WRF with Water Vapor Tracers: A Study of Moisture Sources for the North American Monsoon

The regional atmospheric Weather Research and Forecasting (WRF) Model with water vapor tracer diagnostics (WRF-WVT) is used to quantify the water vapor from different oceanic and terrestrial regions that contribute to precipitation during the North American monsoon (NAM) season. The 10-yr (2004-13) June-October simulations with 20-km horizontal resolution were driven by North American Regional Reanalysis data. Results show that lower-level moisture comes predominantly from the Gulf of California and is the most important source of precipitation. Upper-level (above 800 mb) southeasterly moisture originates from the Gulf of Mexico and Sierra Madre Occidental to the east. Moisture from within the NAM region (local recycling) is the second-most important precipitation source, as the local atmospheric moisture is very efficiently converted into precipitation. However, WRF-WVT overestimates precipitation and evapotranspiration in the NAM region, particularly over the mountainous terrain. Direct comparisons with moisture source analysis using the extended dynamic recycling model (DRM) reveal that the simple model fails to correctly backtrack moisture in this region of strong vertical wind shear. Furthermore, the assumption of a well-mixed atmosphere causes the simple model to significantly underestimate local recycling. However, the direct comparison with WRF-WVT can be used to guide future DRM improvements.