Four years of regional analyses based on the Eta Data Assimilation System (EDAS) are used to document the diurnal cycle and mesoscale features of the components of the atmospheric water cycle in two regions of interest for the Pan-American Climate Studies (PACS) Program. They are the Mexican Monsoon region of the Gulf of California and the United States Great Plains region. EDAS analyses are based on the Eta model that is currently used at the National Centers for Environmental Prediction (NCEP) for operational short term forecasts. Analyses are available at 3-hour intervals with a horizontal resolution of about 48 km and 38 vertical levels (since February 1998, 32 km and 45 levels). The need for regional analyses stems from the fact that global analyses, while helpful in providing a qualitative description of the atmospheric water cycle, do not have the fine spatial/temporal resolution needed for quantitative estimates.
It is found that despite the short period, the results seem stable enough to represent basic aspects of the mesoscale circulations. The nighttime precipitation in the Great Plains can be related to a development of moisture flux convergence associated with an increase of intensity of the low-level jet. Interestingly, the Great Plains nighttime convergence of moisture flux is offset with divergence during daytime and, as a result, overall moisture flux divergence is observed during summer. Also consistent with earlier results, the Eta model forecast precipitation in the Mexican Monsoon region depicts a late afternoon precipitation maximum over land and a minimum over the ocean. The pattern reverses in the early morning, with more precipitation over the ocean and no precipitation over land. The associated moisture flux convergence also reveals a diurnal cycle that agrees with that of precipitation: During the afternoon, the Gulf of California shows a marked divergence, with convergence along the western slopes of Mexico; in the early morning, divergence is observed over the slopes of the Sierra Madre Occidental. The ability of the Eta model to resolve the above mesoscale features of the low-level circulations is a promising aspect for future attempts at estimating the moisture sources for each region.