Regional Eta model experiments: SALLJEX and MCS development

Abstract

The rainfall regime over La Plata Basin is strongly influenced by Mesoscale Convective Systems (MCS) that develop during night time, associated with the Low‐Level Jet (LLJ) to the east of the Andes. Since they are mesoscale systems, their predictability depends on the model resolution and its ability to represent the main features associated with their development. This study aimed at finding the best configuration of the regional Eta model applied to the region of frequent MCS development to the east of the Andes and to analyze the performance of the model at high resolution, simulating specific MCS cases using two convection schemes. Three experiments were performed for the South American Low‐Level Jet Experiment (SALLJEX) period. Performance of the nonhydrostatic version was better than the hydrostatic version in cases of MCS development. The use of estimated instead of climatological soil moisture in the model, improved the results for several synoptic variables. Evaluation of the convection parameterization scheme indicated a better simulation using the Kain‐Fritsch (KF) rather than the Betts‐Miller‐Janjic scheme in the cases of MCS development. Therefore the best configuration of the Eta model for the simulation of the MCS occurrences over northern Argentina is the nonhydrostatic version using KF convection scheme and estimated soil moisture. With this configuration, the model was able to simulate the precipitation and the main atmospheric characteristics associated with MCS development, such as the upper level jet, the LLJ, humidity, and associated mechanisms for ascending motion, 72 hours in advance.