Hydrologic Processes Associated with Cyclone Systems over the United States

Abstract

A substantial amount of precipitation in the midlatitudes occurs in association with extratropical cyclones. Using the data generated by version 1 of the Goddard Earth Observing System (GEOS-1) Data Assimilation 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 cyclones. The area-mean divergence of water vapor flux covering the Great Plains and the eastern region of the United States (80°–105°W, 30°–50°N) was adopted as a hydrologic index. The cyclones over this region that have values of this index smaller than minus one standard deviation over a season were selected for analysis. On average, 15 cases were selected for each season. The composite results show a developing baroclinic wave coupled with a low-level cyclone in which the low-level convergent (divergent) center and the upper-level divergent (convergent) center ahead of the trough (ridge) are linked by an upward (downward) branch of the divergent circulation, consistent with the classic cyclone model. Thus, water vapor converges (diverges) through the low-level divergent circulation of the cyclone wave to maintain precipitation (evaporation) centers ahead of the trough (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 sufficient to account for the summer season runoff by streamflow over this region.