The changing character of precipitation

Kevin E. Trenberth, Aiguo Dai, Roy M. Rasmussen and David Parsons

National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307

voice: (303) 497 1318
fax: (303) 497 1333

24 October, 2002
Bull. Am. Met. Soc.


From a climate perspective, precipitation intensity, duration and frequency are as much of concern as amounts, as these factors determine the disposition of rainfall once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence have enormous impact on the environment and society. Hence advancing understanding and ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital. Hence the intensity of precipitation depends on available moisture, especially for heavy events. As climate warms, the amount of moisture in the atmosphere, which is governed by the Clausius-Clapeyron equation, is expected to rise much faster than the total precipitation amount, which is governed by the surface heat budget through evaporation. This implies that the main changes to be experienced are in the character of precipitation: increases in intensity must be offset by decreases in duration or frequency of events. The timing, duration and intensity of precipitation can be systematically explored via the diuranl cycle, whose correct simulation in models remains an unsolved challenge. Typical problems include the premature initiation of convection, and events that are too light and too frequent. These challenges in observations, modeling and understanding precipitation changes are being taken up in the NCAR "Water-Cycle Across Scales" initiative, which will exploit the diurnal cycle as a test bed for a hierarchy of models to promote improvements in models.
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