Large-Scale Atmospheric Moisture Cycling As Evaluated from NMC Global Analysis and Forecast Products

Eugene M. Rasmusson Department of Meteorology, University of Maryland at College Park, College Park, Maryland

Search for other papers by Eugene M. Rasmusson in
Current site
Google Scholar
PubMed
Close
and
Kingtse C. Mo Climate Prediction Center, NCEP/NWS/NOAA, Washington, DC

Search for other papers by Kingtse C. Mo in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

The broadscale aspects of the annual cycle of monthly mean global water vapor flux, flux divergence, evaporation, and precipitation derived from the National Meteorological Center (NMC) global analysis and forecast products were examined with two objectives in mind: 1) a critical evaluation of the usefulness of recent NMC products for descriptive and diagnostic studies of the global hydrological cycle and in the process and 2) to provide additional information on the behavior of the annual cycle of selected hydrological parameters over the globe in general 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 terms of the rotational and divergent components of the stationary and transient parts of the vapor flux field. Values of the zonally averaged meridional vapor flux derived from the NMC analyses are broadly similar to those presented by Peixoto and Oort, but differences are substantial 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 of the NMC evaporation (E) and precipitation (P) forecasts are examined in terms of differences between the 0–6 h and 12–36 h forecasts. Spinup in P is much larger than spinup in E and is most pronounced (spindown) in the latitudes of the equatorial convergence zones. Comparisons of annual precipitation derived from the NMC 0–6 h forecast with values from the Legates and Willmott and the Jaeger climatologies, and with values from the GPI satellite IR algorithm for the Tropics, are summarized and discussed. Questionable zonally averaged and regional features are identified, but differences are often hard to reconcile due to uncertainties in all the methods of estimation.

The value of the NMC vapor flux analyses for studies of continental-scale hydrology is examined by comparing annual P-E derived from the vapor flux divergence field with annual surface runoff derived from mean annual river discharge. Ale comparison reveals an unrealistic flux divergence field over the continent that appears to be terrain related. A similar bias is not apparent in the forecast P-E field. suggesting that it arises from problems of model resolution and data assimilation.

Abstract

The broadscale aspects of the annual cycle of monthly mean global water vapor flux, flux divergence, evaporation, and precipitation derived from the National Meteorological Center (NMC) global analysis and forecast products were examined with two objectives in mind: 1) a critical evaluation of the usefulness of recent NMC products for descriptive and diagnostic studies of the global hydrological cycle and in the process and 2) to provide additional information on the behavior of the annual cycle of selected hydrological parameters over the globe in general 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 terms of the rotational and divergent components of the stationary and transient parts of the vapor flux field. Values of the zonally averaged meridional vapor flux derived from the NMC analyses are broadly similar to those presented by Peixoto and Oort, but differences are substantial 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 of the NMC evaporation (E) and precipitation (P) forecasts are examined in terms of differences between the 0–6 h and 12–36 h forecasts. Spinup in P is much larger than spinup in E and is most pronounced (spindown) in the latitudes of the equatorial convergence zones. Comparisons of annual precipitation derived from the NMC 0–6 h forecast with values from the Legates and Willmott and the Jaeger climatologies, and with values from the GPI satellite IR algorithm for the Tropics, are summarized and discussed. Questionable zonally averaged and regional features are identified, but differences are often hard to reconcile due to uncertainties in all the methods of estimation.

The value of the NMC vapor flux analyses for studies of continental-scale hydrology is examined by comparing annual P-E derived from the vapor flux divergence field with annual surface runoff derived from mean annual river discharge. Ale comparison reveals an unrealistic flux divergence field over the continent that appears to be terrain related. A similar bias is not apparent in the forecast P-E field. suggesting that it arises from problems of model resolution and data assimilation.

Save