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Warm-season hydroclimate variability over North America: Eugene Rasmusson's seminal contributions
Sumant Nigam, University of Maryland, College Park, MD
Eugene Rasmusson joined the University of Maryland during the warm-phase of the seasonal cycle in 1986; full two seasons before I got there. Eugene's 'Es' – enormous intellect, engaging personality, and enthusiasm for and encouragement of good science – has influenced many Maryland faculty and students in his 20+ years there; including myself whose cold-season interests were gradually supplanted by warm-season ones, largely due to Gene's corridor conversations.
Gene was an interdisciplinary scientist long before it became fashionable or necessary to be one. His pioneering analysis of tropical ocean-atmosphere and extratropical land-atmosphere interactions, with some efforts initiated almost half a century ago, are a testament to his scholarship and vision. Gene's doctoral thesis at MIT, under Victor Starr's guidance, was at the intersections of meteorology, atmospheric general circulation, and hydrology; and forged disciplinary connections that endure. Gene's interests at Maryland came full circle back to his MIT-period callings: hydroclimate. This reaching back was not unlike the full Carnot circuit in thermodynamics, in that interesting work was done in the interim; leading to fundamental advances in our understanding of North American hydroclimate variability.
The role of moisture transports and evapotranspiration in generating warm-season hydroclimate variability over the Great Plains will be reviewed; especially, how appraisals have changed since the seminal 1967-68 papers of Gene Rasmusson. It will be argued that evapotranspiration has an exaggerated role in current analyses, many of which are model based. It is remarkable that the rather significant role of remote water sources (i.e., moisture transports), indicated by modern retrospective atmospheric analyses (ERA-40 and NCEP), was recognized by Gene using only 5 years (1958-63) of relatively sparse aerological data; a tribute to his diagnostic acumen. The importance of transported moisture in water-balance variations over the Great Plains places premium on understanding the low-level circulation links with the adjoining gulfs and oceans; some of which will be discussed. It will shown that notable Great Plains precipitation anomalies arise from the overlap of Pacific and Atlantic basin influences.
Finally, the tendency of many state-of-the-art climate models to vigorously recycle warm-season precipitation over Great Plains will be noted. It will be argued that this not only distorts the regional water-balance but also adversely impacts the land-surface energy budget; compromising regional hydroclimate variability/change predictions. A hypothesis for the recycling propensity of models will be discussed.
Recorded presentationSession 2, Oral Session Part II
Thursday, 18 January 2007, 1:30 PM-5:00 PM, 217C
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