Polarimetric Radar-Based Estimates of Spatial Variability in Characteristic Sizes of Raindrops in Stratiform Rainfall

摘要

Polarimetric X-band radar measurements of differential reflectivity Z(DR) in stratiform rainfall were used for retrieving mean mass-weighted raindrop diameters D(m) and estimating their spatial variability delta D(m) at different scales. The Z(DR) data were calibrated and corrected for differential attenuation. The results revealed greater variability in D(m) for larger spatial scales. Mean values of delta D(m) were respectively around 0.32-0.34, 0.28-0.30, and 0.24-0.26 mm at scales of 20, 10, and 4.5 km, which are representative of footprints of various spaceborne sensors. For a given spatial scale, delta D(m) decreases when the mean value of D(m) increases. At the 20-km scale the decreasing trend exhibits a factor-of-1.7 decrease of delta D(m) when the average D(m) changes from 1 to 2 mm. Estimation data suggest that this trend diminishes as the spatial scale decreases. Measurement noise and other uncertainties preclude accurate estimations of D(m) variability at smaller spatial scales because for many data points estimated variability values are equal to or less than the expected retrieval errors. Even though they are important for retrievals of absolute values of D(m), the details of the drop shape size relation did not significantly affect estimates of size spatial variability. The polarization cross coupling in simultaneous transmission simultaneous receiving measurement mode presents another limiting factor for accurate estimations of D(m). This factor, however, was not too severe in estimations of the size variability. There are indications that tuning the differential attenuation correction scheme might balance off some possible cross-coupling Z(DR) bias if differential phase accumulation is less than approximately 40 degrees.