solid lines, respectively. The location of the outflow boundary is also indicated by thick solid line. The hatched region at the center of the grid encloses an area of upward motion .1.0 m s21. The arrow indicates the direction of gravity wave motion. model divides the atmosphere into two vertical layers is convectively unstable with values of convective avail- with uniform properties in order to determine reasonable potential energy near 500 J kg21 (Fig. 11). Un- able gravity wave characteristics. Selecting the 800-hPa trapped internal gravity waves also are seen to produce level as the level that separates the two vertical model upstream propagation in Stensrud and Fritsch (1994). layers, the gravity wave model yields an upstream Between 2100 UTC on day 2 and 0300 UTC on day 3, propagation speed of 12 m s21 in reasonable agreement the southern portions of the convective lines merge into with the simulated wave propagation speed of ;9ms21 a more cohesive convective cluster (Jirac et al. 2003; [differences of a few m s21 between calculated and Figs. 6e,f). The transition from north–south-oriented con- simulated wave speeds are common (see Eom 1975; vective lines to a single east–west-oriented convective line Stensrud and Fritsch 1993, 1994)]. Using a typical sur- appears to be linked to the creation of a cold pool at the face pressure perturbation from the model simulation, surface (Fig. 12). The cold pool acts to organize the the Eom (1975) model also diagnoses upward motion simulated convection along its leading edge, or outflow between 1.16 and 2.3 m s21, roughly twice the values seen boundary, and is produced by the development of ex- in the simulation (Figs. 8, 12, and 15). A model sounding plicit convective processes in the model simulation upstream of the gravity wave activity (Fig. 11) shows that (prior to this time the majority of the convection in the there is no stable layer to trap the wave energy (Lindzen model is produced by the convective parameterization and Tung 1976). The waves propagate almost parallel to scheme). Rain is consistently produced along and to the the surface–700-hPa winds in a weak-to-moderate wind south of the outflow boundary as the cold pool expands shear environment (900–500-hPa wind shear .2s21)that and intensifies (Fig. 13), leading to the creation of the

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21 FIG. 13. Sea level pressure (hPa, solid lines) and 850-hPa vertical velocity (m s , shaded, see key at bottom) every hour from 0300 to 0600 UTC on day 2. Isolines of sea level pressure every 1 hPa. The hatched semitransparent region outlines accumulated .5 mm precipitation over 1-h periods. northwest–southeast orientation of the main convective Another group of internal gravity waves are seen to the line. While internal gravity waves are still present in the southeast of the cold pool and are moving toward it. warm sector to the north of the outflow boundary and Thus, the evolution of convection in the warm sector is influence convective activity, the cold pool begins to dominated initially by internal gravity waves and later by dominate the convective region evolution as it strengthens the interactions between internal gravity waves and a and grows upscale (Fig. 13). Rainfall is even produced in growing and strengthening cold pool (Figs. 9 and 13). regions of descending motion associated with the grav- The cold pool continues to grow and strengthen over ity waves owing to the dominating influence of upward the coming hours, stretching from near the Atlantic motion along the cold pool outflow boundary. With Ocean to the Andes Mountains by 1200 UTC on day 3 strong northerly environmental winds to the north of (Fig. 14). The cold pool boundary is located nearly the cold pool helping to advect warm and moist air to- 400 km ahead of the cold front, and the cold pool ward the cold pool, the rising motion along the outflow mesohigh is over 2 hPa in magnitude. Upward motion boundary is as large or larger than that associated with along the outflow boundary exceeds 0.5 m s21 in many the gravity waves. Along the eastern edge of the cold locations and continues to be a focus for initiating pool, in the region where the cold pool and gravity waves convective activity as indicated by a vertical cross sec- meet, the values of rising motion are maximized (Fig. 12). tion (Fig. 15). Indeed, the cold pool appears to be the

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FIG. 14. As in Fig. 11, but at 1200 UTC on day 3. Note the 1011-hPa mesohigh located in the center of the plot with a coherent region of upward motion to the north along the edge of the cold pool. dominant feature in controlling the convective devel- fects of internal gravity waves and cold pools control the opment and this explains the shift in the propagation of upstream propagation of serial MCS events over SESA. the convective region to a more northeasterly direction. A conceptual model of satellite-observed serial MCS The important interactions between internal gravity events over SESA by Anabor (2004) and shown in waves and cold pools that together control the evolution Anabor et al. (2008) captures average MCS behavior, of convection is also discussed by Stensrud and Fritsch and shows successive storms developing at roughly 5-h (1993, 1994). They show that while internal gravity intervals 370 km to the northwest of the previous MCS, waves can lead to convection jumping ahead of an active while the whole MCS trajectory is toward the northeast. convective line, thereby forming a new convective line, Low-level cloud bands oriented from southwest to the cold pool plays the dominant role once moist down- northeast are seen in some cases (Fig. 16a), suggesting drafts develop and organize into a cold pool density cur- that upstream-propagating internal gravity waves may rent. The present study outlines a slightly different picture be present. These gravity waves need to propagate up- in which internal gravity waves lead to the initial con- stream at 19 m s21 to explain the average distance in vective development and upstream propagation until time and space between successive storms found by downdrafts from several convective elements merge and Anabor (2004). Since this magnitude of upstream wave organize into a cold pool that grows and slowly domi- propagation speed is possible in the atmosphere (Eom nates the further evolution of convection. Thus, similar 1975), this conceptual model is very suggestive of the to the results of Stensrud and Fritsch (1993, 1994) for importance of internal gravity waves to the develop- North American serial MCS events, the combined ef- ment of successive MCSs in the upstream direction.

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FIG. 15. Cross section of potential temperature (K) with isolines every 5 K and vertical motion (m s21, see key at bottom) across the mesoscale cold pool. Location of the cross section shown in Fig. 12. Note the strong region of upward motion along the northern edge of the cold pool.

However, field campaigns, radar observations, and a upstream propagation of the South American convective high-resolution observational network are needed to region resembles the southward burst convective events evaluate the hypotheses raised in this study. described by Porter et al. (1955) and studied by Stensrud and Fritsch (1993, 1994) over the United States. Serial MCS events over SESA are important contrib- 6. Discussion utors to the local hydrologic cycle as they can provide The mechanisms of upstream propagation during se- roughly half of the total monthly summer precipitation. rial MCS events over SESA are explored using a nu- Thus, the forecasting of these events deserves serious merical simulation of the composite environmental attention in the region. Unfortunately, our conclusions conditions from 10 observed events. Results indicate that that interactions between internal gravity waves and cold the simulation from the composite yields a reasonable pools largely control convective development and evolu- evolution of the large-scale environment throughout the tion suggests that correctly forecasting these events may 3-day simulation and produces a large region of orga- be challenging. The model results presented also may be nized convection in the warm sector over an 18-h period. sensitive to the parameterization schemes selected. Most importantly, the model simulation shows that the While the simulation outlines one mechanism of up- initial convective development and upstream propaga- stream propagation and explains how these serial MCS tion of the early convection is tied to the development events could be produced, a number of questions re- and evolution of untrapped internal gravity waves. As main. What is so special about the environmental con- convective downdrafts develop and begin to merge to ditions in the warm sector that allows for development form a surface cold pool in the simulation, the cold pool of upstream-propagating gravity waves of sufficient and its interaction with the environmental low-level flow amplitude to initiate convection? Is the answer to this starts to play a role in convective evolution. The internal question linked to the presence of the South Atlantic gravity waves and cold pool interact over several hours to anticyclone and the SALLJ that together advect warm influence the convective development, but the cold pool and moist air into the MCS region? Are there other eventually dominates and controls the propagation of mechanisms that could lead to upstream propagation? the convective region by the end of the simulation. This Now that one possible mechanism behind these serial

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FIG. 16. Infrared satellite images suggesting gravity waves on 3 Dec 2005 in the (a) pre-MCS environment. Also shown is the infrared imagery during (f) MCS initiation and at the time of the (g) MCS maximum as based on the criteria of Maddox (1983). The arrow in (a) indicates a region of apparent wave activity extending upstream from a region of early convection. Temperature thresholds are shown in the key.

MCS events has been identified, attention needs to turn Coniglio, M. C., and D. J. Stensrud, 2001: Simulation of a pro- to developing a better understanding of the combination gressive derecho using composite initial conditions. Mon. of environmental conditions that allows such interesting Wea. Rev., 129, 1593–1616. Dudhia, J., 1989: Numerical study of convection observed during convective events to develop. Better observations within the winter monsoon experiment using a mesoscale two- SESA are sorely needed to answer many of these ques- dimensional model. J. Atmos. Sci., 46, 3077–3107. tions. The ability of real-time modeling systems to pre- ——, 1996: A multi-layer soil temperature model for MM5. Pre- dict these events also deserves exploration. prints, Sixth PSU/NCAR Mesoscale Model Users’ Workshop, Boulder, CO, PSU–NCAR, 49–50. [Available from NCAR, P.O. Box 3000, Boulder, CO 80307-3000.] Acknowledgments. The first author was supported by Eom, J. K., 1975: Analysis of the internal gravity wave occurrence the Conselho Nacional de Desenvolvimento Cientı´fico e of 19 April 1970 in the Midwest. Mon. Wea. Rev., 103, 217–226. Tecnolo´ gico (CNPQ; National Council of Scientific and Garreaud, R. D., and J. M. Wallace, 1998: Summertime incursions Technological Development), Brazil. We also gratefully of midlatitude air into subtropical and tropical South America. acknowledge the support given by the ITS group at Mon. Wea. Rev., 126, 2713–2733. Gossard, E. E., and H. W. Hooke, 1975: Waves in the Atmosphere. NSSL. The helpful and constructive comments of three Elsevier, 472 pp. anonymous reviewers are greatly appreciated and led to James, I. N., and D. L. T. Anderson, 1984: The seasonal mean flow an improved discussion of these results. and distribution of large-scale weather systems in the South- ern Hemisphere: The effects of moisture transports. Quart. J. REFERENCES Roy. Meteor. Soc., 110, 943–966. Jirak, I. L., W. R. Cotton, and R. L. McAnelly, 2003: Satellite and Anabor, V., 2004: Descriptive analyses of meso-a convective sys- radar survey of mesoscale convective system development. tems by GOES-8 satellite images. M.S. thesis, Departament of Mon. Wea. Rev., 131, 2428–2449. Remote Sensing, Universidade Federal do Rio Grande do Kain, J. S., and J. M. Fritsch, 1993: Convective parameterization for Sul, Brazil, 78 pp. mesoscale models: The Kain-Fritsch scheme. The Representa- ——, D. J. Stensrud, and O. L. L. de Moraes, 2008: Serial upstream- tion of Cumulus Convection in Numerical Models, Meteor. propagating mesoscale convective system events over south- Monogr., No. 46, Amer. Meteor. Soc., 165–170. eastern South America. Mon. Wea. Rev., 136, 3087–3105. Laing, A. G., and J. M. Fritsch, 1997: The global population of Berbery, E. H., and V. R. Barros, 2002: The hydrologic cycle of the mesoscale convective complexes. Quart. J. Roy. Meteor. Soc., La Plata Basin in South America. J. Hydrometeor., 3, 630–645. 123, 389–405. Byerle, L., and J. Paegle, 2002: Description of the seasonal cycle of ——, and ——, 2000: The large-scale environments of the global low-level flows flanking the Andes and their interannual populations of mesoscale convective complexes. Mon. Wea. variability. Meteorologica, 27, 71–88. Rev., 128, 2756–2776. Campetella, C. M., and C. S. Vera, 2002: The influence of the Lichtenstein, E. R., 1980: La depresion del noroeste Argentino Andes Mountains on the South American low-level flow. (The northwestern Argentina low). Ph.D. dissertation, Uni- Geophys. Res. Lett., 29, 1826, doi:10.1029/2002GL015451. versity of Buenos Aires, Buenos Aires, Argentina, 223 pp.

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