The diurnal cycle of cloud and precipitation associated with the North American Monsoon System (NAMS) has been examined using three-hourly data sets of geostationary IR of Janowiak et al. 2001), the CMORPH satellite precipitation estimates of Joyce et al. (2004), and the Multi-Platform-Merged (MPM) SST analysis of Wang and Xie (2006) for summer 2004. A comprehensive diagnostic study is performed to describe the temporal-spatial structure of the mean state and diurnal cycle of the NAMS cloud/precipitation systems and their relationship to SST over the adjacent oceanic areas. Our results are:

1) Variations of cloudiness and precipitation associated with the North American Monsoon System (NAMS) are dominated by diurnal cycle;

2) Clouds and precipitation start from higher elevation in the morning, move toward the coast as they reach the maximum in late afternoon;

3) The phase of the diurnal cycle is relatively stable, while the magnitude presents changes of synoptic and intraseasonal time scales;

4) Maximum of deep convection and precipitation appears 50-100km west to the mountain crests, and,

5) The intensity of the NAMS convection is positively (negatively) correlated with that of the diurnal amplitude of SSTs over the Gulf of Mexico (the Gulf of California).

Further work is underway to examine how the NAMS diurnal cycle and its relationship to SST are simulated by NCEP operational climate forecast models.