Polarimetric and Thermodynamic Observations of Hydrometeor Refreezing

Open Access
Author:
Tobin, Dana Marie
Graduate Program:
Meteorology
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 14, 2016
Committee Members:
  • Matthew Robert Kumjian, Thesis Advisor
  • Jerry Y Harrington, Thesis Advisor
  • Yvette Pamela Richardson, Thesis Advisor
Keywords:
  • ice pellets
  • microphysics
  • refreezing
  • polarimetric
  • dual-polarization
  • radar
  • wet-bulb temperature
Abstract:
Recent studies document a polarimetric radar signature of refreezing. The signature is characterized by a low-level enhancement in differential reflectivity (ZDR) and decrease in co-polar correlation coefficient (ρhv) within a region of decreasing radar reflectivity factor at horizontal polarization (ZH) toward the ground. The signature is examined for the first time in conjunction with crowdsourced precipitation reports and thermodynamic data. Time-averaged quasi-vertical profiles (QVPs) retain the most prominent features of the signature, which appear to line up with certain thermodynamic features among cases. The height at which the low-level ZDR values begin to increase towards the ground corresponds to wet-bulb temperatures (Tw) = -5 °C, and may indicate the height at which freezing begins. Humidity profiles suggest the local generation of anisotropic crystals growing at the expense of small liquid drops via evaporation and vapor deposition. These crystals may promote the refreezing of fully melted hydrometeors via contact nucleation. Some regions suggest the rapid cooling of hydrometeors, that may act to freeze drops quickly and reduce their fall speeds, which may be responsible for producing an enhancement in ZH near the top of the refreezing layer (RFL) present in some cases. The evolution of the signature is also examined during three winter storms in which surface precipitation type transitions from ice pellets to freezing rain. Synoptic analyses reveal similarities among these cases in the location of nearby surface features associated with an approaching low-pressure system, namely an advancing warm front and an inverted surface trough, and strong warm-air advection aloft at 850 hPa and weak temperature advection at the surface. During transition events, the refreezing signature descends monotonically and intersects the ground at the time of the transition. A simple method for forecasting this changeover time from quasi-vertical profiles (QVPs) of polarimetric radar data is developed and compared to crowdsourced precipitation reports and surface precipitation types derived from Rapid Refresh (RAP) model wet-bulb temperatures (Tw) and a recently developed background precipitation classification algorithm. This simple technique shows greater skill in predicting changeover times as compared to forecasts based on RAP Tw profiles. The repeatability of the refreezing signature’s descent in precipitation transition events from ice pellets to freezing rain suggests the potential for its use in operational settings.