Diagnosis and sensitivity study of two severe storm events in the Southeastern Andes

Abstract

Two cases of severe convection have been studied by means of a set of mesoscale numerical simulations using the Pennsylvania State University-National Center for Atmospheric Research model. These events were caused by two convective systems developing during 23–24 December 2004 and 4–5 January 2005 in the province of Mendoza, Argentina, in the eastern Andes. Two S-band radars registered maximum reflectivities of over 60 dBZ during more than 5 h and the hailpads in the study zone registered hailstones with a diameter of up to 3 cm on one of the study days. The synoptic patterns were similar in both cases. The arrival of a trough from the west, crossing the border with Chile, and the presence of the Northwestern Argentinean Low outlined a favorable environment for the development of convection on the lee side of the Andes. The high diurnal temperatures and the topography of the area favored the formation of thermal mesolows on the lee side of the mountain range. Model output fields suggest that a low-level jet stream carrying warm and moist air from the north impinging on the eastern side of the Andes, together with a mid and high-level cold advection associated to the trough, generated a convergence line with areas of convective instability and water vapor flux convergence. In both cases, the mesoscale model forecasts the spatial distribution of the precipitation field relatively accurately but underestimates the total precipitation estimated by radars. The Andes Mountain Range and the solar radiation could have played an important role in triggering convection in the study zone. A factor separation technique is used to explore this issue. For both events the combined effect of topography and solar radiation is decisive to explain the distribution of precipitation in the study area.