Depolarization Estimates from Linear H and V Measurements with Weather Radars Operating in Simultaneous Transmission– Simultaneous Receiving Mode

Circular depolarization ratio (CDR) is a polarimetric parameter, which, unlike linear depolarization ratio (LDR), does not exhibit significant dependence on hydrometeor orientation and can be used for particle type identification and shape estimation if propagation effects are small. The measurement scheme with simultaneous transmission and simultaneous reception (STSR) of horizontally and vertically polarized signals is widely used with research and operational radars. The STSR scheme does not provide direct measurements of depolarization. This study presents an estimator to obtain depolarization ratios from STSR complex voltages in radar receivers. This estimator provides true CDR if the phase shift on transmission, (beta), is equal to (plus-minus)90° and the phase shift on reception, (gamma), equals -(beta). Even if these conditions are not satisfied, depolarization estimates are still possible if (beta) + (gamma)= 0° (though such estimates deviate slightly from true CDR varying between CDR and slant-45°LDR). The sum (beta) + (gamma)represents the initial differential phase shift offset and can be accounted for. The use of this depolarization estimator is illustrated with the data from the NOAA X-band radar. The measurements in ice clouds demonstrate the utility of near-CDR estimates to identify dendritic crystals and their gradual aggregation within the cloud. Illustrations are also given for near-CDR estimates in rain. An important advantage of depolarization estimates in the STSR mode is that these estimates are obtained from two "strong" channel returns. This greatly relaxes the radar sensitivity requirements compared to radar systems that utilize direct depolarization measurements as the power ratio of radar echoes measured in "strong" and "weak" receiving channels that represent two orthogonal polarizations.