Tuesday, 11 January 2005: 9:15 AM
Atmospheric moisture cycling over the southwestern US
Abstract. In this talk we use a suite of surface and upper-air based observations, reanalysis products, and regional model simulations to examine the large-scale summertime hydrologic cycle over the southwestern United States. At the climatological scale, it is found that 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, making the southwestern US a seasonal source of moisture to the atmosphere. 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 (<800mb). Based upon the balances found in this region, we develop a new “recycling” metric of precipitation (which is defined as the ratio of locally-derived precipitation to total precipitation) to better quantify the contributions of these various budget terms to the climatological rainfall. While a traditional method for estimating the recycling rate gives a ratio of about 0.25, the new metric indicates a recycling rate of 0.80. This suggests that about 75-85% of the area-averaged precipitation is the result of evaporative processes, indicating a much greater importance of locally-derived rainfall in generating climatological precipitation than previously thought. As part of this talk we will discuss how the newly-developed regional moisture-cycling metric differs from traditional metrics and how it may provide greater insight into the regional feedback mechanisms that affect the response of seasonal rainfall variability to local and external forcing factors.