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Mark Smalley

Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison

Precipitation Aggregation and the Local Environment

Room 811 AOSS, March 28, 2016, 3:30 PM


The details of large-scale spatial structures of precipitation have only recently become apparent with the advent of high-resolution near-global observations from space-borne radars. As such, the relationships between these structures and the local environment and global climate are just beginning to emerge in the scientific community. Precipitation aggregates on a wide variety of scales, from individual boundary layer instabilities to extra-tropical cyclones. Separate aggregation states have been associated with widely varying precipitation rates and atmospheric states, motivating the inclusion of spatial information in hydrologic and climate models. This work adds to the body of knowledge surrounding large-scale precipitation aggregation and its driving factors by describing and demonstrating a new method of defining the spatial characteristics of precipitation events. The analysis relies on the high sensitivity and high resolution of the CloudSat Cloud Profiling Radar for the identification of precipitation with near-global coverage. The method is based on the dependence of the probability of precipitation on search area. Variations in this relationship are caused by variations in the principal characteristics of event spatial patterns: the relative spacing between events, the number density of events, and the overall fraction of precipitating scenes at high resolution. The relationship is modeled by a stretched exponential containing two coefficients, which are shown to depict seasonal general circulation patterns as well as local weather. NASA’s Modern-Era Retrospective analysis for Research and Applications is then used to place those spatial characteristics in the context of the local and large-scale environment. At regional scale, precipitation event density during the Amazon wet season is shown to be dependent on zonal wind speed. On a global scale, the relative spacing of shallow oceanic precipitation depends on the combination of 700 mbar wind speed and relative humidity, while the number density of events depends on the combination of lower tropospheric stability and sea level pressure. The methods and results of this work have applications in diverse topics, such as downscaling and improving the representation of complicated precipitation processes in general circulation models, increasing the accuracy of soil moisture and stream flow models, and improving meso- to synoptic-scale moist circulation theory.

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