HYDROLOGIC PROCESSES ASSOCIATED WITH CYCLONE SYSTEMS OVER THE UNITED-STATES

A substantial amount of precipitation in the midlatitudes occurs in as sociation with extratropical cyclones. Using the data generated by ver sion 1 of the Goddard Earth Observing System (GEOS-1) Data Assimilatio n System for 1985-89, hydrologic processes and the water vapor budget over the United States were analyzed to illustrate the maintenance of precipitable water and precipitation associated with extratropical cyc lones. The area-mean divergence of water vapor flux covering the Great Plains and the eastern region of the United States (80 degrees-105 de grees W, 30 degrees-50 degrees N) was adopted as a hydrologic index. T he cyclones over this region that have values of this index smaller th an minus one standard deviation over a season were selected for analys is. On average, 15 cases were selected for each season. The composite results show a developing baroclinic wave coupled with a low-level cyc lone in which the low-level convergent (divergent) center and the uppe r-level divergent (convergent) center ahead of the trough (ridge) are Linked by an upward (downward) branch of the divergent circulation, co nsistent with the classic cyclone model. Thus, water vapor converges ( diverges) through the low-level divergent circulation of the cyclone w ave to maintain precipitation (evaporation) centers ahead of the troug h (ridge). It is estimated that the amount of water vapor accumulating in the Great Plains and the eastern United States throughout winter ( November-March) could be converged by typical cyclones within a month. During summer (May-September), it would take only about half a month for typical cyclones to converge water vapor toward this region suffic ient to account for the summer season runoff by streamflow over this r egion.