1 A dual-polarization radar hydrometeor classification algorithm for ...

489490491492493494495496497498499500501502503504505506507508509510511A transition zone from convective rain to snow associated with a vertical bright band(Stewart 1992) propagated eastward through central Oklahoma prior to blizzard conditions on 24December 2009. The vertical bright band was easily detected in a reconstructed CASA radarRHI as an organized region of wet snow extending toward the ground in Fig. 10. The HCAanalysis matches METAR observations across this domain. Rain was reported at the radar sitewhile a transition to freezing rain, sleet, and then brief instances of snow were observed inadvance of the main vertical bright band (10-12 km range), with dry aggregated snowflakesconsistently falling in the cold air at increasing range. This illustrates the ability of the algorithmto differentiate heavy rain from wet snow, both of which produce high Z h and Z dr , based ondifferent expected ranges of ! HV . It also shows the melting layer detection algorithm’s versatilityin identifying a descending bright band by accounting for varying ML heights with range. Rainand sleet were classified as a function of temperature and therefore height using the 12 UTCupstream KOUN sounding. A refreezing signature indicative of sleet as described in Kumjian etal. (2013) was briefly observed ahead of the vertical bright band passage near the 0°C level in adifferent radar scan not shown, but the algorithm did not correctly classify this near-surfaceprocess as previously explained.Once the surface precipitation type changed to snow within the radar domain during thennext hour, particularly high K dp up to 2.6° km -1 was observed within a classic DGZ in Fig. 11.This exceeds the maximum X-band K dp expected for dendrites by 0.6° km -1 , perhaps becausethese dendrites exceeded the maximum diameter allowed in our scattering model (1 cm). Thehigh K dp -Z dr “pocket” and HCA dendrite identification in Fig. 11 was collocated with the -15°Cisotherm from a nearby sounding three hours prior. Wet snow is included in this HCA because atemperature inversion was still present but not strong enough to prevent snow from reaching the23