Freezing Rain: An Observational and Theoretical Study

Abstract

Data from a Doppler radar, an instrumented aircraft, and several rawinsonde observations during freezing rain and ice pellet events have been analyzed for this study. From these data, 34 soundings were obtained that characterized the vertical structure of the atmosphere at the time of the freezing precipitation. These soundings were analyzed to determine the general nature of the vertical structure and the environmental conditions common to these types of winter precipitation events. The state of the hydrometeors reaching the surface (i.e., supercooled liquid or ice) was critically dependent upon the melting and refreezing characteristics. To better understand these characteristics, two theoretical models were used: 1) a heat transfer model to simulate the melting of a dendritic snowflake and 2) a separate heat transfer model to simulate the refreezing characteristics of a liquid droplet. These two models were run using typical values for lapse rates and temperatures as determined from the soundings. In addition to this, model runs were made using the characteristics from each of the soundings as the initialization parameters. The theoretical predictions agreed quite well with the observed nature of the surface precipitation. The implications for the remote detection of freezing precipitation are discussed and the application of this research to a forecasting environment is considered.