SEMINAR SERIES and COLLOQUIA

Winter precipitation in all forms can have serious consequences for the areas they affect. Study of precipitation in different phases is important to understanding the physical processes that occur in storms, as well as improving their representation in numerical weather prediction models. The ice phase is important not only in winter events, but also for warm season precipitation in which clouds rise above the freezing level. The University of Oklahoma operates a 2D Video Disdrometer (2DVD) at the Kessler Farm Field Laboratory approximately 30 kilometers southwest of the polarimetric weather radar (KOUN) operated by the National Severe Storms Laboratory (NSSL). The 2DVD measures the size, shape, orientation, and velocity of particles that pass through its measurement volume. This data is used in conjunction with measurements of Z, ZDR, and ρhv from KOUN radar, which can also reveal information on the predominant hydrometeor species within the resolution volume. The 2DVD and KOUN radar were used to observe six winter precipitation events during the 2006-2007 winter. These events contained periods of rain, snow, and mixed-phase precipitation. Five-minute particle size distributions were generated from the disdrometer data and fitted to the gamma distribution; polarimetric radar variables were then calculated for comparison to KOUN data. Two simple melting models were developed to simulate observed snow distributions falling through a melting layer. It is found that the snow PSDs generally undergo a similar transition, regardless of the method of calculation. The number of large particles decreases when transformed to a melted DSD, shortening the distribution's tail, resulting in distributions similar to those from periods of rain. Finally, radar microphysics retrievals are performed, and the resulting distributions compare reasonably well to observed disdrometer data.

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