11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Friday, 7 June 2002
Spectral Polarimetric Measurements in the Mixed Phased Cloud
J. Verlinde, Penn State Univ., University Park, PA; and D. Moisseev, N. Skaropoulos, S. Heijnen, F. V. D. Zwan, and H. Russchenberg
Poster PDF (644.6 kB)
Bright band radar measurements taken with the advanced polarimetric, S-band, FMCW radar from the Technical University of Delft are presented. These measurements were taken during the Baltex/Bridge Cloud campaign (BBC) on September 19, 2001 during a 18 hour continuous rain period at the Royal Dutch Meteorological Institute research site at Cabauw, Netherlands. The linearized polarimetric radar alternately transmitted horizontally and vertically polarized signals, and received both co- and cross-polar returns. The collection of the raw beat-signal returns allowed detailed processing and analysis. Full velocity power spectra were calculated at each range gate (15 meter wide) for all polarization states. In this paper details of 4 minutes of data, taken at an elevation angle of 45o during a period of little change, will be presented. The melting layer is taken as a convenient laboratory of mixed phased hydrometeors to evaluate the ability of spectral polarimetry of identify habits.

Profiles of the standard polarimetric variables revealed typical melting layer characteristics: ZDR and LDR peaks approximately 200 m and 245 m below the (co-polar) reflectivity peak, and a 10 dB difference between the reflectivities in the bright band (37.5 dBZ) and in the rain below (27.5 dBZ). In addition to these variables we also determined several other variables to characterize the properties of the melting layer processes: the co- and cross-polar half-power signal width, the velocity distribution of ZDR and LDR, and the mean deviation of the ZDR in each spectrum. Both the co- and cross-polar spectral width profiles revealed peaks in the lower part of the bright band: however, these peaks were at different heights, and the cross-polar peak was twice the magnitude of the co-polar peak. These differences suggest microphysical influences (changes in permittivity) rather than mechanical processes (turbulence caused canting) as the cause. Furthermore, this faster fall-off in power in the co-polar signal strongly influences the spectral distribution of LDR, the value of which increases from the center of the spectrum where values are around -12 dB the values of 0 dB at the edges of the spectrum. Moreover, the spectral representation of ZDR reveals deviations from the mean of twice the gate value - revealing the random nature of the orientation of the melting particles relative to the polarization. At the upper and middle parts of the bright band the slowest falling particles are characterized by positive ZDR (+1 dB), while the fastest falling particles have negative values (-1 dB), suggesting significant particle shape and melting characteristics changes with size. In the lower part of the bright band the ZDR of the slower falling particles increases to values of +4 dB, but the variability also increases. At the same time, the peak in spectral ZDR shifts towards the bigger particles as more particles become completely melted and take on the well-known size-shape characteristics of drops.

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