Understanding the physical processes that lead to the formation of freezing drizzle are of critical importance to forecasting aircraft icing conditions in cloud resolving and meso-scale forecast models. Freezing drizzle is normally formed by either classical or non-classical mechanisms. The classical mechanism is where ice precipitation falls through a warm melting layer and subsequently falls into a cooler sub-freezing layer. Such situations often occur during winter time warm fronts in the mid-west and eastern U.S. The non-classical mechanism is where drizzle forms by condensation of cloud droplets followed by the collision and coalescence. This process is often termed the warm-rain process. This mechanism typically occurs within the temperature range of 0 C. to -10 C. where significant cloud water exist and little natural ice nuclei are active. The current thought is that the non-classical mechanism is not sufficiently represented by current microphysical parameterizations used in operational forecast models.

This paper will explore the application of recent theoretical developments of the warm-rain process to the forecasting of freezing drizzle in cloud resolving and meso-scale forecast models. The present work is inspired by the warm-rain parameterization schemes of Cohard and Pinty (Q.J.R.M.S., 1999) and Khairoutdinov and Kogan (M.W.R., 2000) who have developed warm-rain parameterizations base upon extensive comparisons of detailed bin spectral models. These works however introduce additional variables of cumulative cloud condensation nuclei, and number concentrations for cloud and rain fields. It is realized that such additional variables and microphysical complexities can easily over load the computer processing limitations of an operational forecast model. The goal of this work it to develop a simple hierarchy in which various physical options can be tested to access their potential to improve the forecasting of freezing drizzle in cloud resolving and meso-scale models.

Modeling results will be presented and compared with available observations of winter-time freezing drizzle events.