Thursday, 6 June 2002
Modeling Freezing Drizzle in Cloud and Meso-scale models.
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.