The Impact of the Radar Differential Phase upon Transmission on the Polarimetric Variables

The WSR-88D weather radars employ simultaneous transmission and reception of orthogonally polarized waves. The radars feature two separate channels for the waves. The difference in the waveguide lengths in the channels leads to a difference in phases between the transmitted waves. This phase called a differential phase upon transmission (PT) is typically not accounted for in the calculations of differential reflectivity, the differential phase (DP), and correlation coefficient (CC). The phase PT is silently assumed to be zero.

It is shown that the transmission phase PT affects all polarimetric radar variables. The maximal impact is observed in the values of measured differential phase and correlation coefficient. Nonzero phase PT explains large differential phases observed in ice clouds, different CC values in hail observed with adjacent radars, and anomalous DP and CC values in the melting layer. The theoretical results are demonstrated with patterns collected with WSR-88D radars in the US.

Results of PT measurements on WSR-88D KOUN located in Norman, OK are also presented. It is shown that the PT value depends on the difference in the waveguide lengths and radar frequency. The impacts of the KOUN’s PT on measured polarization variables are discussed.