Fine structures of refractivity in the boundary layer reveled with a polarimetric WSR-88D

Enhancements to data collection and signal processing such as application of covariance estimators, angular and range oversampling, coherent summation, and noise despeckling allow power measurement of weak echoes from Bragg scatterers having equivalent reflectivity factors as low as -25 dBZ at distance of 10 km from the WSR-88D radar. This level of sensitivity allows revealing a fine structure of refractivity field in the boundary layer and polarimetric measurements of Bragg scatterers.

Measured differential reflectivities ZDR of Bragg scatterers lie between -0.08 and 0.06 dB which supports the hypothesis that the intrinsic ZDR is 0 dB. Measured copolar correlation coefficients have distributions peaked at about 0.998-1.0. If insects and birds are spatially separated from Bragg scatterers, the dual-polarization capability of the WSR-88D allows distinguishing echoes from these two types of scatterers since ZDR from biota is significantly larger than 0 dB. In mixtures of Bragg and biota scatter, polarimetric spectral analysis shows differences in portions of the H and V spectra belonging to birds/insects with large positive spectral ZDR and Bragg scatterers with zero spectral ZDR.

Utilization of polarimetric WSR-88Ds to distinguish Bragg scatter from echoes from insects and birds has the potential to provide information on convective boundary layer depth. Fields of refractivity in the boundary layer are discussed and compared with data obtained from the wind profilers and rawinsondes. Given that the WSR-88D can be used to monitor the temporal and spatial changes of water vapor near the earth using backscatter from fixed ground objects, additional information on the depth of the CBL from a polarimetric WSR-88D could provide an important constraint on the changes in water vapor and turbulence within the boundary layer.