Investigation of Southern Great Plains atmospheric moisture budget for CLASIC

The Cloud and Land Surface Interaction Campaign (CLASIC) was conducted over the Southern Great Plains (SGP) ARM Climate Research Facility (ACRF) during June 2007. One of the primary foci of the CLASIC Science Plan is to understand the interactive roles of horizontal moisture advection and land surface processes for cumulus convection. Our moisture budget analysis for CLASIC, conducted over an expanded region surrounding the SGP during CLASIC and three other contrasting May-June periods, provides a bulk approach for relating cloud properties to larger-scale atmospheric conditions. Moisture budget analysis is an important tool for studying land-atmosphere interactions, since the linkages among atmospheric dynamics, water vapor, surface conditions, and precipitation are constrained by the moisture continuity equation.

Record breaking rainfall during CLASIC led to a uniformly saturated land surface marking an extreme condition over the SGP that heavily damaged the maturing winter wheat crop. In our study, we included three other contrasting May-June periods: 2006 with extreme dryness over the SGP; 1998 with extreme upstream dryness over Texas; and 2002 with a more normal rainfall regime over our entire study area. Despite the large variation in rainfall and land surface cover among all four study periods, our investigation revealed fundamental commonalities among their moisture budget components and related variables. Fig. 1 shows these moisture budget terms composited by rainfall amounts (P). For daily P < 4 mm/day (the P categories of most relevance to CLASIC), there is moist horizontal advection (HA<0) and horizontal divergence in the presence of moisture (HD>0). As P rises above this level, HD decreases dramatically and becomes convergent, indicating stronger synoptic forcing. This increase in the HD contribution to the total moisture convergence is accompanied by a decrease in HA contribution. This paradoxical decrease in moist horizontal advection with increasing P can be explained by the finding that there is a very small phase difference between HA and P. Also included in this Fig. 1 is a recycling estimate (P_E/P) that uses the equation developed in our recent Midwestern moisture budget study (Zangvil et al. 2004) where P_E is the P derived from local evapotranspiration. Our moisture recycling methodology involves the advected and locally evapotranspired origins of P being expressed in terms of an inflow/outflow (“bulk”) formulation that is defined at the boundaries of the study area and not in terms of the more traditional moisture flux divergence (“process”) formulation. The recycling ratios for P<4 mm/day are consistently higher for 2007 when there was copious rainfall and a saturated land surface.

Zangvil, A., D. H. Portis and P. J. Lamb, 2004: Investigation of the large-scale atmospheric moisture field over the Midwestern United States in relation to summer precipitation. Part II: Recycling of local evaporation and association with soil moisture and crop yields. J. Climate, 17, 3283-3301.