AUGUST 2008 ANABOR ET AL. 3087

Serial Upstream-Propagating Mesoscale Convective System Events over Southeastern South America

VAGNER ANABOR Laboratório de Física da Atmosfera, Departamento de Física, Universidade Federal de Santa Maria, Santa Maria, Brazil

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

OSVALDO L. L. DE MORAES Laboratório de Física da Atmosfera, Departamento de Física, Universidade Federal de Santa Maria, Santa Maria, Brazil

(Manuscript received 2 August 2007, in final form 25 October 2007)

ABSTRACT

Serial mesoscale convective system (MCS) events with lifetimes over 18 h and up to nearly 70 h are routinely observed over southeastern South America from infrared satellite imagery during the spring and summer. These events begin over the southern La Plata River basin, with individual convective systems generally moving eastward with the -layer-mean . However, an important and common subset of these serial MCS events shows individual MCSs moving to the east or southeast, yet the region of convective development as a whole shifts upstream to the north or northwest. Analyses of the composite mean environments from 10 of these upstream-propagating serial MCS events using NCEP–NCAR reanalysis data events indicates that the synoptic conditions resemble those found in mesoscale convective complex environments over the United States. The serial MCS events form within an environment of strong low- level warm advection and strong moisture advection between the surface and 700 hPa from the Amazon region southward. One feature that appears to particularly influence the low-level flow pattern at early times is a strong surface anticyclone located just off the coast of Brazil. At upper levels, the MCSs develop on the anticyclonic side of the entrance region to an upper-level jet. Mean soundings show that the atmosphere is moist from the surface to near 500 hPa, with values of convective available potential energy above 1200 J kgϪ1 at the time of system initiation. System dissipation and continued upstream propagation to the north and northwest occurs in tandem with a surface high pressure system that crosses the Andes Mountains from the west.

1. Introduction Rodgers et al. 1983, 1985). An important and well- studied subset of these systems are defined as meso- Since the 1980s, a number of papers have docu- scale convective complexes (MCCs) by Maddox (1980, mented the environments, life cycles, and observational 1983) and are characterized by their large size, long characteristics of mesoscale convective systems (MCSs) lifetimes (Ͼ6 h), and the quasi-circular cloud shields over the Great Plains of the United States. These sys- they produce as observed by satellite at infrared wave- tems are important because they produce a large frac- lengths (see Table 1 for the definition and physical tion of warm season rainfall (Fritsch et al. 1986; Heide- characteristics of MCC). Over 400 MCCs occur across man and Fritsch 1988) and often are associated with the globe each year and are observed on every conti- severe (Maddox et al. 1982; Wetzel et al. 1983; nent except Antarctica (Laing and Fritsch 1997). They commonly occur over land in the lee of major mountain ranges and in association with low-level jets, and likely Corresponding author address: Vagner Anabor, Laboratório de Física da Atmosfera, Departamento de Física, Universidade Fed- make significant contributions to local and global hy- eral de Santa Maria, 97119.900 Santa Maria, RS, Brazil. drologic budgets (Laing and Fritsch 1997). E-mail: [email protected] Rawinsonde data from 10 MCC events are used to

DOI: 10.1175/2007MWR2334.1

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TABLE 1. Definition and physical characteristics of MCCs. Adapted from Maddox (1980).

Size criteria: 1) Cloud shield with infrared Յ32°C, must have an area Ն100 000 km2 and 2) interior cold cloud region with temperatures ՅϪ52°C must have an area Ն50 000 km2 Initiation: Size definitions 1 and 2 area first satisfied Max MCC extent occurs when contiguous cold cloud shield with infrared ՅϪ32°C reaches its maximum size Shape: Eccentricity (minor axis/major axis) Ն0.7 at time of maximum extent Dissipation: Size definitions 1 and 2 no longer satisfied produce composites of the typical large-scale synoptic environment associated with these events during their initiation, maturation, and dissipation stages (Maddox 1983). Results indicate that MCCs develop in condi- tionally unstable environments in regions of strong low- level warm advection and in association with a weak midlevel short-wave trough and a quasi-stationary fron- tal boundary. A low-level jet is present at initiation and strengths slightly as the system matures, helping to pro- duce low-level convergence and rising motion. The low- level jet disappears by the time of system dissipation when the low-level warm advection is replaced by cold advection. A slightly different long-lived meso-␣-scale convec- tive system (Orlanski 1975), called a persistent elon- gated convective system (PECS), is described by FIG. 1. La Plata and Amazon river drainage basins (light and Anderson and Arritt (1998). These systems meet the dark gray, respectively), with topographic features 500 m or more size and duration criteria for MCCs and develop and above sea level shown by the white line [Andes Mountains (west) grow in environments similar to those found for MCCs, and Brazilian heights (east)]. Rectangle outlines the area shown in but have a more elongated infrared cloud shield shape the composites of Figs. 6, 9, 12, and 15. (minor axis/major axis Յ0.7) and lifetimes Ͼ12 h. These elongated systems are observed more frequently over A careful analysis of a single southward burst event the United States than the more circular MCCs, typi- indicates that while the individual MCSs in general cally initiating in environments characterized by deep, move downstream with the cloud-layer-mean wind, the synoptic-scale ascent associated with continental-scale convective area as a whole propagates upstream into baroclinic waves. Jirak et al. (2003) use a 3-yr warm the low-level flow of warm, moist air provided by the season to show that PECS are highly cor- low-level jet (Stensrud and Fritsch 1993). Thus, the con- related with negative lifted-index values and produce vective activity is produced by a series of individual more and severe weather than MCCs. downstream-moving MCSs that interact to produce up- Analysis of radar data indicates that more than half of stream propagation of a convective region. the PECS develop through an areal merger or combi- The existence of MCCs over South America is docu- nation merger of smaller convective systems. Merging mented by Velasco and Fritsch (1987), indicating a lo- convective clusters occur in more than 70% of the cal maximum in occurrence over western Colombia PECS studied (Jirak et al. 2003). during the Northern Hemisphere summer (as also seen A type of serial MCS event observed over the United by Mapes et al. 2003) and a continental maximum in States is the southward burst (Porter et al. 1955). These MCC occurrence over the La Plata River basin region MCS events are set apart from other convective events (Fig. 1) of southeastern South America during the because they propagate upstream relative to the flow at Southern Hemisphere summer. In contrast, the Ama- all levels of the troposphere and either move away from zon River basin shows a curious lack of MCC activity or along the frontal boundaries near which they initiate. in the summer season (Velasco and Fritsch 1987). A

Unauthenticated | Downloaded 09/26/21 04:53 PM UTC AUGUST 2008 ANABOR ET AL. 3089 similar pattern of MCS occurrence during the Southern infrared satellite imagery. Real-time, full-resolution (4 Hemisphere summer is found by Salio et al. (2007). km), calibrated and navigated Geostationary Opera- Interactions between convective systems and cold- tional Environmental Satellite-12 (GOES-12) infrared frontal incursions are examined by Siqueira et al. satellite images are obtained online from the National (2005). No studies of PECSs over South America have Aeronautics and Space Administration (NASA) God- been conducted. dard Space Flight Center (GSFC; see online at http:// While MCCs are observed via satellite over South goes.gsfc.nasa.gov/goeseast/argentina). The time inter- America, there is little information on the large-scale val between GOES-12 images is about 30 min, and 256 environmental conditions of these systems. It may be gray levels are available for the brightness temperature that the composite environment defined by Maddox conversion. The area of this study extends from roughly (1983) for events in North America also describes the 20° to 45°S over the South American continent. Data typical environments of South American MCC events. are downloaded from May 2005 to September 2006 and Velasco and Fritsch (1987) indicate that the average MCSs are identified and tracked in order to identify characteristics of South American MCCs are similar to serial MCS events. their North American counterparts, although the South There are some differences in the literature concern- American systems tend to develop later in the diurnal ing the temperature thresholds used to define MCSs cycle, last longer, and are 60% larger. Velasco and from satellites, but most researchers agree that infrared Fritsch (1987) offer a number of hypotheses for the temperature thresholds lower than Ϫ30°C represent larger South American MCCs, but the lack of observa- deep convective regions (Maddox 1983; Velasco and tions hampers any assessments. Fritsch 1987; Machado and Rossow 1993; Machado et Using satellite data, Starostin and Anabor (2004) ap- al. 1992, 1998; Jirak et al. 2003; Siqueira and Machado ply a semiautomatic imaging process to document the 2004; Zipser et al. 2006). Maddox (1983) uses Ϫ32° and structure and evolution of 49 long-lived convective sys- Ϫ52°C thresholds to determine MCCs over the United tems over South America between 1999 and 2001. They States, whereas Velasco and Fritsch (1987) use Ϫ40° find that these convective events are a combination of and Ϫ62°C for MCCs over South America and Ander- several MCSs or MCCs with a total average lifetime of son and Arritt (1998) use a single threshold tempera- ϳ32 h. Often the cluster of multiple MCSs existing at ture of Ϫ52°C for PECS. Machado et al. (1998) and the same time appears from the satellite to form a Machado and Rossow (1993) explore the use of tem- single large PECS as described by Anderson and Arritt perature thresholds from Ϫ21° to Ϫ67°C and find that (1998). Starostin and Anabor (2004) show that at the deep convective cloud systems are associated with a time of maximum extent, 70% of these serial MCS sys- region of infrared brightness temperatures ϽϪ28°C tems are classified as PECSs. These long-lived MCS that at some time have embedded areas with infrared events also resemble the coherent precipitation episodes brightness temperatures ϽϪ55°C. Thus, three tempera- shown by Carbone et al. (2002) over the United States. ture thresholds, Ϫ38°, Ϫ45°, and Ϫ50°C, are chosen to The aim of this study is provide further documenta- identify MCSs from GOES-12 data in the South Ameri- tion of these long-lived serial MCS events over South can midlatitudes for this study. Sensitivity tests on the America and evaluate the synoptic patterns and envi- present dataset indicate that MCS identification is not ronmental conditions at several times during their life very sensitive to temperature threshold changes of a cycle. Owing to the lack of upper-air observations over few degrees. southeastern South America, the National Centers for Satellite image animation is used to preselect long- Environmental Prediction–National Center for Atmo- lived MCS events with Ϫ45°C threshold areas Ͼ100 000 spheric Research (NCEP–NCAR) reanalysis data are km2 and lifetimes Ն18 h from the time of the first used. The data and methodology used in this study are storms. A polygon is used to select the MCS area in described in section 2. Section 3 documents the typical each satellite image, allowing for the separation of the evolution of these serial MCS events. The composite midlatitude MCSs from tropical convection and from environments are described in section 4, followed by a peripheral storms that can be near the MCSs but clearly discussion in section 5. are not part of them. A simple comparative classifier is used to identify and estimate the area enclosed by the 2. Data and methodology Ϫ38°, Ϫ45°, and Ϫ50°C brightness temperatures thresholds in the infrared GOES-12 scenes. This a. Satellite data manual area selection is used to identify the events for The classification and characterization of midlatitude a small number of serial MCS cases. South American MCSs is accomplished by evaluating Serial MCS events are tracked to determine their

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FIG. 2. Infrared satellite imagery at various UTC times 14–16 Jan 2006 that illustrate the evolution of one of the 10 serial MCS events studied. Cloud-top temperature thresholds of Ϫ38°, Ϫ45°, and Ϫ50°C are shown in black, gray, and white, respectively. The letters F, I, M, and D refer to the times of first storms, MCS initiation, MCS maximum, and MCS dissipation, respectively. typical evolution. The position of the geometric center events (examples are shown in Figs. 2 and 3) are found of a MCS or cluster of MCSs during its life cycle is that develop to the north or northwest, many of which defined using the following rules: become PECS at some stage in their life cycle. Some of these events persist for several days (Fig. 2), while oth- Ϫ 1) If infrared temperatures less than 40°C are not ers last less than a day (Fig. 3). present (as happens for the initial storm stages), the Satellite animations are used to define the location of center of the convective region is the geometric cen- the MCS region at the time of four life stages: first Ϫ ter of the 38°C pixels. storm position (FSP), initiation system position (ISP), Ϫ 2) Once a temperature less than the 45°C threshold is maximum system position (MSP), and dissipation sys- present, the center of the convective region is the tem position (DSP). The FSP is the position of the first geometric center of the Ϫ45°C pixels. pixel at Ϫ38°C present in the preconvective area that 3) If the area of the Ϫ50°C pixels occupies at least 25% later develops into the MCS (Figs. 2 and 3). The others of the area of the Ϫ45°C pixels, then the center of stages mimic the MCC life cycles from Maddox (1983), the convective region is calculated using the geomet- in which the ISP is the position of the first MCS that ric center of the Ϫ50°C pixels. reaches the size criteria of the Ϫ45°C area greater than 100 000 km2 and the Ϫ50°C area greater than 50 000 This last condition is necessary because calculating km2, the MSP is the mean position of the MCS(s) at the the geometric center of an MCS region using only the time of the maximum extent of the (combined) Ϫ38°C Ϫ50°C threshold generates very noisy system trajecto- cloud shield, and the DSP is the mean position of the ries, due to the development and decay of intense, MCS(s) when the minimum size criterion is no longer smaller-scale convective elements within the MCS. met. The MCS systems studied and the time, size, and From May 2005 to September 2006, 18 serial MCS locations of the FSP, ISP, MSP, and DSP for each sys-

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FIG. 3. As in Fig. 2, but for the 24–25 Apr 2006 serial MCS event. tem are listed in Table 2. These serial MCS events last MCS events varies (Table 2), the 6-h reanalysis data for anywhere between 19 and 69 h and during this time allow us to capture the large-scale conditions near the period at least one MCC is observed for each case. The four life cycle times for each event. The averaging ap- lifetimes of these serial MCS events are similar to the proach used to create the composite allows for easier lifetimes of the coherent precipitation episodes docu- identification of the important mesoscale and large- mented by Carbone et al. (2002) over North America. scale features that are common to the serial MCS events as done in Maddox (1983) for individual MCCs. b. Reanalysis data However, the amplitudes of these common features are Once each case is documented using the infrared typically damped compared to those seen in individual GOES-12 imagery, NCEP–NCAR reanalysis I data events. In addition, features that are only found in one (Kalnay et al. 1996) with 2.5° ϫ 2.5° spatial resolution or two events are largely removed from the resulting and 6-h time resolution are used to create composite composite. large-scale environmental conditions at each of the four Further examination of the 18 serial MCS cases in- serial MCS life cycle stages. While features with length dicates that 8 of the cases developed under different scales of less than 1200 km (ϳ4 ⌬x) are poorly repre- large-scale patterns than the other 10. Thus, only data sented in the reanalyses, the lack of upper-air observa- from the 10 serial MCS cases with a similar large-scale tions leads to the reanalysis data being the best source pattern are used in the composite analysis. This subjec- of synoptic-scale data over southeastern South tive evaluation of the synoptic setting of these events America. However, the use of the relatively coarse re- provides greater confidence that the resulting compos- analysis data means that mesoscale features, such as ites are a realistic representation of the large-scale en- low-level jets and moist axes, tend to be smoothed vironment of this type of serial MCS event. (Vernekar et al. 2003; Salio et al. 2007). A grid with dimensions 50° ϫ 50° is centered on the MCS geometric center and reanalysis data obtained at 3. Evolution of serial MCS events the standard levels of 1000, 925, 850, 700, 600, 500, 400, 300, 250, and 200 hPa. This is done for the reanalysis The convective storms that lead to the development times closest to the times of FSP, ISP, MSP, and DSP of the first MCS develop within a region of low-level for each serial MCS case. The resulting mean field from over either the southern La Plata river basin the serial MCS events is then centered on the MCS (Fig. 1) or between this basin and the Atlantic Ocean geometric center. Although the longevity of the serial (Figs. 2 and 3). The time of these first storms varies

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TABLE 2. Dates, times (UTC), and sizes (103 km2) of the three cloud-top temperature thresholds, time after first storms, and lat and lon values of the MCS centroid from the 10 serial MCSs studied over South America at each life cycle stage (i.e., first storms, initiation, maximum extent, and dissipation). Mean values from each life cycle stage are also indicated.

FSP Date Time (UTC) Ϫ38°C Ϫ45°C Ϫ50°C Lat (°) Lon (°) 1 9 Sep 2005 0245 — 4.64 0.00 0.00 Ϫ31 Ϫ56 2 29 Sep 2005 1139 — 0.10 0.00 0.00 Ϫ30 Ϫ60 3 3 Oct 2005 0640 — 3.86 0.00 0.00 Ϫ30 Ϫ58 4 4 Nov 2005 1539 — 0.05 0.00 0.00 Ϫ27 Ϫ61 5 22 Nov 2005 0939 — 22.22 0.29 0.00 Ϫ34 Ϫ66 6 3 Dec 2005 0240 — 17.76 0.00 0.00 Ϫ40 Ϫ65 7 11 Jan 2006 2209 — 10.98 0.45 0.00 Ϫ37 Ϫ61 8 14 Jan 2006 1339 — 7.73 0.00 0.00 Ϫ35 Ϫ56 9 24 Apr 2006 2345 — 0.03 0.00 0.00 Ϫ33 Ϫ60 10 14 Aug 2006 0110 — 0.11 0.00 0.00 Ϫ32 Ϫ57 Mean 1058 — 6.75 0.07 0.00 Ϫ33 Ϫ60 ISP Time after FSP 1 10 Sep 2005 0245 24 282.10 93.50 6.62 Ϫ33 Ϫ55 2 30 Sep 2005 0639 19 281.49 90.59 23.82 Ϫ25 Ϫ56 3 3 Oct 2005 1339 6.98 147.44 61.06 4.02 Ϫ33 Ϫ56 4 4 Nov 2005 2345 8.1 137.71 60.24 24.22 Ϫ22 Ϫ61 5 22 Nov 2005 2139 12 129.18 50.29 0.05 Ϫ35 Ϫ63 6 4 Dec 2005 0740 29 154.21 55.81 3.46 Ϫ31 Ϫ59 7 12 Jan 2006 1910 21 205.57 80.48 3.79 Ϫ32 Ϫ60 8 14 Jan 2006 2345 10 155.42 57.22 8.67 Ϫ35 Ϫ60 9 25 Apr 2006 0539 5.9 135.58 50.90 2.24 Ϫ32 Ϫ59 10 14 Aug 2006 0339 2.48 190.18 61.02 0.29 Ϫ35 Ϫ55 Mean 1250 12.00 181.89 66.11 7.72 Ϫ31 Ϫ59 MSP Time after ISP 1 10 Sep 2005 1015 7.5 631.38 254.40 52.43 Ϫ33 Ϫ52 2 30 Sep 2005 1339 7 412.43 113.49 2.82 Ϫ25 Ϫ53 3 4 Oct 2005 1015 20.6 701.12 328.32 151.76 Ϫ29 Ϫ53 4 5 Nov 2005 0545 6 340.59 199.44 74.75 Ϫ22 Ϫ59 5 24 Nov 2005 0710 33 593.65 355.73 194.45 Ϫ27 Ϫ57 6 5 Dec 2005 1009 26.48 507.02 278.40 87.94 Ϫ29 Ϫ57 7 12 Jan 2006 2010 1 265.33 111.58 7.47 Ϫ32 Ϫ59 8 16 Jan 2006 1010 13.4 452.61 225.66 41.06 Ϫ31 Ϫ63 9 25 Apr 2006 1309 7.5 377.70 150.27 28.43 Ϫ30 Ϫ57 10 14 Aug 2006 1310 9.5 773.86 198.30 2.45 Ϫ34 Ϫ49 Mean 1123 13.19 506 222 64 Ϫ29 Ϫ56 DSP Time after MSP 1 11 Sep 2005 0945 23.5 444.72 51.49 0.00 Ϫ30 Ϫ49 2 1 Oct 2005 0209 12.5 191.78 53.98 0.69 Ϫ21 Ϫ53 3 5 Oct 2005 1539 29.4 457.82 88.66 0.35 Ϫ24 Ϫ50 4 5 Nov 2005 1009 4.4 265.87 65.60 2.27 Ϫ22 Ϫ57 5 24 Nov 2005 2345 16.58 244.86 58.88 11.22 Ϫ22 Ϫ50 6 5 Dec 2005 2345 13.6 338.62 206.82 103.65 Ϫ23 Ϫ54 7 13 Jan 2006 0709 10.9 129.57 56.53 7.17 Ϫ27 Ϫ64 8 16 Jan 2006 2345 10.6 236.94 102.21 17.42 Ϫ22 Ϫ64 9 25 Apr 2006 1909 6 229.34 53.73 0.96 Ϫ28 Ϫ57 10 16 Aug 2006 1410 49 327.62 57.38 0.00 Ϫ26 Ϫ53 Mean 1456 17.6 286.72 79.53 14.37 Ϫ24 Ϫ55 greatly, but is most frequent near 0000 UTC1 (Fig. 4). ganized over the next 3–29 h, with individual smaller The convective region grows upscale and becomes or- convective elements generally moving with the cloud- layer-mean wind to the east or southeast. Some of the events are a succession of individual MCSs that remain 1 Local time is UTC Ϫ3h. stationary or propagate upstream and that in combina-

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FIG. 4. Number of occurrences of FSP, ISP, MSP, and DSP within 6-h time windows throughout the diurnal cycle for the 10 events studied. Times are in UTC. tion produce an even larger upstream propagation of the serial MCS events show only a single maximum the entire region of convection (Fig. 2), while other near 2100 LT. The time of maximum cloud shield ex- events are mergers of individual MCSs into a larger tent is again similar, occurring at 0600 LT for MCCs system that propagates upstream (Fig. 3). The time of and near 0900 LT for serial MCS events. In comparison the maximum cloud shield extent is most often at 1200 with MCC life cycles over the United States (Maddox UTC, an average of 13 h after the first MCS reached its et al. 1986), the life cycles of these South American initiation size criterion, and may consist of cloud shields events are very different. A maximum in the frequency from several individual MCSs that have merged. The of first storms occurs at 1500 LT for MCCs over the time from initiation to maximum extent varies from 1 to United States, near the time of the local minimum in 33 h, again highlighting the different evolutions seen in first storms over South America. Maximum cloud these upstream-propagating systems. In contrast to the shield extent occurs near local midnight over the smaller convective elements that move with the mean United States, compared to 0600–1200 LT over South cloud-layer wind, the larger MCSs and MCCs tend to America. The differences in the local times of the four remain more stationary during their initiation and ma- life cycle stages suggests that the processes that lead to ture stages and the development of new convection oc- convective system initiation and influence system evo- curs to the west-northwest of the existing systems (Fig. lution may be somewhat different in South and North 3). After another 18 h, on average, the cloud shield America. associated with the MCS(s) begins to dissipate. In many A conceptual model is developed from the most com- respects, these events resemble the coherent precipita- mon type of the serial MCS events in which successive tion episodes analyzed by Carbone et al. (2002), al- MCSs develop toward the northwest at roughly 5-h in- though they propagate in a very different direction. Co- tervals (Starostin and Anabor 2004; Fig. 5 in the current herent precipitation episodes over the United States paper). As shown in the next section, the mean cloud- often begin over the higher elevations of the lee side of layer during the 10 serial MCS events are from the Rocky Mountains and propagate downstream to the northwest, yet successive MCSs in these 10 cases the east and over lower elevations, whereas the South develop to the north or northwest over time. This up- American serial MCS events begin near the coast and stream development of MCSs is reminiscent of the propagate upstream to the northwest and over higher southward burst squall lines over the United States dis- elevations. cussed by Porter et al. (1955) and Stensrud and Fritsch The serial MCS event life cycle (Fig. 4) shares a num- (1993). Southward burst systems propagate upstream ber of similarities with the MCC life cycle over midlati- with respect to the flow at all levels of the atmosphere, tude South America documented by Velasco and and Stensrud and Fritsch (1993) conclude that this oc- Fritsch (1987). Both have a double peak in the time of curs via a combination of cold pool propagation and first storms, with one peak near local noon and the internal gravity waves that move ahead of the convec- other in the evening. A minimum in first storms occurs tive line and initiate new convective development near 1800 local time (LT). Velasco and Fritsch (1987) While the datasets available for these 10 cases are not also show a double peak in the time of MCC initiation, sufficient to analyze the physical mechanisms for the with local maximums near 1800 and 0200 LT, whereas upstream propagation, the overall similarities to the up-

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FIG. 5. Typical characteristics of serial MCS events over South America showing (a) a histogram of the relative position of the new MCS (MCS-02) to the preceding MCS (MCS-01), (b) histogram of time interval between successive MCS development (h), and (c) conceptual model of serial MCS development from Anabor (2004). New MCSs develop in succession toward the northwest of the preceding MCS. As the MCSs mature, they tend to merge into a larger convective cluster. From Anabor (2004). stream-propagating MCS events over North America characteristics and a geographical location similar to as documented in Stensrud and Fritsch (1993) are strik- the northwestern Argentinean low, which is frequently ing. observed in the summer and highly intermittent in the winter (Lichtenstein 1980; Seluchi et al. 2003). Strong meridional pressure gradients on both sides of the sur- 4. Large-scale environmental conditions face low are captured. The strong gradient to the west a. First storm position of the low may be an artifact of the calculation to re- duce surface pressure to sea level pressure over the 2 The composite analysis centered at the FSP shows a Andes Mountains. The strong gradient to the east is meridionally elongated low pressure system in the lee due to the close proximity of a high pressure system of Andes Mountains near 30°S (Fig. 6a). This low has over the South Atlantic just off the Brazilian coast. This high also is seen at 850 hPa and is important to the development of the low-level flow that supports advec- 2 ϫ Only the center 33° 33° portion of the composite grid is tion into the developing convective region. shown in the figures to more easily identify the key features near the MCS location. Even this smaller domain would stretch over The flow associated with the surface low generates a North America from the eastern slopes of the Rocky Mountains broad area of convergence centered to the west of the to the east coast and from Texas to the Hudson Bay. FSP, with warm temperature and moisture advection

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FIG. 6. NCEP–NCAR composite reanalysis fields averaged over the 10 serial MCS events and centered on the MCS centroids (black triangle) at the time of the first storms, showing (a) surface pressure every 1 hPa (solid lines), 1000-hPa wind barbs, and 1000-hPa divergence (10Ϫ5 sϪ1, shaded); (b) 1000-hPa temperature (°C, dashed lines) and specific (g kgϪ1, shaded); (c) 850-hPa geopotential height (m, solid lines), temperature (°C, dashed lines), wind barbs, and specific humidity (g kgϪ1, shaded); (d) 700-hPa geopotential height (m, solid lines), temperature (°C, dashed lines), wind barbs, and specific humidity (g kgϪ1, shaded); (e) 500-hPa geopotential heights (m, solid lines), temperature (°C, dashed lines), wind barbs, and vorticity (10–5sϪ1, shaded); and (f) 250-hPa geopotential heights (m, thick solid lines), 500-hPa vertical velocity omega (Pa sϪ1, light lines, with negative values dashed), wind barbs, and wind speed (m sϪ1, shaded). A full wind barb is 10 m sϪ1.

Unauthenticated | Downloaded 09/26/21 04:53 PM UTC 3096 MONTHLY WEATHER REVIEW VOLUME 136 into the region. While Velasco and Fritsch (1987) show surface mixing ratios between 11 and 15 g kgϪ1 for MCC events over South America, the present analysis indicates mixing ratios at the lower end of this range for these serial MCS events. However, these lower surface mixing ratio values are typical for MCC events in the United States (Maddox 1983; Velasco and Fritsch 1987). Two regions of higher moisture values are ob- served to the north:one near the Brazilian coast and the other adjacent to the Andes Mountains. The surface winds suggest advection of moisture toward the FSP from both of these regions is occurring. A trough is present at 850 hPa just to the east of the Andes Mountains (Fig. 6c) with a high pressure region just off the coast. Strong warm advection and moisture advection are present ahead of the trough and into the region of FSP with mixing ratios above8gkgϪ1 over the FSP. The low-level jet, which is often present in the environments of MCCs over the United States (Mad- dox 1983; Augustine and Howard 1991; Anderson and Arritt 2001) and of long-lived MCSs over South America (Nicolini et al. 2002; Saulo et al. 2004; Salio et al. 2007) is not evident, likely owing to the coarseness of FIG. 7. Lifted-index (°C) values analyzed from NCEP–NCAR the reanalysis data (Verneker et al. 2003; Salio et al. reanalysis data at time of first storms. Black triangle designates 2007). However, the gradient in the geopotential height MCS centroid. Calculated as mean of all 10 cases. suggests a strengthening low-level flow over the FSP region. Even without a clear low-level jet signal, 850- hPa warm advection reaches 3.4°C (12 h)Ϫ1, roughly 1979; Uccellini and Kocin 1987; Salio et al. 2007). The the same value seen in MCC environments (Maddox importance of the upper-level jet is suggested by the 1983). Salio et al. (2007) find that subtropical MCSs are values of omega (vertical velocity; dP/dt in isobaric co- much more common in environments with low-level ordinates) at 500 hPa, where a broad area of rising Ϫ Ϫ1 jets than in environments without them. motion approaching 0.1 Pa s is present over and to A 700-hPa short-wave trough is located to the west of the west of the FSP region. the FSP and is negatively tilted with the northern edge Not surprisingly, the FSP occurs near a local mini- mum in the lifted index (Fig. 7). While the lifted-index farther east than the southern edge. With the Andes values are mainly positive, indicating that surface par- Mountains extending above 4000 m, this trough likely is cels lifted to 500 hPa are colder than the environment at influenced by the higher terrain to the west. The atmo- this level, the values are small (between 0 and 1) sug- sphere remains relatively moist, with mixing ratios Ϫ gesting that only small modifications of the environ- above5gkg 1 ahead of the trough. The winds veer ment are needed before deep convection can occur. from 850 to 700 hPa, with the winds becoming more The composite sounding from the FSP yields a slightly zonal with height. Warm advection is present over the different perspective, with parcels lifted from 925 hPa FSP at this level, implying that a deep layer of warm producing a lifted index of Ϫ4°C and convective avail- advection from the surface to 700 hPa is associated with Ϫ able potential energy (CAPE) of 946 J kg 1 (Fig. 8). In these serial MCS events. The trough to the west of the addition, the atmosphere below 500 hPa is fairly moist FSP is still clearly seen at 500 hPa in association with at all levels. Several studies indicate that cloud-layer cyclonic vorticity advection, while to the north of the moisture has a positive influence on convective devel- FSP the wind barbs show a closed anticyclonic circula- opment (Ferrier et al. 1995; Tompkins 2001). tion. The analysis at 250 hPa shows the FSP region is po- b. Initiation system position sitioned on the anticyclonic side of the entrance region to the upper-level jet (Fig. 6f), a region considered fa- At the time the first MCS develops (Fig. 9), the sur- vorable for storm development (Uccellini and Johnson face low is over 2 hPa deeper than at the time of the

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mean motion of the cells comprising the convective sys- tem, which is highly correlated with the mean 850–300- hPa wind. The propagation component is defined as the rate and location of new cell development relative to the existing cells, which is directly proportional but op- posite to the speed and direction of the low-level jet. Applying this technique to the reanalysis data yields a predicted MCS motion of 8 m sϪ1 downstream toward 24°. The failure of this technique to predict an upstream propagation to the convective region is likely due to the underestimation of the low-level jet in the reanalysis data. The results of Corfidi et al. (1996), however, clearly emphasize the importance of the low-level jet to convective system propagation. The short-wave trough at 700 hPa remains just to the west of the ISP, although it clearly has moved eastward since the first storms began and is now positively tilted (Fig. 9d). A closed geopotential height isoline now sur- rounds the high to the north over eastern Brazil. Warm advection continues over the ISP region. The midlevels remain moist with mixing ratios above6gkgϪ1 in a small region over the ISP. The flow pattern at 500 hPa has changed only slightly since the time of the first storms, although there is a clear signal of increased moisture at and downwind of the region of deep con- FIG. 8. Skew T–logp diagram of environment at the MCS geo- metric center at time of the first storms. Thicker gray and black vection. Above the ISP, upward motion is still present lines show parcels lifted from the surface and 925 hPa, respec- in association with the entrance region of the jet streak. tively. Surface lifted-index values (Fig. 10) have become negative over the ISP region, in comparison to the small but positive values found at the time of the first storms. first storms, is quasi-circular, and has moved slightly to A relative minimum of lifted-index values stretches the north. The ISP region is within the zone of the northward from the ISP. A composite sounding (Fig. strongest gradients in sea level pressure. The region of 11) again provides additional information, with parcels high pressure over the Atlantic Ocean has not moved from 925 hPa yielding a lifted index of Ϫ5°C and a or changed in intensity. Warm advection and moisture CAPE value of 1262 J kgϪ1. The atmospheric column advection are still present, with surface mixing ratios from the surface to 500 hPa is moister than at the times Ϫ now above 11 g kg 1 in the ISP region. Both the first of the first storms. The wind barbs on the sounding storms and the initial MCS develop in the warm sector highlight that the winds throughout the cloud layer ahead of any frontal boundary or wind shift line. have a westerly component, further emphasizing the At 850 hPa, the trough is deeper and has sharpened, upstream propagation of the convective region seen and the gradients in geopotential height have strength- from satellite. ened over the ISP region. The ISP is within the region of largest warm advection, with values now exceeding Ϫ c. Maximum system position 4.3°C (12 h) 1, and is associated with mixing ratios above 10 g kgϪ1, an increase of 25% since the time of The time of the maximum system extent is on aver- the first storms. The low-level jet remains absent in the age 13 h after the initial system develops (ϳ0800 LT), wind field, as expected, although the geopotential and the surface low has moved eastward and is less height gradient suggests a zone of higher wind speeds intense than at the time of the initial MCS development feeding into the convective region from the northwest. (Fig. 12). The sea level pressure gradient is weaker, A technique to predict the propagation of MCCs is although a broad area of convergence is still present developed in Corfidi et al. (1996) and Corfidi (2003) as around and to the southeast of the low. The high pres- a simple vector addition of advection and propagation sure system in the Atlantic has finally shifted farther components. The advection component is simply the out to sea. Surface mixing ratios are up to 12 g kgϪ1 at

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FIG. 9. As in Fig. 6, but for the time of MCS initiation.

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FIG. 10. As in Fig. 7, but for the time of MCS initiation.

FIG. 11. As in Fig. 8, but for the time of MCS initiation. the MSP and strong warm advection persists. The mix- ing ratio field indicates that the moister air from the and may be due in part to the development of an upper- northwest, along the eastern slopes of the Andes level divergent circulation produced by the persistent Mountains is being advected into the MCS region. mesoscale convection (Maddox et al. 1981). The satel- The 850-hPa trough is weaker than previously ob- lite data used in this study indicate that the total mean served, yet shows persistent northwesterly winds of up Ϫ MCS Ϫ38°C cloud shield at this time has an area equal to 15 m s 1 flowing into the MSP bringing warm, moist to 505 568 km2, while for a threshold temperature of air southeastward. Warm advection has decreased to 2 Ϫ Ϫ45°C this total area decreases to 221 560 km (Table 2.8°C (12 h) 1, and is weaker than observed at the ear- 2). These areas are larger than some of the countries in lier times. A zone of cold advection in the southwestern this region of South America. portion of the domain is expanding northward behind Lifted-index values at the time of the MSP remain the trough, although the region of convective activity in below zero surrounding and to the north of the convec- the mean still occurs in the warm sector ahead of the tive region, but are not as negative as observed at the trough. The short wave at 700 hPa is becoming progres- time of the initial MCS development (Fig. 13). The sively more positively tilted, with the northern end of composite sounding (Fig. 14) again indicates that a the wave remaining just to the west of the MSP. The deep moisture layer lies over the convective region, but short wave is clearly identified in the vorticity field at the instability for the most unstable parcel from 925 500 hPa, directly south of the MSP region over the hPa has been reduced. The lifted-index value is only southern portion of the domain and is oriented nearly Ϫ4°C and the CAPE has been reduced to 980 J kgϪ1. south–north at this level. Continued high values of mix- Thus, the environment is starting to become less favor- ing ratio are seen downstream of the convective region. able for deep convection. Winds within the cloud layer The 500-hPa omega field is a minimum just south of the continue to have a westerly component. MSP and has a distinct circular shape with minimum values less than Ϫ3PasϪ1. This feature may be a re- d. Dissipation system position sponse to the development of persistent deep convec- tion as seen in Maddox (1983) and the values of omega The dissipation stage typically begins 18 h after the are similar to those found associated with MCCs. The MSP. The surface low has disappeared and has been flow at 250 hPa is divergent downstream of the MSP, replaced by a broad surface trough, with weak winds

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FIG. 12. As in Fig. 6, but for the time of MCS maximum.

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FIG. 13. As in Fig. 7, but for the time of MCS maximum.

FIG. 14. As in Fig. 8, but for the time of MCS maximum. and a more diffuse convergence field (Fig. 15). The high pressure system over the Atlantic Ocean has moved the DSP appears to be weakening. The weaker winds even farther to the east. To the west, another high pres- further suggest that less moisture is being brought into sure system is beginning to cross the Andes in the southwestern portion of the domain, with cold advec- the convective region. tion across much of southern South America. Equator- The winds at 700 hPa have shifted to more westerly ward incursions of midlatitude air east of the Andes near the DSP, with little warm advection apparent. The enhances convection as a passive response to the in- low-level wind shear observed at earlier times has dis- tense low-level convergence, provided that the thermo- appeared, with the 850- and 700-hPa wind directions dynamic conditions are conducive to its development nearly equal. At the upper levels, the short wave has (Garreaud and Wallace 1998). Transient cold anticy- moved farther eastward and is strongly positively tilted. clones over the subtropical plains are a key element of The winds at 250 hPa remain divergent to the east of the equatorward migration of the atmospheric systems the convective region, with the jet streak moving out (Gan and Rao 1991; Siqueira et al. 2005), leading to the over the Atlantic. The 500-hPa omega values are mini- equatorward movement of convection at this time in mized just to the south of the DSP, but are not as large most of the 10 cases. While warm advection and mois- in magnitude as those at the time of maximum system ture advection are still present at the DSP, the wind extent. The 250-hPa trough along the southern edge of field is much weaker and leads to smaller values of Fig. 16f is deeper than expected based upon continuity advection. The moist axis extending northwestward with earlier times and may be an artifact of the analysis from the convective region is also more diffuse and approach. weaker than seen at earlier times. The lifted-index values remain negative over the The 850-hPa trough is broader than at earlier times, DSP region, but are located farther north than at earlier although still located to the west of the convective re- times (Fig. 16) as the convection also has moved north- gion. Warm advection has ceased at this level, with the ward. The composite sounding (Fig. 17) indicates that winds blowing nearly parallel to the isotherms in the while the lower-troposphere remains moist, instability DSP region. While the moist axis stretching northwest- has continued to decrease with a lifted-index value of ward remains, the pool of moisture to the northwest of Ϫ2°C and a CAPE value of only 518 J kgϪ1.

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FIG. 15. As in Fig. 6, but for the time of MCS dissipation.

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FIG. 16. As in Fig. 7, but for the time of MCS dissipation.

FIG. 17. As in Fig. 8, but for the time of MCS dissipation. 5. Discussion

Serial MCS events over southeastern South America region southward. One feature that appears to particu- are seen routinely in infrared satellite imagery during larly influence the low-level flow pattern at early times the spring and summer. These events begin over the is a strong surface anticyclone located just off the coast southern La Plata river basin, with individual systems of Brazil. At upper-levels, the MCSs develop on the moving to the east or southeast, yet the region of con- anticyclonic side of the entrance region to an upper- vective development as a whole shifts to the north or level jet, a result also seen in South American MCS northwest. This upstream propagation of the convec- environments by Salio et al. (2007). Mean soundings tive region resembles the southward burst convective show that the atmosphere is moist from the surface to events described by Porter et al. (1955) and studied by near 500 hPa, with CAPE values above 1200 J kgϪ1 at Stensrud and Fritsch (1993) over the United States. In the time of system initiation. The convective region dis- general, the convective region propagates upstream in a sipates as the environmental conditions become less fa- direction nearly opposed to the low-level jet, as succes- vorable, with decreasing amounts of warm advection sive MCSs develop to the northwest, such that this re- and upper-level rising motion, and reduced values of gion moves into the low-level flow of warm, moist air. CAPE. System dissipation and continued upstream However, for the South American events, the initial propagation to the north and northwest occurs in tan- region of convective development occurs in the warm dem with a surface high pressure system that crosses sector of a cyclone near the Atlantic coast and not the Andes Mountains and may explain some of the along a frontal boundary as typically seen in the United northwestward movement of the MCSs as a wind shift States. moves northward along the eastern slopes of the Andes Analyses of the composite mean environments from (Garreaud and Wallace 1998; Siqueira et al. 2005). 10 of these serial MCS events using NCEP–NCAR re- These serial MCS events observed over South analysis data events indicates that the synoptic condi- America represent an interesting subset of all MCS and tions resemble those found in MCC environments over MCC events in the region. While the lack of in situ the United States (Maddox 1983). The MCSs form observations, particularly radar and rawinsondes, ham- within an environment of strong low-level warm advec- pers a more detailed analysis of the mechanisms in- tion and strong moisture advection from the Amazon volved in this upstream propagation, the long-lived na-

Unauthenticated | Downloaded 09/26/21 04:53 PM UTC 3104 MONTHLY WEATHER REVIEW VOLUME 136 ture of these events, with convective region lifetimes tions of convective storms in different large-scale environ- varying from 19 to 69 h, suggests that they are impor- ments and comparisons with other bulk parameterizations. J. tant contributors to the local weather over extended Atmos. Sci., 52, 1001–1033. Fritsch, J. M., R. J. Kane, and C. R. Chelius, 1986: The contribu- periods. The prediction of these events would be help- tion of mesoscale convective weather systems to the warm- ful in providing guidance to the citizens of several season precipitation in the United States. J. Climate Appl. South American countries. An assessment of the ability Meteor., 25, 1333–1345. of numerical models to reproduce these events is Gan, M. A., and R. B. Rao, 1991: Surface cyclogenesis over South needed. America. Mon. Wea. Rev., 119, 1293–1302. Garreaud, R. D., and J. M. Wallace, 1998: Summertime incursion of midlatitude air into subtropical and tropical South Acknowledgments. We gratefully acknowledge the America. Mon. Wea. Rev., 126, 2713–2733. support given by the ITS group at NSSL. Dr. Ernani de Heideman, K. F., and J. M. Fritsch, 1988: Forcing mechanisms and Lima Nascimento (SIMEPAR/LEMMAUFPR) is other characteristics of significant summertime precipitation. thanked for initiating the contact with NSSL for the Wea. Forecasting, 3, 115–130. internship, and the assistance of John F. Mejia (NOAA/ Jirak, I. L., W. R. Cotton, and R. L. McAnelly, 2003: Satellite and radar survey of mesoscale convective system development. NSSL/CIMMS) is gratefully acknowledged. The Mon. Wea. Rev., 131, 2428–2449. NCEP–NCAR reanalysis data were provided by the Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Re- NOAA/OAR/ESRL/PSD, Boulder, Colorado, from analysis Project. Bull. Amer. Meteor. Soc., 77, 437–470. their Web site at http://www.cdc.noaa.gov/, and their Laing, A. G., and J. M. Fritsch, 1997: The global population of help is appreciated. Robert Hart, Assistant Professor, mesoscale convective complexes. Quart. J. Roy. Meteor. Soc., 123, 389–405. Department of , The Florida State Univer- Lichtenstein, E. R., 1980: La depresion del noroeste Argentino sity, is thanked for developing the Skew T–logp func- (The northwestern Argentina low). Ph.D. dissertation, Uni- tion for GrADS. The first author was supported by the versity of Buenos Aires, 223 pp. Conselho Nacional de Desenvolvimento Científico e Machado, L. A. T., and W. B. Rossow, 1993: Structural charac- Tecnológico (CNPQ; National Council of Scientific and teristics and radiative properties of tropical cloud clusters. Mon. Wea. Rev., 121, 3234–3260. Technological Development), Brazil, Dept. de Física. ——, M. Desbois, and J. P. Duvel, 1992: Structural characteristics Finally, we thank Drs. Anatoli Starostin and Rit Car- of deep convective systems over tropical Africa and the At- bone, and an anonymous reviewer for their comments lantic Ocean. Mon. Wea. Rev., 120, 392–406. and suggestions, which greatly improved this manu- ——, W. B. Rossow, R. L. Guedes, and A. W. Walker, 1998: Life script. cycle variations of mesoscale convective systems over the Americas. Mon. Wea. Rev., 126, 1630–1654. Maddox, R. A., 1980: Mesoscale convective complexes. Bull. REFERENCES Amer. Meteor. Soc., 61, 1374–1387. ——, 1983: Large-scale meteorological conditions associated with ␣ Anabor, V., 2004: Descriptive analyses of meso- convective sys- midlatitude, mesoscale convective complexes. Mon. Wea. tems by GOES-8 satellite images. M.S. thesis, Departament Rev., 111, 126–140. of Remote Sensing, Universidade Federal do Rio Grande do ——, D. J. Perkey, and J. M. Fritsch, 1981: Evolution of upper Sul, 78 pp. tropospheric features during the development of a meoscale Anderson, C. J., and R. W. 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CORRIGENDUM

Because of a processing error, a figure was mistakenly published in low resolution in ‘‘Serial Upstream-Propagating Mesoscale Convective System Events over Southeastern South America,’’ by V. Anabor et al., which was published in Monthly Weather Review, Vol. 136, No. 8, 3087–3105. On p. 3095, Fig. 6 should have been published in higher resolution, as shown below. The staff of Monthly Weather Review regrets any inconvenience this error may have caused.

DOI: 10.1175/2008MWR3014.1

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FIG. 6. NCEP–NCAR composite reanalysis fields averaged over the 10 serial MCS events and centered on the MCS centroids (black triangle) at the time of the first storms, showing (a) surface pressure every 1 hPa (solid lines), 1000-hPa wind barbs, and 1000-hPa divergence (1025 s21, shaded); (b) 1000-hPa temperature (8C, dashed lines) and specific humidity (g kg21, shaded); (c) 850-hPa geo- potential height (m, solid lines), temperature (8C, dashed lines), wind barbs, and specific humidity (g kg21, shaded); (d) 700-hPa geo- potential height (m, solid lines), temperature (8C, dashed lines), wind barbs, and specific humidity (g kg21, shaded); (e) 500-hPa geopotential heights (m, solid lines), temperature (8C, dashed lines), wind barbs, and vorticity (10–5 s21, shaded); and (f) 250-hPa geopotential heights (m, thick solid lines), 500-hPa vertical velocity omega (Pa s21, light lines, with negative values dashed), wind barbs, and wind speed (m s21, shaded). A full wind barb is 10 m s21.

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