Convective-resolving regional climate simulations for the Amazon Basin: Comparison to TRMM precipitation data

With increasing computational power, regional climate simulations are now becoming possible on convective-resolving grids, thus eliminating the need for parameterized convection. In the present study, a series of seasonal calculations using the Weather Research and Forecasting model are computed at 4-km grid spacing, which is adequate to resolve at least the broad structures of most precipitating convective systems. Simulations are computed for both the DJF and MAM seasons as averaged over 2005-2008, with a simulation domain covering the majority of the Amazon Basin and the adjacent South American coastline. Precipitation statistics are computed and compared to satellite rainfall retrieval data from the 13-year Tropical Rainfall Measuring Mission (TRMM 3B42 and 2A25). A set of companion simulations with 12-km grid spacing are computed for comparison, using the Betts-Miller convective parameterization. As compared to the 12-km runs, the convective-resolving simulations show significant improvements in the overall mean rain rate (-15% error versus TRMM, compared to 80% error at 12-km), as well as the rain rate probability distributions, the diurnal evolution and timing of precipitation, and the overall seasonal dependence. Compared to the TRMM retrievals, the 4-km case rains somewhat too infrequently (4.5% of events, versus 7.5% in the retrieval data) but is much more likely to produce rain at high rain rates, with the result being an average rain rate similar to that of the data. Both the high- and low-resolution cases capture the progating precipitation features near the Atlantic coastline, as well as the basin-wide diurnal oscillation in the continental interior. However, the 4-km case shows significantly better timing and evolution statistics, with peak rainfall occurring 2 to 3 hours too early at 4-km, versus 5 to 6 hours too early at 12-km. Compared to TRMM, the 4-km case produces too little rain activity at peak rainfall, but maintains the resulting stratus layer for too long the following day, thus likely reducing the shortwave forcing. Overall, the present calculations show significant promise for computing regional rainfall patterns on convective-resolving grids.