Evaluation of the Atmospheric Water Budget Following an Intense Cold-Air Outbreak over the Gulf of Mexico—Application of a Regional Forecast Model and SSM/I Observations

The atmospheric water budget is examined for a 12-day period following an intense cold-air outbreak over the Gulf of Mexico. Budget terms are compared using analyses from the U.S. National Meteorological Center's operational Nested Grid Model (NGM) and using precipitable water and surface wind speed estimated from the Special Sensor Microwave/Imager (SSM/I) instrument aboard the defense meteorological satellite F8. The atmospheric-storage term, determined from the areal-averaged total precipitable water, does not differ significantly between that obtained from the NGM and that obtained from SSM/I data. The storage increases by a factor of more than 3 during the initial five days following the passage of the surface high over the Gulf. Horizontal flux divergence of water vapor computed from the full vertical structure in the NGM output is well approximated by the substitution of the surface-700-mb mean wind and the total precipitable water for the vertical profiles along the boundaries of the atmospheric volume. Evaporation from the sea surface is determined using GOES surface temperatures and NGM surface air conditions. The impact of satellite-derived surface winds on the areal-average evaporation is determined by replacing NGM wind speeds with those estimated from the SSM/I data. The relative importance of precipitation on the water budget is assessed from model estimates. During the onset of airmass modification, evaporation appears to be the dominant mechanism in producing the observed atmospheric moistening. As evaporation diminishes after one to two days, evaporation and flux convergence are of similar magnitude. Together, these terms underestimate the amount of moistening observed during the first five days.