We examine the three-dimensional structure of the atmospheric water budget components over the central U.S. as part of our efforts in support of GCIP water budget studies. We analyze the time series of monthly atmospheric vapor flux convergence (VFC) derived from twice daily radiosonde data. Both mandatory level and vertically integrated monthly fluxes are examined for over a twenty-year period. This paper focuses on the analysis of the vertical structure of the vapor flux convergence and the relative contributions of the mean and eddy components. We also discuss relative importance of the zonal and meridional vapor flux components to the atmospheric water budget of the central United States.
The 23-year mean annual cycle of the vapor flux convergence shows that seasonal changes in VFC are most significant below 700 hPa. In the long-term mean, VFC due to transient eddies dominates the VFC due to the mean flux. However, our analysis shows that during prolonged dry periods over the Central United States, it is anomalies of the mean VFC which are usually related to the precipitation anomalies. In particular, the analysis shows that the 1988 drought was associated with anomalous weak mean atmospheric moisture inflow from the Gulf of Mexico with relative little change in the eddy component. In contrast, there was no observed significant increase of mean vapor fluxes through the southern U.S. boundary associated with the wet years.