Mesoscale moisture analysis of the North American monsoon

The regional circulations that contribute moisture to the large precipitati on over northwestern Mexico, the core region of the North American monsoon, are investigated using three summer seasons (July-September 1995-97) of Et a Model mesoscale analyses and forecasts. Analyses are produced by the Era Model`s own four-dimensional data assimilation system that includes a diver se mix of observations. Comparison of the forecast precipitation with satel lite estimates and previous observational studies shows similarity in locat ion. shape, and scale of the patterns over northwestern Mexico. the magnitu de of the precipitation over the slopes of the Sierra Madre Occidental is a lso similar to that from climatologies based on rain gauge observations. Ex amination of the morning and evening forecast precipitation also reveals ag reement with equivalent estimates from high-resolution satellites. Excessiv e model forecast precipitation is found over the Isthmus of Tehuantepec in eastern Mexico. which seems related, at least in part. to deficiencies in t he convective parameterization scheme. Special attention is given to the diurnal cycle that is needed to resolve t he interactions between circulation and precipitation. The Gulf of Californ ia exhibits evaporation through the entire diurnal cycle. In contrast, mois ture flux divergence has a marked diurnal cycle with the largest magnitude over the gulf during the afternoon: this divergence is associated with the afternoon sea and valley breezes that favor a net transport of moisture tow ard the western slopes of the Sierra Madre Occidental. At the same time, la rge convergence of moisture flux develops over the slopes of the Sierra Mad re Occidental. and is followed by intense afternoon-evening precipitation. The reverse circulation during nighttime and early morning results in moist ure flux convergence near the coastline and over water, where early morning precipitation develops. Large divergence of moisture flux is found over the northern sector of the Gulf of California at ail times, and it results almost equally from transie nts and the time mean flow. The time mean flow is characterized by a nightt ime and predawn low-level jet whose intensity is weaker than the Great Plai ns counterpart, but still appears to transport a significant amount of mois ture into the southwestern United States. Northward transport of moisture i s also accomplished by the transient fluxes that include, but are not limit ed to, the episodic northward moist surges frequently discussed in the lite rature.