Dominant balances and exchanges of the atmospheric water cycle in the reanalysis 2 at diurnal, annual, and intraseasonal time scales

Abstract

Output from the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) Reanalysis 2 (R2) is passed through a broadband filter to determine the normalized covariances that describe the variance of the atmospheric water cycle at diurnal, annual, and intraseasonal (∼7-80 days) time scales. Vapor flux convergence is residually defined to close the water cycle between successive 3-hourly output times from 2002 to 2004, resulting in a balance between precipitation, evaporation, precipitable water tendency, and vertically integrated vapor flux convergence. The same balance holds at each time scale, allowing 100% of each variable's temporal variance to be described by its covariance with other water cycle components in the same variance category. Global maps of these normalized covariances are presented to demonstrate the unique balances and exchanges that govern temporal variations in the water cycle. The diurnal water cycle is found to be dominated by a land-sea contrast, with continents controlled thermodynamically through evaporation and the oceans following dynamic convergence. The annual time-scale features significant meridional structure, with the low latitudes described mostly through variability in convergence and the extratropics governed by the properties of advected continental and maritime air masses. Intraseasonal transients lack direct solar oscillations at the top of the atmosphere and are characterized by propagating dynamic systems that act to adjust the precipitable water content of unsaturated regions or exchange directly with precipitation in saturated areas. By substituting the modeled precipitation with observation-based fields, a detailed description of the water cycle's exchanges relating to the nocturnal precipitation maximum over the Midwest is obtained. © 2008 American Meteorological Society.