A Comparison of X-band Polarization Parameters with In-Situ Microphysical Measurements in the Comma Head of Two Winter Cyclones

Radars with polarization capabilities provide the ability to retrieve information on the microphysical properties of hydrometeors, including dominant particle habits, within the radar sample volume. Since the advent of dual-polarization radars, much work has been put into implementing a means of classifying hydrometeors by type given a suite of polarization variables at each radar gate. While existing methods of hydrometeor classification (HC) are computationally efficient, their deterministic approach of assigning only one dominant habit to a radar sample volume does not properly consider the distribution of habits present in that volume.

During the Profiling of Winter Storms (PLOWS) field campaign the NSF/NCAR C-130 aircraft, equipped with in-situ microphysical probes, made multiple passes within a single vertical plane through the comma head of two cyclones as the comma head region moved over the Mobile Alabama X-band (MAX) dual-polarization radar. The radar continually performed range-height indicator scans in the same plane as the C-130 flight track.

Using two cyclones sampled on 14-15 February and 21-22 February 2010, 579 and 202 coincident data points, respectively, were identified when the plane was within 10 s (~1 km) of the radar gate. Using the morphological features of the in-situ imaged crystals, the habit of each particle was identified. For each radar reflectivity (Z) – differential reflectivity (Z_dr) binned interval, the reflectivity-weighted contribution of each habit was determined. Comparing against the distribution of axis ratio and √Area/Perimeter derived for each habit gives information on crystal properties for each Z-Z_dr bin. The results indicate that habits with less circular shapes (dendrites, aggregates, and irregular crystals) are more likely to occur when radar measurements of Z are greater than 10 dBZ and Z_dr > 2 dB. The presence of both dendrites and aggregates for similar Z and Z_dr support previous studies that state dendrites are the favored crystal species for aggregate formation. While irregular particles make the largest contributions to the observed shapes, the general lack of dominance of a given habit for a particular Z and Z_dr suggests that determining the probability of specific habits in the radar volume may be more suitable than the deterministic methods currently used.