Some Mechanisms Leading to the Formation of an Argentinian Dryline

RUIZ, JUAN; SALIO, PAOLA VERÓNICA; BECHIS, HERNÁN

San Francisco

Conferencia; American Geophysical Union Fall Meeting; 2016

American Geophysical Union

Drylines are boundaries that separate air masses with different moisture content. Although they are known to form in different parts of the world, the most studied, by far, are those who develop in the Great Plains of the United States during the warm season. Drylines are also frequently observed during the austral summer over the eastern slope of the Andes, in central Argentina. These drylines are sometimes associated to convection initiation in the afternoon and evening. Despite the importance of this phenomena for the regional weather and climate, the mechanisms associated with these systems, as well as their importance in convection initiation have not been previously studied. Large-scale tomicro-scale formation mechanisms have been documented over the Great Plains, but geographical features of the region are strongly different, presenting new challenges in order to understand the generation processes.In this work an objective and multiparameter dryline identification algorithm is developed and applied to the detection of drylines over central Argentina during two summer seasons. The synoptic and mesoscale environment leading to the formation of a typical dryline observed in this region during January 2016 is also analyzed. The terms of the frontogenesis function, applied to the specific humidity field, are evaluated for this particular case. We found that the shear-deformation term is the main contributor to the dryline genesis, with a smaller contribution of the confluence term. Backward trajectories of air parcels starting at both sides of the dryline are computed, in order to identify the origins of the air masses. The hot, dry air mass south of the dryline appears to have been originated over the Southern Pacific, and experienced subsidence after crossing the Southern Andes elevations.The contrast with a much more humid air mass advancing from subtropical South America sets up the environment for the formation of the dryline.