Characteristics of a Hailstorm over the Andean La Paz Valley

Abstract. The iconic hailstorm and flash flood episode of 19 February 2002 over La Paz city is numerically investigated in this article. Large scale atmospheric circulation is dynamically downscaled in order to take into account the complex orography forcing and local features. Satellite observations suggests late morning shallow convection over the Altiplano that becomes deep convection in the early afternoon around complex orography. The control simulation captures well the cloud evolution and suggest a two-stage precipitation mechanism. First, early convection occurred around 1200 LST and originated from thermodynamic instability combined with lake breeze and orographic lifting. Rainfall discharge then generated cold pools. During the second stage, cold pools around complex orography were propagated by lake breeze and encountered the La Paz Valley breeze, triggering the deep convection near La Paz city around 1400 LST. We assess the importance of local features through numerical experiments, which include modification of orography, suppression of surface heat fluxes, changes of surface lake temperature and removal of the lake. We show the importance of orographic configuration as triggering mechanism for convection initiation and for mesoscale circulation, the role of lake temperature for frontal breeze and propagation of cold pools, and of surface heat fluxes for atmospheric instability. This study highlights the complex interaction between lakes, surface heating and orography that favour deep convection and hailstorm formation, which is especially relevant around the Titicaca lake region.