Comparative study of atmospheric water vapor budget associated with precipitation in Central US and eastern Mediterranean

摘要

Water vapor budget (WVB) analysis is a powerful tool for studying processesleading to precipitation (P), since the linkages among atmospheric dynamics,water vapor fields, surface conditions, and P are constrained by themoisture continuity equation. This paper compares WVB calculations over theUS Midwest (MW), the US Southern Great Plains (SGP), and the easternMediterranean Sea (EM) during their seasons of maximum P. Despite theinter-regional differences in time of year, size of region, and surfacecharacteristics, the WVBs over these regions have common features. First,the change in precipitable water (dPW) is highly correlated with themoisture flux divergence (MFD) and not evaporation (E), implying thatatmospheric humidity is affected more by the large-scale atmosphericcirculation than land-atmosphere interactions. Second, P is positivelycorrelated with moisture inflow (IF/A). However, a pronounced differenceexists between the North American and the Mediterranean study regions withrespect to the processes associated with increased P. For the MW and theSGP, increased P is associated with moisture flux convergence (−MFD) due toincreased IF/A. In contrast, increased P over the EM is not associated with−MFD, since both the outflow (OF/A) and IF/A increase at similar rates.Recycling ratio (R) estimates were calculated for each region using anequation previously developed. The moisture recycling methodology involvesthe externally advected versus locally evaporated contributions to P beingexpressed in terms of a "bulk" formulation in which IF/A and OF/A aredefined at the boundaries of the study area. Due to its scale dependence, Rcannot be directly compared among the different regions, and a normalizationprocedure was developed for this comparative study. Its results suggest thenormalized R ranges between 12-25% for the study regions, with the valuefor the oceanic EM being somewhat larger than over the continental MW andSGP.