Ingredients Supporting the Upscale Growth of Organized Convection in Subtropical South America

Extreme convection tends to form in the vicinity of mountain ranges, and the Andes in subtropical South America help spawn some of the most intense deep convection in the world. Research using the Tropical Rainfall Measuring Mission (TRMM) satellite’s Precipitation Radar provides a unique perspective of the climatological nature and variability of moderate to intense orographic convection in subtropical South America. The synoptic environment and structures of the convection and mesoscale convective systems (MCSs) in subtropical South America are similar to those found in other regions of the world, especially the United States. In subtropical South America, however, the topographical influence on the convective initiation and maintenance of the MCSs is unique. The Andes and other mountainous terrain of Argentina focus deep convective initiation in the foothills of western Argentina. Subsequent to initiation, the convection often evolves into propagating MCSs, similar to those seen over the U.S. Great Plains, sometimes producing damaging tornadoes, hail, and floods across a wide agricultural region.

The ingredients supporting the upscale growth of organized convection were hypothesized by Romatschke and Houze (2010), Rasmussen and Houze (2011, 2016), and Rasmussen et al. (2014) using TRMM satellite observations and reanalysis composites. These ingredients include an enhanced South American Low-Level Jet, lee cyclogenesis, strong synoptic forcing, and a strong capping inversion formed from mechanical flow over the Andes that descends in the lee. Recent observations from the RELAMPAGO field campaign have provided new insights into the importance of the hypothesized ingredients, including their role in supporting upscale growth of convection in Argentina, and will be presented. In addition, radar data from the field campaign and the mesoscale organization of convection will be shown to provide context to the satellite observations. Finally, the relationship of upscale growth to the terrain will be examined and compared to previous terrain modification experiments to provide new insights into some of the most intense convection on Earth.