Flash flood event over central Argentina: A case study.

PAOLA SALIO; YANINA GARCIA SKABAR; MATILDE NICOLINI

Cancún. Mexico.

Congreso; 15th International Conference on Clouds and Precipitation.; 2008

International Commission on Clouds and Precipitation

Flash flood events occur within minutes or hours of excessive rainfall, can destroy buildings and trigger catastrophic situations due to sudden water stream. The central region of Argentina presents a flat terrain with a weak slope toward the Atlantic Ocean. March 26 to April 1, 2007 was a week characterized by the presence of successive convective systems over central Argentina that generated strong rain rates, flooding large areas and producing important damages and lost of lives.     A primary goal of the present work is to describe the synoptic and mesoscale characteristics of the environment associated with a flash flood over the central region of Argentina, with a special emphasis in the relationship between the behaviour of convective precipitation and the evolution of the low level jet. In order to achieve this objective a numerical simulation was performed with the Brazilian Regional Atmospheric Modeling System (BRAMS), nesting up to a 1.25 km-fine resolution grid and an evaluation of the performance of the model over the region and for this particular case is also discussed. The evolution of the successive mesoscale convective systems that affect the area of interest and their impact on precipitation rain rates have been studied using  satellite images every half hour and 4km-resolution. Hourly satellite precipitation estimation (CMOrPH) represents an important data source given the lack of hourly rain rate data over the area. 24 hour accumulated precipitation from the raingauge network is used to validate CMORPH data. A clusterization and tracking technique called ForTraCC is employed to determine the life-cycle of the systems. A threshold in precipitation intensity is used to separately analyze stratiform and convective structures of precipitation in each system. The role of the large scale processes is essential in the development of deep moist convection. This situation is characterized by the presence of a strong trough centered on 75°W, which generates an important advection of cyclonic vorticity over the central part of the country which favors large scale vertical ascent. This system remains stationary over the whole studied period. A cold front associated with these large scale features advances toward north-east from 42°S and crosses the studied area on April 1st.. The thermodynamic environment is characterized by strong CAPE, low CINE and the presence of a deep northerly flow with a low-level jet profile. This unstable environment favored by large scale vertical ascent presents the ideal conditions for the formation of convection.     Convection activity over central Argentina during this event was characterized by the presence of six prefrontal mesoscale convective systems. They generated and decayed over the same region and produced strong rain rates over the flooded area. ForTraCC analysis considering the IR brightness temperature threshold of 235 K shows that all systems tended to generate during the beginning of the night and to decay during daytime. Maximum extension varied from small size to the larger that covered the whole area on  March 29 at 8Z. The extreme estimated rain rate registered over the studied period occurred on March 27 at 8 and again at 4 UTC associated with the evolution of different systems, but the most important rain rate estimations were close to 70 mm h-1 on March 29