Dual polarization signatures of melting hydrometeors: 23 August 2013

Overview

Around 2005 MDT on 22 August 2013 (0205 UTC on 23 August), the thunderstorm activity that had developed during the afternoon hours was beginning to dissipate. In areas where the convective vertical motions were no longer active, the precipitation became more stratiform in character. An RHI scan done by the CSU-CHILL radar on an azimuth of 101 degrees intercepted stratiform rain at closer ranges, and a thunderstorm convective core at more distant ranges. Dual polarization indications of melting hydrometeors occurred at differing height levels in these two echo regimes. The following plot of the reflectivity data observed in the 2.3 degree PPI shows the general situation. The solid white line shows the location of the RHI scan data that was collected ~2 minutes after the PPI data. (Note: The CSU-CHILL X-band system was out of service on this date. All of the data is from the 11 cm wavelength / 3 GHZ channel.)

RHI data

The first plot shows the vertical structure of the reflectivity field. A horizontally-oriented brightband echo was present in the nearer-range / light stratiform rain area. The water coating that develops as frozen hydrometeors descend through the 0C level is a major factor in generating the local reflectivity enhancement. When the snow particles completely melt, they collapse into raindrops with smaller diameters and higher terminal fallspeeds than their "parent" melting hydrometeors. This shift to smaller particle diameters and lower volumetric concentrations reduces the reflectivity at heights below the bright band. At ranges beyond ~35 km, the strong updrafts in a thunderstorm has lead to the growth of many large-diameter hydrometeors, yielding significantly higher reflectivity levels.

The next plot shows the corresponding radial velocity field. The 18 and 48 dBZ reflectivity contours are overlaid to help identify the bright band and convective core regions. Convergence and a probable updraft are implied near X=37, Z=4 km. Outflow from the convective cell's downdraft is responsible for the enhanced inbound (negative in sign) velocities near the surface.

The next plot shows the corresponding differential reflectivity (Zdr) field. In the bright band, the most positive Zdr values are restricted to a narrow height layer where the developing water coating on the descending ice crystals is significant. This water coating enhances the particles dielectric constant, making their mean horizontal orientation more apparent to the radar and thus increasing Zdr. When the particles fully melt, the resultant drops are less oblate, producing less positive zdr. Within the convective echo core, the Zdr patterns have a much less layered appearance. Positive values extend to ~5 km AGL in general association with the updraft near X=37 km. Oblate raindrops that are lofted above the 0C level by updrafts typically contribute to such positive zdr "columns". At greater ranges (~X=40 km) in the main precipitation shaft, the development of positive zdr values is delayed to heights of ~3 km AGL (i.e., several hundred meters below the brightband level). This melting level depression is due to the longer time required for melting to proceed in the more massive ice particles (graupel and small hailstones) in the convective precipitation core. The higher terminal fallspeeds of these particles, aided by downdrafts in the echo core, also cause zdr enhancements due to melting to appear at lower heights.

The co-polar correlation coefficient between the horizontal (H) and vertical (V) signal returns (multiplied by 10) is shown in the next plot. rhoHV is decreased when the radar pulse volume contains a diverse variety of hydrometeor shapes, orientations, and thermodynamic phases. Melting regions, where quasi-spherical, fully melted drops coexist with large, irregular, partially melted ice particles are characterized by locally reduced rhoHV. Correlation coefficient reductions to ~0.9 and below occur in the mixed phase region of the bright band. A second, vertically-oriented correlation minimum is also found in the reflectivity maximum along the near edge of the convective precipitation area (X=35 to 37 km) where raindrops are mixed with melting graupel.

The final dual polarization variable presented is linear depolarization ratio (Ldr). Ldr is the ratio of the cross-polar and co-polar received signal powers. The cross-polar signal is increased when the major axis of non-spherical hydrometeors is oriented out of the polarization plane of the incident radar waves. This