17.6 The mesoscale organization and dynamics of extreme convection in subtropical South America

Friday, 9 August 2013: 2:45 PM
Multnomah (DoubleTree by Hilton Portland)
Kristen L. Rasmussen, University of Washington, Seattle, WA; and R. A. Houze Jr. and A. Kumar

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 convection in the world. An investigation of the most intense storms for 11 years of TRMM Precipitation Radar (PR) data shows a tendency for squall lines to initiate and develop in this region with the canonical leading convective line/trailing stratiform structure. The synoptic environment and structures of the extreme convection and 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 a narrow region. Subsequent to initiation, the convection often evolves into propagating mesoscale convective systems similar to those seen over the Great Plains of the U. S. and produces damaging tornadoes, hail, and floods across a wide agricultural region.

Numerical simulations conducted with the NCAR Weather Research and Forecasting (WRF) Model extend the observational analysis and provide an objective dynamical evaluation of storm initiation, development mechanisms, dynamics and microphysics. A capping inversion in the lee of the Andes is important in preventing premature triggering. The South American Low Level Jet impinging on low mountains to the east of the main Andes ranges triggers extremely deep and intense convection. Mesoscale organization into leading line/trailing stratiform system occurs as the storms propagate eastward. The simulated mesoscale systems closely resemble the storm structures seen by the TRMM satellite as well as the overall shape and character of the storms shown in the GOES satellite data. A sensitivity study removing small-scale topographic features that are hypothesized to focus deep convective initiation determines the role of the topography in triggering and geographical focusing the extreme convection. Results of the numerical experiment in which smaller mountain features are removed will be presented at the conference. In addition, insights into the mesoscale processes that keep the South American MCSs tied to the topography following upscale organization and development longer than those in the U.S. will be highlighted and shown.

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