The sensitivity of WRF downscaled precipitation in Puerto Rico to cumulus parameterization and interior grid nudging

The sensitivity of the Weather Research and Forecasting Model (WRF) simulated precipitation
over Puerto Rico is evaluated using multiple combinations of cumulus parameterization (CP)
schemes and interior grid nudging. NCEP-DOE AMIP-II reanalysis (R-2) is downscaled to 2-
km horizontal grid spacing with both convective permitting simulations (CP active only in the 49 middle and outer domains) and CP schemes active in all domains. The results generally show
lower simulated precipitation amounts compared to the observations, regardless of WRF
configuration. However, activating the CP schemes in the inner domain improves the annual cycle, intensity, and placement of rainfall compared to the convective permitting simulations.
Furthermore, the use of interior grid nudging techniques in the outer domains improves the
placement and intensity of rainfall in the inner domain. Incorporating a CP scheme at convective
permitting scales (< 4 km) and grid nudging at non-convective permitting scales (> 4 km)
improves the island average correlation of precipitation by 0.05 to 0.2 and reduces the island
average RMSE by up to 40 mm on average over relying on the explicit microphysics at
convective permitting scales with grid nudging. Projected changes in summer precipitation between 2040-2042 and 1985-1987 using WRF to downscale CCSM4 ranges from a 2.6 mm
average increase to 81.9 mm average decrease, depending on the choice of CP scheme. The differences are only associated with differences between WRF configurations, which indicates
the importance of CP scheme for projected precipitation change as well as historical accuracy.