The Summertime Atmospheric Hydrologic Cycle over the Southwestern United States

Abstract

In this paper the authors examine the large-scale summertime hydrologic cycle associated with the northwestern branch of the North American monsoon, centered on the southwestern United States, using a suite of surface-and upper-air-based observations, reanalysis products, and regional model simulations. In general, it is found that on an area-averaged basis, seasonal precipitation is balanced predominantly by evaporation; in addition, this evaporation also supports a net, vertically integrated moisture flux divergence from the region of the same magnitude as the precipitation itself. This vertically integrated large-scale moisture flux divergence is the result of an offsetting balance between convergence of low-level moisture and divergence of moisture aloft (<750 mb). Over the western portion of the domain, most of this low-level moisture convergence is related to advection from the Gulf of California and eastern Pacific; over the eastern portion of the domain, low-level moisture convergence is related to advection from the Gulf of Mexico. The low-level moisture, supplied both by evaporation and advection, is carried aloft primarily by convection (as opposed to large-scale vertical velocities), which then feeds both the precipitation and large-scale divergence fields. The large-scale divergence augments the anticyclonic circulation of moisture aloft, resulting in enhanced exiting fluxes over the Great Plains. A new metric for measuring recycling of moisture in convective semiarid areas is introduced; this metric is designed to better capture the importance of evaporative processes for supporting regional precipitation in these types of environments. Using this metric, it is shown that about 70%?90% of the area-averaged precipitation is the result of evaporative processes, while the remaining 10%?30% is related to low-level convergence of moisture.