Extreme Precipitation Events Associated with Summer Rains in the Western Slope of the Peruvian Andes Using a Numerical Modeling and Weather Radar Data: Case Studies

Abstract

Precipitation forecasting is a challenge in general for any part of the world, but in Lima it is particularly difficult due to its unusual nature and the mechanisms that can generate it. So, it is of great interest to study the mechanisms that generate it when it exceeds historical averages. This work analyzes the synoptic and local circulation conditions that gave rise two precipitation events over the Rimac river basin, in order to characterize the physical processes related to those events. In the first case, the rain affected the city of Lima, while in the second case the precipitation occurred mainly in the upper part of the basin. In the investigation, surface precipitation measurements, weather radar and satellite information, as well as the WRF (Weather Research and Forecasting) model outputs were used. For the analysis of the synoptic-scale general circulation prevailing during both events, data from the Global Forecast System were used (GFS). As a result, the role played by the humid Eastern Amazon flow was confirmed, but in this case, the important role played by the local circulation of sea daytime breezes and its interaction with Amazon flow. Associated with this interaction, the presence of gravity waves and their importance in strengthening cloud systems was observed. At the same time, it was detected that the daytime sea breeze does not change direction during the night, as it generally does, but it stays from the sea towards the land, although somewhat weaker. The weakening of the Eastern flow from the Amazon was observed to be related to the retreat to the east of the ridge of the South Atlantic Anticyclone. Also, the importance of anticyclonic circulation at high levels over the region was confirmed. At the same time, it was found that the WRF model acceptably describes the mechanisms of formation of these events.