Precipitation Processes in Convection and over Mountains

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Under prior NSF support, the Principal Investigator has advanced scientific understanding of precipitation processes in mid-latitude, Great Plains, mesoscale convective systems. In addition to continuing this research, the Principal Investigator will focus on precipitation processes in orographic convective storms. Such storms are known for their potential to produce extreme flooding events, but due to complexities of the atmospheric dynamics and the difficult environment for field work, much is still unknown about these systems. Polarimetric radar data from already completed field projects will be analyzed to identify microphysical processes occurring in the buoyant, entraining, expand-ing, bubbles ("particle fountains") that constitute the building blocks of most convective systems. Ongoing studies have developed a con-ceptual model that helps explain the convective precipitation process and serves as a basis for testing numerical forecast models.In addition to advancing and refining the convec-tive precipitation system conceptual model, this project will focus on precipitation processes in orographic set-tings. Field observations in research meteorology have tended to emphasize precipitating cloud systems in non-orographic situations, to simplify the observations by eliminating the difficulties of carrying out radar and air-craft studies in complex terrain and to simplify the anal-ysis by eliminating the complex orographic modifications of the airflow, precipitation growth, and fallout. This study addresses the orographic problem by using all of the above-mentioned advances in radar meteorology and by taking advantage of the sophisticated theory and modeling of dry flow over terrain, which has developed over the last several years. One regime to be studied is coastal northern California. Within the mountain ridges are important river valleys known for their flooding. Using both operational and special data sets, the Principal Investigator will construct composites of the reflectivity and radial velocity data which will lead to identification, on a climatologi-cal basis, of the orographic modification of the dynamics and microphysics of frontal rainstorms moving onshore from the Pacific.The studies of orographic precipitation processes will be furthered by the field phase of the Mesoscale Alpine Programme (MAP) in northern Italy and sur-rounding countries in the Fall of 1999. This region is a natural laboratory for studying orographic precipitation and the Principal Investigator will benefit from the data to be collected by other researchers. Successful completion of this research should lead to a better understanding and forecasting of very heavy, localized precipitation in mountains.