STUDIES OF NORTH-AMERICAN CONTINENTAL-SCALE HYDROLOGY USING ETA-MODELFORECAST PRODUCTS

In this study, the forecast products of the National Meteorological Ce nter's Eta model during the period August 1993 to March 1994 are criti cally examined to gain insight into the quality of the first guess fie lds used in the model's four-dimensional data assimilation and the use fulness of the forecast products in diagnostic studies involving unmea sured hydrological variables. We find that the Era model 12-36 hour fo recasts produce patterns of monthly precipitation that are a reasonabl y good approximation of the monthly observed precipitation fields for the period under consideration. The diurnal cycle of the water vapor f luxes and their seasonal changes obtained from the model forecasts are also in good agreement with observations. During the warm months the stationary component of the forecast fluxes includes a good representa tion of the low-level jet (LLJ) that is an important means for the tra nsport of moisture from the Gulf of Mexico into the Great Plains. The LLJ decays during autumn, when it is replaced by the transient compone nt of the fluxes as the means of transport of water vapor from the Gul f of Mexico region into the United States. Finally, evaporation estima tes were derived from the convergence of the fluxes of the forecast fi elds and model and observed values of precipitation. All estimates agr ee within 0.5 mm d(-1) and showed a reasonable seasonal cycle, with ma ximum evaporation during the warm season and a minimum evaporation dur ing February.