Wednesday, 17 January 2007: 4:45 PM
Simulations of Warm Season Diurnal Cycle of Precipitation over the Continental United States and Northern Mexico in General Circulation Models
214B (Henry B. Gonzalez Convention Center)
The diurnal cycle is a fundamental component of the warm season climate of the continental United States and northern Mexico. Distinctive signals are in the large-scale observed circulations such as the low-level jets, mountain-valley winds, and land-ocean circulations, all contribute to a complex diurnal evolution of precipitation that serves to define the warm season climate in that region. Simulated diurnal cycle of precipitation was analyzed in three global climate models from NCEP, GFDL, and NASA/GMAO. The results for each model were based on an ensemble of 5 summer simulations forced with climatological sea surface temperatures. The models exhibit substantial regional deficiencies in the seasonal mean rainfall amount that appear to be related to problems with the diurnal cycle. The models show a general tendency to rain in the early afternoon - several hours earlier than observed. Especially prominent are the discrepancies in the diurnal cycle of precipitation over the eastern slopes of the Rocky Mountains and adjacent Great Plains, including the failure to adequately capture the observed nocturnal peak. Moreover, the observed eastward propagation of convective systems initiated from the mountains and migrated into the Great Plains is not adequately simulated. Large errors in both the phase and amplitude of the diurnal cycle in precipitation remain even at the highest resolution (half-degree) runs. The model biases, in general, tend to be associated with unrealistically strong coupling of the convection to the daytime boundary-layer heating. Improved simulations of the diurnal cycle of precipitation require further improvements in the parameterizations of moist convection process, such as its coupling with the boundary layer and free-atmospheric forcing.