LARGE-SCALE ATMOSPHERIC MOISTURE CYCLING AS EVALUATED FROM NMC GLOBALANALYSIS AND FORECAST PRODUCTS

The broadscale aspects of the annual cycle of monthly mean global wate r vapor flux, flux divergence, evaporation, and precipitation derived from the National Meteorological Center (NMC) global analysis and fore cast products were examined with two objectives in mind: 1) a critical evaluation of the usefulness of recent NMC products for descriptive a nd diagnostic studies of the global hydrological cycle and in the proc ess and 2) to provide additional information on the behavior of the an nual cycle of selected hydrological parameters over the globe in gener al and over the United States in particular. The 2-year period August 1991-July 1993, was chosen for study. The global-scale characteristics of the NMC vertically integrated vapor flux fields are described in t erms of the rotational and divergent components of the stationary and transient parts of the vapor flux field. Values of the zonally average d meridional vapor flux derived from the NMC analyses are broadly simi lar to those presented by Peixoto and Oort, but differences are substa ntial in the Southern Hemisphere. The magnitude of the NMC meridional flux is generally larger in the middle latitudes, partly because of a larger transient flux. The seasonally varying spinup characteristics o f the NMC evaporation (E) and precipitation (P) forecasts are examined in terms of differences between the 0-6 h and 12-36 h forecasts. Spin up in P is much larger than spinup in E and is most pronounced (spindo wn) in the latitudes of the equatorial convergence zones. Comparisons of annual precipitation derived from the NMC 0-6 h forecast with value s from the Legates and Willmott and the Jaeger climatologies, and with values from the GPI satellite IR algorithm for the Tropics, are summa rized and discussed. Questionable zonally averaged and regional featur es are identified, but differences are often hard to reconcile due to uncertainties in all the methods of estimation. The value of the NMC v apor flux analyses for studies of continental-scale hydrology is exami ned by comparing annual P-E derived from the vapor flux divergence fie ld with annual surface runoff derived from mean annual river discharge . The comparison reveals an unrealistic flux divergence field over the continent that appears to be terrain related. A similar bias is not a pparent in the forecast P-E field, suggesting that it arises from prob lems of model resolution and data assimilation.