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 Ó 2009 American Meteorological Society Unauthenticated | Downloaded 09/29/21 12:42 PM UTC JULY 2009 A N A B O R E T A L . 2145 (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. Unauthenticated | Downloaded 09/29/21 12:42 PM UTC 2146 MONTHLY WEATHER REVIEW VOLUME 137 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 con- ditions are created using a simple composite approach on the 10 serial upstream-propagating MCS events docu- mented by Anabor et al. (2008). This approach is selected to capture the features that are common to the environ- ments of these serial MCS events, while removing fea- tures that are unique to individual cases.