Doppler Weather Radar Research and Development
NSSL Project 5 – Investigation of the Use of Dual-Polarization Radar to Improve Quantitative Precipitation Estimation for Improving Flash Flood and Flood Detection, Warnings, and Forecasts:
Identification of Aircraft Icing Conditions with Polarimetric Radars
Funding Type: CIMMS Task II
Objectives
Investigate the utility of using polarimetric radar data to identify aircraft
icing conditions.
Accomplishments
Data from the polarimetric KOUN WSR-88D radar are being analyzed to investigate
polarimetric signatures associated with aircraft icing conditions.
A database of 23 potential icing events has been created by classifying
all polarimetric KOUN WSR-88D radar data collected over a 3 year period
(from the spring of 2002 through the spring of 2005) according to precipitation
type. From this, several events that exhibited extensive regions of
stratiform precipitation were identified, encompassing both cold- and
warmseason precipitation events. All pilot reports (PIREPS) within
150 km of the KOUN radar for these events were then obtained from a
database maintained by the National Center for Atmospheric Research.
From this search, approximately 120 PIREPS, with reported icing conditions
ranging from trace to heavy, have been identified at times when coincident
KOUN data are available.
Based on preliminary observations, polarimetric WSR-88D radar signatures at locations immediately above the freezing level in stratiform clouds, as well as in the radar bright band itself, might be useful for diagnosing the presence or absence of supercooled liquid water (SLW). For example, in systems where SLW is present and aircraft icing potential high, water mass in the 0 to -10ºC layer (a region where aircraft icing is most prevalent) is acquired by ice particles through accretion, resulting in particles that are on average more heavily rimed, slightly more spherical in shape, and have a much lower aggregation rate than would be the case if SLW were not present. This typically results in a gradual increase in radar reflectivity Z with decreasing height and, in general, a rather weak radar bright band signature. On the other hand, in stratiform regions where SLW is not present, no accretion takes place and aggregation is more intense, resulting in a much more steep increase in Z with decreasing height and, by comparison with a stratiform cloud where SLW is present, a more intense radar bright band signature.
This project is ongoing.
Conceptual model describing hypotheses for microphysical differences, as shown by idealized profiles of polarimetric variables, between stratiform clouds with low icing potential (solid lines) and high icing potential (dashed lines). Radar reflectivity is depicted in red, differential reflectivity in blue, specific differential phase in green, and correlation coefficient in purple. Dotted lines in differential reflectivity and correlation coefficient low icing potential profiles show structure believed to be associated with enhanced depositional growth in the -10 and -20ºC layer, resulting in stellars and dendrites that grow by aggregation as they fall.