Spatial variability of rain drop characteristic sizes as estimated from X-band polarimetric radar

Polarimetric X-band radar measurements of differential reflectivity, ZDR, were used for retrievals of mean mass-weighted rain drop diameters, Dm, and estimates of their spatial variability &DeltaDm at different spatial scales. The measurements were performed in stratiform winter-time rainfall observed in California's American River basin. Prior to retrievals, the ZDR data were calibrated and corrected for differential attenuation effects. Examination of the characteristic drop diameters revealed greater variability in drop sizes for larger spatial scales. Mean values of &DeltaDm, were around 0.32-0.34 mm, 0.28-0.30, and 0.24-0.26 mm at scales of 20 km, 10 km and 4.5 km, respectively. For a given spatial scale, &DeltaDm, decreases when the mean value of the drop mean mass-weighted diameter increases. At a 20 km scale, which was the largest scale available due to the geometry of scanning, the diminishing trend results in approximately a factor of 1.7 decrease of &DeltaDm, when the average value of Dm at this resolution changes from about 1 to 2 mm. Estimation data suggest that this trend diminishes as the spatial scale decreases. The measurement noise and other uncertainties preclude accurate estimations of the variability in drop characteristic sizes at smaller spatial scales because for many data points the estimated variability values were equal or less than the expected retrieval errors. Even though they are important for retrievals of absolute values of the characteristic drop size, the details of the drop shape - size relation did not significantly affect estimates of this size spatial variability. The polarization cross-coupling present in the simultaneous transmission - simultaneous receiving measurements mode presents another limiting factor in accurate estimations of characteristic drop sizes. This factor, however, was not too severe in estimations of the size variability during the measurements used in this study. Tuning coefficient in the differential attenuation correction scheme can balance off the cross-coupling differential reflectivity bias.