Impact of model resolution on the prediction of summer precipitation over the United States and Mexico

Summer seasonal simulations for selected years were performed using the NCEP Global Forecast System (GFS) at high (T126L28) and low (T62L28) resolutions, and the NCEP 80-km regional spectral model (RSM) nested in the T62 model outputs (RSM80/T62). All models have 28 levels in the vertical. All experiments were performed with prescribed observed sea surface temperatures to ensure that simulation errors came from model deficiencies. While the T126L28 model does not have a statistically significant advantage in simulating 500-hPa height anomalies over the Pacific North American domain, it yields better monsoon precipitation forecasts and interannual variability. The T62L28 model simulations are too dry over the Southwest and northwestern Mexico when compared to observations and do not properly capture interannual variations of monsoon rainfall. The RSM80/T62 nesting improves the overall rainfall simulations somewhat but is not able to overcome deficiencies of the T62L28 global model to capture interannual variations in monsoon precipitation. Results indicate that a high-resolution version of the global model is needed for seasonal forecasts of monsoon precipitation. Both models capture the low-level jet from the Great Plains (GPLLJ) and rainfall anomalies associated with the 1993 summer floods and the 1988 summer drought, although the simulated rainfall maxima are often weaker and shifted spatially when compared to observations. The impact of horizontal resolution is largely local and is limited to areas over the western region of North America. The T126 model is able to capture the low-level jet from the Gulf of California (GCLLJ), while the T62 model is too coarse to resolve the Gulf of California (GOC). Moisture surges along the GOC are not properly simulated by the T62 model. Overall, the T62 model simulates a very dry Southwest and a weaker monsoon.