Observations of snow particle size distribution with 2-D video disdrometer and polarimetric radar
Sean Luchs, University of Oklahoma, Norman, OK; and G. Zhang, A. V. Ryzhkov, M. Xue, L. Ryzhkova, and Q. Cao
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. In situ measurement is crucial in understanding the microphysical of processes of precipitation. However, airborne observations are expensive and infrequent. An alternative and more viable method would be to combine ground-based obaservations with radar measurements whenever possible. The University of Oklahoma operates a 2D Video Disdrometer (2DVD) at the Kessler Farm Field Laboratory approximately 28.5 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 hv 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. The disdrometer data was processed in one-minute particle size distributions (PSDs) or combined to determine the PSD for a longer period. A total of 7752 one-minute PSDs were collected from these six events. From these PSDs, five-minute particle size distributions are generated and fitted to the gamma distribution, and polarimetric radar variables are then calculated. A melting model is used to calculate a corresponding raindrop size distribution (DSD) for the snow PSDs in two different approaches. One approach assumes that mass is conserved as the frozen particles melt into liquid drops. The other method takes particle velocity into account and assumes that the mass flux is conserved. In addition, the melted DSD is determined in two ways. One way involves applying the melting model to the measured data, and then applying a new gamma distribution fit. The other derives a formula to transform the gamma distribution fitted to the measured data into a new melted gamma distribution. This results in four distributions – two assuming mass conservation and two assuming mass flux conservation.
The snow PSDs generally undergo a similar transition, regardless of the method of calculation. Snow PSDs frequently appear as very similar to exponential distributions, particularly when the number of particles is large. The number of large particles decreases when transformed to a melted DSD, shortening the distribution's tail. When the formula is used to transform a gamma distribution fitted to a snow PSD, the number of small drops stays very similar. However, when the measured snow data is transformed into a rain DSD, which then has a new gamma distribution fitted to it, the number of small drops usually decrease, resulting in distributions similar to those fitted to data from periods of rain.
Extended Abstract (196K)
Poster Session 2, Observing Systems
Wednesday, 14 January 2009, 2:30 PM-4:00 PM, Hall 5
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