Radar Doppler polarimetry applied to precipitation measurements: introduction of the spectral differential reflectivity

The differential reflectivity Zdr is sensitive to the shape and orientation of the hydrometeors. The value of this parameter is related to a mean shape, which is not always representative of the measured particles. The differential reflectivity of spherical particles is 0 dB. Even in the case of spheroidal droplets, Zdr values of 0 dB can occur. For slant profiles the differential reflectivity corresponds to the mean spheroidal shape of raindrops. It indicates the presence of particles with different shapes and orientations in the different meteorological layers of the precipitation: rain, melting layer and precipitating cloud.

A major improvement for understanding the microstructure of precipitation can be achieved by combining simultaneous Doppler and polarimetric information. Two power Doppler spectra, hh and vv must be measured simultaneously in order to obtain the Doppler velocity spectrum of Zdr. The spectral differential reflectivity sZdr is thus defined for each Doppler velocity and provides detailed measures of the microstructure of precipitation.

The Doppler velocity can be related to the fall speed of the hydrometeors in the case of vertical profiles. For slant profiles, there is a Doppler contribution due to the horizontal wind, which differs as a function of height. In case of rain, the Doppler velocity can be related to the size of the hydrometeors. Because sZdr is function of the Doppler velocity and sensitive to the shape of hydrometeors, this enables the study of the shape-size relationship.

Therefore using Doppler polarimetric measurements, linear relations, sZdr versus Doppler velocity, have been measured in rain for slant profiles of light precipitation. These relations, sZdr versus Doppler velocity are the purpose of this paper. The measurement to obtain them and the definition of the spectral differential reflectivity sZdr will be given. The relations are going to be presented and explained. Their possible use for measurement of turbulence will be investigated.