Extreme rainfall event in the Northeast coast of Brazil: a numerical sensitivity study

This study investigates an extreme rainfall event which occurred in Northern Brazil (NEB) between 20 and 30th 2017 May causing several deaths and making thousands homeless. Based on a suite of microphysics and planetary boundary layer (PBL) schemes based on the WRF model, it is demonstrated that anomalous weather conditions are characterized by significant upward and eastward wind flow. Omega differences with respect to climatological conditions showed values up to - 0.04 Pa/s and wind up to 6-8 m/s in consonance with higher precipitation in May in the NEB coast. The cumulative rainfall for 11 days was higher than 500 mm in some locations, as measured by weather stations. These conditions were simulated by the WRF model under different physics parameterization schemes. In total 24 experiments with WRF were implemented. Non-local PBL demonstrated better performance than the local PBL. Moreover, the rainfall was concentrated in small portions of the region, and the local scheme limited the WRF conditions to estimate the correct maximum precipitation location. The Morrison scheme performed better compared to the other schemes. Results presented here show that the correct choices of the microphysics and PBL parameterizations are fundamental to obtain good simulation/forecast, especially for extreme rainfall events. This study demonstrates that regional modeling is crucial to provide accurate information to forecasters and decision makers to plan actions which hamper catastrophic situations such as landslides and floods in high-risk regions.