Investigation of microphysical processes of rain formation using UHF wind profilers and S-band polarimetric radar

In this study, characteristics of the snow microphysics for a snowfall event that occurred in central Oklahoma on January 27, 2007 are investigated using two UHF wind profilers, a polarimetric WSR-88D radar, Oklahoma Mesonet surface observations, and upper-level soundings. The polarimetric weather radar is used to retrieve microphysical properties of hydrometeors. Complementary measurements from the UHF wind profilers are used to estimate the wind fields aloft. When hydrometeors are present, their fall speeds can be obtained from Doppler spectral data, under the assumption that the vertical component of the wind is zero or negligibly small. This study represents the first time that such an instrumentation set has been used for the study of the microphysics of snow formation.

The two UHF (404 and 915 MHz) wind profilers used are both located at the University of Oklahoma (OU) Kessler Farm Field Laboratory (KFFL). The 404-MHz radar is part of the NOAA Profiler Network while the 915-MHz radar is a boundary layer radar (BLR) operated through OU's Atmospheric Radar Research Center (ARRC). The polarimetric radar data are provided by the NOAA National Severe Storms Laboratory KOUN WSR-88D radar, which is located approximately 30 km north of the KFFL research site. The beam orientation of the ARRC BLR was directed vertically and both spectral and moment data were recorded with time and height resolutions of 20 seconds and 250 m, respectively. The NOAA profiler observed the three-dimensional wind components with time and height resolutions of 6 minutes and 250 m, respectively. Here only moment data are available.

A snowband with a width of approximately 40 km passed over KFFL between 1730Z and 2030Z on the day of the snowfall event. Sounding data from KOUN at 12Z on the 27th and 0Z on the 28th show that the heights of the freezing level are located at approximately 1.9 km and 0.7 km, respectively. During this event, both profilers measured net downward Doppler velocities below a height of about 4 km and the Doppler spectra from the BLR displayed dramatic changes (such as broadening and bi-modality) as the snow particles fell through the freezing level. This was accompanied by a rapid decrease in differential reflectivity Z_DR recorded with KOUN. Concurrent with these observations, the 404-MHz wind profiler indicated northeasterly wind below and westerly-northwesterly wind above a height of 1.8 km. We hypothesize that the observed polarimetric and Doppler signatures are attributed to intense aggregation of snowflakes which was facilitated by dynamically induced turbulence in the layer, of enhanced wind shear. We present data from the three radars in support of this hypothesis. Furthermore, using the Mesonet and sounding data, we show that an observed cooling at the surface might be attributed to evaporation of hydrometeors in the later stages of the snowband as they pass through several dry layers located below a height of 2.5 km.