Union Lecture: Opportunities, Challenges, and Benefits of Spaceborne Precipitation Estimates
Presiding: T Bullen, U.S. Geological Survey; M Steiner, Princeton University
U25A-01 INVITED 17:20h
Opportunities, Challenges and Benefits of Spaceborne Precipitation Estimates
With a successful TRMM mission, multiple new radiometer missions and planned radar missions, it is natural to ask what have we learned, what key issues still face the field of satellite rainfall estimation, and what benefits one may envision in the short as well as longer term? This talk will explore these issues from three evolving subdisciplines: climate and climate change; rainfall products for applications in data sparse regions; and data assimilation aimed at improved forecasting skills. While each subdiscipline has made tremendous progress over the past decade, a central challenge to all is the fundamentally underconstrained nature of spaceborne rainfall retrievals. If insufficient properties of the rainfall can be measured directly, then assumptions must be made about rain system properties that may have large impacts on the solution. This fundamental problem limits the confidence we have in our climate record, the applicability of satellite estimates to regional applications as well as the error models needed for data assimilation. Nonetheless, the availability of active and passive microwave sensors in space, together with ground-based sensors, does allow for progress to be made if the goals are properly defined. If differences among sensors are viewed in terms of the underlying rainfall regimes instead of merely as "errors" in one sensor or the other, then new insights about precipitation physics may be derived from such information. Examples are appearing in the literature. Water vapor concentrations can be used to explain much of the difference in the climate trends between TRMM radar and radiometer. SST appears well correlated with precipitation efficiency of cumulus congestus clouds. Unusually large aerosol concentrations can lead to excessive cloud water that is interpreted as precipitation by radiometers. Properly interpreted, the nature of the observed differences, and their relationship to dynamical and microphysical properties of precipitating clouds appears to hold tremendous promise to turn individual space- and ground-based measurements into a coherent framework that advances climate research as well as applications.