The Sensitivity of WRF Downscaled Precipitation in Puerto Rico to Cumulus Parameterization and Interior Grid Nudging

Located in the Northern Caribbean Sea, Puerto Rico is in a region dominated by the easterly trade winds. The trade winds combined with the topography lead to dramatic changes in rainfall over short distances over the island. The coarse projections available for the Caribbean cannot represent the precipitation regime over the island of Puerto Rico. Therefore, there is a need for high resolution projections of climate change for Puerto Rico to assist conservation and adaptation decision making. The purpose of this study is to test the ability of the Weather Research and Forecasting Model (WRF) as a regional climate model to simulate the precipitation within Puerto Rico by downscaling NCEP-DOE Reanalysis to 2-km horizontal resolution. Eight annual simulations were performed using different combinations of cumulus parameterization schemes and interior grid nudging techniques to assess the sensitivity on the island precipitation. The set of simulations includes a comparison of convective permitting simulations to those simulations that rely on the cumulus parameterization schemes. The results show that while the WRF precipitation is lower than the observations in Puerto Rico, activating cumulus parameterization schemes in the inner domain improves the timing, intensity, and placement of rainfall compared to the convective permitting simulations. The results also show that using interior grid nudging techniques in the outer domains improves the placement and intensity of rainfall in the inner domain when simultaneously using a convective parameterization scheme at convective permitting scales. Careful attention is required when testing a regional climate model's ability to simulate rainfall at convective permitting scales, including both activating a cumulus scheme and application of interior grid nudging in the outer domains.