Polarimetric radar/aircraft observations and modelling of ice crystals in winter storms

One of the first results of winter time precipitation study using polarimetric radar, instrumented aircraft, and a microphysical cloud model is presented.; Two winter storm case studies, 27 March 1991 and 27 November 1987, analyze formation and growth of ice crystals, onset of aggregation and riming, and transformations from pristine crystals to aggregates. Both case studies compare extensively the simultaneous collection of coordinated radar and in situ measurements made by aircraft penetrations through winter storms. In addition, 27 March 1991 and 27 November 1987 case studies provide a unique opportunity to examine winter storm phenomena in two different climate regimes, namely-Northern Colorado (Continental USA) and Southern Germany (Continental Europe) using CSU-CHILL S-band and DLR C-band polarimetric radars, respectively.; Two numerical models, electromagnetic scattering and snow growth, are used to provide a framework for microphysical interpretation and validation of polarimetric radar and aircraft observations of winter storms. The electromagnetic scattering model computes radar observables such as reflectivity ({dollar}Zsb{lcub}HH,VV{rcub}),{dollar} differential reflectivity {dollar}(Zsb{lcub}DR{rcub}),{dollar} cross correlation coefficient {dollar}(rhosb{lcub}HV{rcub}),{dollar} and specific differential phase shift {dollar}(Ksb{lcub}DP{rcub}){dollar} from an averaged Mueller matrix and is coupled to the output of a 3-dimensional time dependent snow growth model to study radar profiles of snowbands consisting of both pristine ice crystals and aggregates. The snow growth model developed for both rimed and unrimed steady-state homogeneous stratiform snowfall provides insight into both the evolution of ice crystal growth processes and predicts snow growth in terms vapor deposition, aggregation, and riming as well as cloud updrafts and ice particle breakup.; 27 March 1991 case study derives an empirical Z-S (S = snowfall rate) relationship for a deep cyclonic snowstorm observed in Northern Colorado. 10-11 March 1993 case study examines propagation effects using CSU-CHILL radar and correlates oriented crystals with {dollar}Ksb{lcub}rm DP{rcub}{dollar} observations during ice-phase precipitation. Also, an intercomparison is given of S-band polarimetric signatures taken during a shallow upslope storm observed simultaneously by CSU-CHILL and NCAR CP-2 radars on 11 March 1993. (Abstract shortened by UMI.)...