Using WRF with Water Vapor Tracers to Study the Moisture Sources for the North American Monsoon

Abstract:

The North American Monsoon (NAM) accounts for approximately 70% of the total annual precipitation in northwest Mexico, and 40-50% of annual precipitation in the Southwestern US. Until recently, the broad consensus about the sources for NAM precipitation was that the Gulf of California and eastern tropical Pacific contributed to moisture at lower levels (below 850mb) and the Gulf of Mexico and Caribbean Sea at upper levels. However, using an analytical 2D Lagrangian model (the DRM) we have recently shown that approximately 40% of the moisture that contributes to NAM precipitation is of terrestrial origin. However, the DRM cannot provide information about the vertical structure of moisture as it assumes a well-mixed atmosphere. To alleviate this problem, in this work we use the weather research forecast (WRF) model with the recently added capability of water vapor tracers, to study the moisture sources of NAM precipitation and their vertical structure. We find that the Gulf of California contributes to low-level moisture and the Gulf of Mexico contributes to upper-level moisture, as previously hypothesized. However, we also show the important role of regional moisture recycling from the NAM region at lower levels and upper level moisture from eastern Mexico that has crossed the Sierra Madre Occidental mountain chain. The tracers provide a very detailed picture of the complex moisture transport processes in the NAM region.