Radar and Remote Sensing

Polarimetric Characteristics of Warm-Rain Precipitation Growth Processes

Nick Carr

23 April 2015, 1:15 PM

National Weather Center, Room 1350
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Flooding caused by excessive precipitation is a significant natural hazard that results in numerous fatalities and significant amounts of damage annually. A particular type of precipitation event characterized microphysically, by efficient precipitation growth through warm-rain processes (i.e. collision-coalescence), is particularly dangerous from a flooding perspective. The flooding potential of these events is enhanced due to both the relative efficiency of the precipitation-growth process and the tendency for spaceborne and terrestrial sensors to produce poor and unreliable precipitation estimates. The relatively poor performance of radar precipitation estimates in warm-rain is primarily due to the dependence of the parameters of the reflectivity to rain rate (Z-R) relationship, on the microphysical properties of precipitation, and the microphysical properties of warm-rain precipitation are notably distinct from the microphysical properties of typical mid-latitude convective and stratiform precipitation.

The recent dual-polarization upgrade to the United States’ ground radar network provides an excellent opportunity to analyze and quantify precipitation microphysical properties and processes. Since, the microphysical characteristics of precipitation grown primarily via warm-rain processes are hypothesized to be distinct from those associated with ice and mixed-phased processes, an analysis of the three-dimensional profiles of polarimetric radar variables, will enable an improved identification of the dominant precipitation growth processes, and this improved identification would result in improved precipitation estimates.

This study attempts to identify the radar characteristics associated with enhanced warm-rain processes by analyzing radar and ancillary environmental data for numerous warm-rain dominant precipitation events over the CONUS for the 2014 warm season (April to September). Specifically the polarimetric characteristics associated with a variety of warm-rain events, such as a landfalling tropical cyclone, orographically enhanced precipitation, a springtime Mesoscale Convective Complex, and intense convective precipitation are examined. Early results show that these events are largely characterized by the following polarimetric signatures: low Differential Reflectivity values, low Specific Differential Phase values, low echo tops, an increase in the values of the radar variables toward the surface, and a sharp decrease in radar reflectivity above the freezing level. These characteristics are primarily visualized through the use of the Gorgucci microphysical parameter space. Current precipitation classification algorithms for identifying warm-rain dominant precipitation are also discussed, along with potential methods of incorporating the observed polarimetric characteristics into these existing algorithms. Finally, potential methods to improve satellite precipitation estimates in warm-rain events are briefly discussed.

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