A Study of the 24-26 March 2015 Rainfall Event over the Atacama Desert (Chile) using WRF Simulations

On 26 March 2015, catastrophic floods occurred over the Northern Chilean regions of Atacama, Antofagasta and Coquimbo, all located in the Atacama Desert, one of the driest regions on Earth. Preliminary investigations using NCEP/NCAR reanalysis data suggest the weakening of the high pressure system (HPS) that usually presents over Northern Chile during austral summer is responsible, allowing a cut-off low (COL) pressure system over the subtropical southeast Pacific to move unusually further north-east, toward the Atacama Desert. The presence of Andes on the East and a ridge of HPS developing south-southwest of the area caused the COL to trap in the proximity of the central Chilean coastline. This, combined with northwesterly winds advecting moist air generated by a prior rapid warming of the eastern tropical Pacific, led to the record-breaking rainfall over the desert. In 3 days the precipitation over Copiapo and other areas was equivalent to the amount over several years. To further study the dynamical and thermo-dynamical features of this unique event, high resolution (3km grid) numerical simulations with Weather Research and Forecasting (WRF) model were conducted. A possible blocking influence of the Andes on the COL's lifecycle was investigated. WRF model capabilities in properly representing air flow and precipitation patterns over this challenging topography (the mountain height rises from sea level to more than 6km within 230km) were investigated. Different physical options, including several microphysics and cumulus parameterization schemes, are tested to find the optimal model set-up that might be used as the backbone of a future flood warning system. In this regard, surface distribution of precipitation was analyzed in detail for the case of Río Salado watershed, an almost unmonitored basin in high altitude complex terrain and one of the most severely damaged areas by the flood. Research funded by ND-PUC Seed Fund, and Cigiden, Conicyt/Fondap Grant 15110017