THE MOISTURE BUDGET OF THE CENTRAL UNITED-STATES IN SPRING AS EVALUATED IN THE NCEP NCAR AND THE NASA/DAO REANALYSES/

The moisture budget of the central United States during May is examine d using multiyear (1985-89) assimilated datasets recently produced by NASA/DAO and NCEP/NCAR. Intercomparisons and comparisons with station observations are used to evaluate the limitations of the assimilated p roducts for studies of the atmospheric component of the U.S. hydrologi c cycle. Attempts are made to reconcile differences in terms of dispar ities in the analysis systems. Both reanalyses overestimate daily mean precipitation rates by a factor of almost 2 over the southeastern Uni ted States. This is associated with much larger than observed afternoo n convective rain and a substantial overestimate of the number of days with precipitation. Both products capture the transition to the much drier conditions over the western United States, though the NCEP/NCAR product extends moderate rain rates too far to the northwest. Over the Great Plains, the reanalyses capture observed synoptic-scale precipit ation events quite well, but the variability of the daily mean precipi tation is underestimated; this is particularly true for the NASA/ DAO analysis, which has difficulty capturing the extreme rain rates. The N CEP/NCAR product shows generally higher correlation's with the observe d precipitation, though the fluctuations in the two assimilation produ cts are more similar to each other than they are to the observations. The moisture transport in the reanalyses compares favorably to gridded rawinsonde data though there are some significant regional difference s particularly along the Gulf Coast. Examination of the overall moistu re budget for the central United States shows that the observations ac t as a significant local source of moisture, reflecting model bias in the first-guess fields. In both products the analysis increments act t o remove water over much of the northern and western part of the count ry, apparently counteracting excessive evaporation in those regions, e specially in the NASA/DAO. Perhaps most disturbing are the substantial differences between the two reanalyses in the moisture divergence fie lds since these are the most strongly constrained by the observations. Both reanalyses capture the basic temporal and structural characteris tics of the Great Plains low-level jet (LLJ) documented in previous ob servational studies. Composites of the nocturnal fluxes of moisture du ring LLJ events reveal a horizontally confined region of strong southe rly transport to the east of the Rocky Mountains that is sandwiched be tween well-defined synoptic-scale cyclonic (anticyclonic) circulations to the northwest (southeast). Low-level inflow from the Gulf of Mexic o increases by more than 50% over nocturnal mean values in both reanal yses, though the excess inflow is more than 30% stronger in the NCEP/N CAR reanalysis. While both analyses underestimate the nocturnal maximu m in precipitation over the Great Plains, the pattern of precipitation anomalies associated with LLJ events compares favorably to observatio ns.