Pa. Dirmeyer et Kl. Brubaker, Contrasting evaporative moisture sources during the drought of 1988 and the flood of 1993, J GEO RES-A, 104(D16), 1999, pp. 19383-19397
Using hourly observed precipitation data, National Centers for Environmenta
l Prediction 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 S
tates during the drought year of 1988 and the flood year of 1993. These res
ults are compared to calculations using a bulk-transport approach and month
ly mean data. We find that about 41% of precipitation over the Mississippi
River basin originated as evaporation from the same basin during April-July
1988 and 33 % during 1993. During the July peak of the 1993 flood the recy
cling ratio was considerably lower than in other months, while the source o
f moisture from the western Gulf of Mexico and Caribbean Sea increased enor
mously. By contrast, at the June climax of the 1988 drought, the recycling
ratio reaches a maximum. We also find a small source region in the eastern
subtropical Pacific Ocean during both years, which decreases significantly
with the onset of the Mexican monsoon around the beginning of July. The bac
k-trajectory approach represents an improvement over the bulk recycling app
roach by rejecting the assumption of uniform distributions of rainfall in s
pace and time, instead focusing specifically on the sources and transport o
f moisture contributing to observed rainfall events.