Unusually high differential attenuation at C band: Results from a two-year analysis of the French trappes polarimetric radar data

Abstract

The differential phase (FDP) measured by polarimetric radars is recognized to be a very good indicator of the path integrated by rain. Moreover, if a linear relationship is assumed between the specific differential phase (KDP) and the specific attenuation (AH) and specific differential attenuation (ADP), then attenuation can easily be corrected. The coefficients of proportionality, gH and gDP, are, however, known to be de- pendent in rain upon drop temperature, drop shapes, drop size distribution, and the presence of large drops causing Mie scattering. In this paper, the authors extensively apply a physically based method, often referred to as the "Smyth and Illingworth constraint," which uses the constraint that the value of the differential reflectivity ZDR on the far side of the storm should be low to retrieve the gDP coefficient. More than 30 convective episodes observed by the French operational C-band polarimetric Trappes radar during two summers (2005 and 2006) are used to document the variability of gDP with respect to the intrinsic three- dimensional characteristics of the attenuating cells. The Smyth and Illingworth constraint could be applied to only 20% of all attenuated rays of the 2-yr dataset so it cannot be considered the unique solution for at- tenuation correction in an operational setting but is useful for characterizing the properties of the strongly attenuating cells. The range of variation of gDP is shown to be extremely large, with minimal, maximal, and mean values being, respectively, equal to 0.01, 0.11, and 0.025 dB o-1. Coefficient gDP appears to be almost linearly correlated with the horizontal reflectivity (ZH), differential reflectivity (ZDR), and specific differ- ential phase (KDP) and correlation coefficient (rHV) of the attenuating cells. The temperature effect is negligible with respect to that of the microphysical properties of the attenuating cells. Unusually large values of gDP, above 0.06 dB o-1, often referred to as "hot spots," are reported for 15%-a nonnegligible figure-of the rays presenting a significant total differential phase shift (ΔΦDP>30o). The corresponding strongly attenuating cells are shown to have extremely high ZDR (above 4 dB) and ZH (above 55 dBZ), very low rHV (below 0.94), and high KDP (above 48 km-1). Analysis of 4 yr of observed raindrop spectra does not re- produce such low values of rHV, suggesting that (wet) ice is likely to be present in the precipitation medium and responsible for the attenuation and high phase shifts. Furthermore, if melting ice is responsible for the high phase shifts, this suggests that KDP may not be uniquely related to rainfall rate but can result from the presence of wet ice. This hypothesis is supported by the analysis of the vertical profiles of horizontal re- flectivity and the values of conventional probability of hail indexes. © 2009 American Meteorological Society.