Simulation of Extreme Rainfall Events in Uruguay: Role of Initialization and Large Scale Dynamics

We investigate the role of initial conditions in the simulation of extreme rainfall events in summer (December–January-February) in Uruguay using RegCM4.7. Although extreme events are defined for the south of the country, considering the dimensions of these kind of events, the validation is performed in a larger region. Firstly, the quality of the observational database is analyzed and filtered. Then, RegCM4.7 is run in two different modes. The first mode is weather like (WL), where the model is initialized with reanalysis 24 h before the extreme event date. The second one is climate mode (CM), that simulates 5 months (starting 1st October) and their initial conditions are realistic only for the first day. Then, simulated rainfall were compared to the observations with Taylor diagrams, Spearman correlations and accumulated values. The difference in the ability of both modes is associated to the role of initial conditions. Overall, half of the extreme events were simulated with good skill in WL while CM was able to skillfully simulate 28% of the events. This difference is attributed to the better simulation of atmospheric circulation in WL, especially in low levels. However, in some particular cases, relevant atmospheric variables were more realistic in CM and its rainfall simulations did also. Finally, we found that in CM, most of the model's skill comes from the large scale circulation anomalies associated with ENSO teleconnections in subtropical South America.