Calibration of operating modes of mm-wavelength zenith-pointing radars

We focus on measurements of circular depolarization ratio (CDR) in rain with the Atmospheric Radiation Measurement (ARM) Millimeter wave cloud radar (MMCR) in search for signatures due to non-axisymmetric oscillation of raindrops. If the depolarization ratio is obtained from the so-called polarimetric mode (5 main channel and 6 weak channel), CDR enhancements in the lower part of the troposphere are well visible, but are due to main channel saturation. If the depolarization ratio is obtained from the precipitation mode (4 main channel and 6 weak channel), then a correlation between CDR and max drop diameter and between CDR and rain rate can be observed. Potentially, the sensitivity of mm-wave radars to depolarization from oscillating raindrops may be used for quantitative estimation, provided the cross-polar isolation of the antenna is improved to better than -30 dB and the receiver architecture is designed to provide truly dual-pol measurements (simultaneous reception of co-pol and cross-pol channels). CDR measurements of rain are interpreted by means of T-matrix simulations of horizontally oriented oblate spheroids with a canting angle width between 7˚ and 10˚. Surprisingly, such model cannot explain the observed depolarization. We then resorted to assume ellipsoidal equilibrium raindrop shapes to account for the non-axisymmetric oscillations. The non axisymmetric model for raindrop shape is substantiated by DDA (Discrete Dipole Approximation) simulations.

In the analysis, the invariance of the antenna ICPR (cross-polar isolation) is used to derive a method for the relative calibration of the precipitation mode with respect to the general mode. Relative calibration of the precip mode with respect to the general mode can be made with an accuracy of tenths of dBZ (~0.2 dBZ).