Analyses of moisture sources and sinks in the atmosphere have historically used mean monthly data on rainfall and circulation, and bulk formulations of moisture recycling. However, transients in moisture advection are extremely important in determining the actual transport. Using hourly observed precipitation data, NCEP reanalyses at 6-hourly intervals, and a quasi-isentropic back trajectory algorithm, we have examined the transport and surface sources of moisture supplying rainfall during the spring and summer over the United States during several noteworthy years, including the drought year of 1988 and the flood year of 1993.
We find that on average, about 1/3 of precipitation over the Mississippi-Missouri river basin originates as evaporation from the same basin. This is compared to higher values derived from bulk recycling calculations using the same data. In the northern part of the basin, terrestrial sources dominate oceanic sources. In the south, the opposite is true. During the peak of the 1993 flood, the recycling ratio dropped to about 1/4. At this same time, the source of moisture from the western Gulf of Mexico and Caribbean Sea increased by a factor of five over climatological values. We also find a small source region in the eastern subtropical Pacific ocean, which decreases significantly with the onset of the Mexican monsoon around the beginning of July