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

8081828384858687888990919293949596979899100101102systems such as ASOS or METAR, rapidly disseminated model output, and upper-air soundingscannot independently discern different snow crystal types, rain, freezing rain, or sleet with muchconfidence for several reasons (Elmore et al. 2011, Schuur et al. 2012). These observationalmethods do not provide the temporal or spatial resolution available from radar either.Hydrometeor classification algorithms combining atmospheric soundings withpolarimetric radar observations have been successful for warm-season, convective precipitation(Liu and Chandrasekar 2000, Zrnic et al. 2001, Ryzhkov et al. 2005a, Dolan and RutledgehenceforthDR09, Park et. al 2009, Chandrasekar et al. 2011) because the freezing level does notvary much in space or time. This is not the case for winter precipitation though, which motivatesuse of a polarimetric radar-based melting layer detection algorithm to identify wet or meltingsnow and then inform additional classification steps below and above this radar bright band layer(Giangrande et al. 2008 and Boodoo et al. 2010 - henceforth G8B1).To date, wintertime polarimetric algorithms using this melting layer detection techniqueand external temperature information from either a sounding or model forecast have attempted toidentify winter hydrometeor types with varying levels of success (Kouketsu and Uyeda 2010,Elmore et al. 2011, Schuur et al. 2012). Elmore et al. (2011) showed that the radar’s inability toidentify the refreezing of raindrops and errors in the melting layer detection algorithm led toHCA failures and poor overall performance in diagnosing surface weather conditions. Schuur etal. (2012) produced satisfactory results using an algorithm based on rapidly updated modeloutput temperature and moisture fields along with polarimetric radar data. The methodologypresented in Schuur et al. (2012) is particularly valuable at far ranges where the radar resolutionis degraded, and below the lowest elevation angle scan where surface weather conditions cannotbe diagnosed by the radar at all.5