Understanding and characterizing rain microphysics are important for accurate quantitative precipitation estimation (QPE) and quantitative precipitation forecasts (QPF). Recent advancements in disdrometer and radar observation techniques make such study more interesting and fruitful. A two-dimensional video disdrometer (2DVD) measures size, shape, orientation and falling speed of each precipitating particle, yielding detailed information about rain microphysics. A UHF wind profiler reveals the reflectivity-weighted motion of hydrometeors at various heights. A polarimetric radar provides a large spatial coverage of measurements of reflectivity, differential reflectivity, specific differential phase and cross-correlation coefficient that depend on cloud/precipitation physics. The 2DVDs, profiler and polarimetric radars are complementary ideal to study rain microphysics and storm evolutions.

In collaboration with NOAA's National Severe Storms Laboratory (NSSL), National Center for Atmospheric Research (NCAR), and NSF Collaborative Adaptive Sensing of the Atmosphere (CASA) program, the University of Oklahoma (OU) organized a field experiment in central Oklahoma during the spring storm season of 2007. NSSL ran a S-band polarimetric weather radar (KOUN), a prototype for the polarization upgrade of the national WSR-88D network. CASA was operating its Integrative Project One (IP1) testbed in southwest Oklahoma that is composed of four X-band polarimetric radars. Figure 1 shows disdrometer and radar sites. A disdrometer from NCAR was deployed near Lake Thunderbird, Norman, Oklahoma, since April 3rd, on the land of Edward Jessup (35o13'50.68”N, 97o16'34.52”W), a site under a dual-Doppler lobe formed by the KOUN polarimetric radar and the Oklahoma City WSR-88D radar (KTLX). OU's disdrometer was deployed at Cement, Oklahoma, on the land of Tommy Harris (34o56'19.50”N, 98o4'31.08”W) from April 24 to July 9, 2007, together with a UHF boundary layer wind profiler. The site is roughly at the center of the triangle formed by three CASA IP1 radars (KSAO, KCYR and KRSP) located at Chickasha, Cyril, and Rush Springs, respectively, hence has triple Doppler radar coverage. The two disdrometer sites are at short range (20 km) and medium range (76 km) from the polarimetric KOUN radar, allowing disdrometer-radar comparisons for different resolution volume.

The two 2DVDs recorded more than fifteen thousand minutes of raindrop size distribution (DSD) data. KOUN radar collected data for about 90% of the rain events. The CASA radars also operated in a field experiment mode from April 9 to June 10. A variety of storms occurred over the disdrometer sites. The datasets are being processed to characterize rain microphysics and microphysical processes including evaporation and accretion. Also calculated from rain DSDs are the S- and X-band polarimetric radar variables. The polarimetric KOUN and CASA radar measurements are compared and verified with the DSD calculations and with that from the profiler measurements and retrievals. The potential of polarimetric radars in microphysical parameterization will be demonstrated through rain DSD retrieval. The relationship between precipitation microphysics and storm evolution is being studied using the dual- and triple-Doppler radar data.

Figure 1: Disdrometer sites and polarimetric radar locations for the 2007 field experiment.