Interactions between the Convective Lifecycle and Terrain 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.

To test hypotheses developed using satellite observations, numerical simulations conducted with the NCAR Weather Research and Forecasting (WRF) Model provide insight into the nature of the orographic control on deep convection initiation in subtropical South America. The capping inversion in the lee of the Andes is important in preventing premature initiation in the simulations, allowing for stronger convection once the inhibition is overcome. The impingement of the South American Low Level Jet (SALLJ) on the foothills and low mountains to the east of the main Andes range triggers deep and intense convection, in addition to supporting back-building storms that persist throughout the nocturnal hours. Sensitivity studies removing and/or reducing various topographic features have shown the profound influence of the terrain on the initiation and upscale growth of the subsequent MCSs in this region. A conceptual model for convective storm environments leading to convection initiation that was developed for subtropical South America will be presented.