School of Meteorology (Defense)

Polarimetric signatures of enhanced warm rain precipitation growth processes

Nick Carr

OU School of Meteorology

24 July 2015, 3:00 PM

National Weather Center, Room 3620
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Mid-latitude precipitation events, in which precipitation is generated primarily below the freezing level via warm-rain processes, have traditionally presented a significant challenge for radar quantitative precipitation estimation (QPE) algorithms. Specifically these precipitation events are characterized by atypical drop size distributions and precipitation vertical structures relative to other types of mid-latitude precipitation events. Radar QPE in these events can be improved if they are correctly identified/classified prior to precipitation estimation. Precipitation classification algorithms can be improved by incorporating polarimetric radar data, which provide more detailed information on precipitation microphysical characteristics than single-polarization radar data. However, prior to developing and implementing a polarimetric warm-rain classification scheme, the typical characteristics of the polarimetric radar variables in mid-latitude warm-rain precipitation events must first be documented and then compared with the polarimetric characteristics associated with non warm-rain events.
Taking advantage of the 2013 NEXRAD dual-polarization upgrade, the three dimensional profiles of the polarimetric radar variables: Z, ZDR, KDP, and ρhv were analyzed for; an intense tropical cyclone, 48 mid-latitude warm-rain events, and 42 non warm-rain precipitation events over the eastern CONUS for the 2014 warm season. The analysis focused on both the values of the polarimetric variables and their vertical variation, and significant results are as follows: Nearly all warm-rain precipitation events were characterized by relatively low median values of Z, ZDR and KDP compared to the non warm-rain cases. Analysis of the vertical profiles of the polarimetric variables revealed that droplet coalescence was the dominant warm microphysical process in the majority of warm-rain and non warm-rain convective events, while in non warm-rain stratiform events, evaporation and breakup appeared to be the dominant warm microphysical processes. Most warm-rain events were also associated with a sharp decrease in reflectivity with height above the freezing level coincident with low echo top heights, and freezing level ZDR and KDP values near 0, indicating limited ice and mixed-phase precipitation growth processes. These polarimetric signatures were particularly magnified in the tropical cyclone case, perhaps indicating a greater relative importance of warm-rain growth processes in tropical cyclones relative to mid-latitude events. The precipitation microphysical characteristics were also visualized using a parameter space of KDP,Z, and ZDR, and the warm-rain events were generally found to lie in a distinct region of this parameter space relative to non warm-rain convective events. These preliminary results support the conclusion that three-dimensional polarimetric radar data can be used to identify and classify precipitation generated primarily via warm-rain growth processes, and these results could have important implications for future precipitation classification and radar QPE algorithms.

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