Please login first
Drop size distribution retrievals for light rain and drizzle from S-band polarimetric radars
* 1 , 1 , 2 , 2, 3 , 3 , 4
1  Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, CO 80523, USA
2  NASA GSFC Wallops Flight Facility, Wallops Island, VA 23337, USA
3  Science Systems and Applications, Inc., Lanham, MD 20706, USA
4  Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA
Academic Editor: Anthony Lupo


Polarimetric radars use differential reflectivity (Zdr) in addition to the radar reflectivity (Zh) to determine the two main parameters governing rain drop size distributions (DSD), typically the mass-weighted mean diameter, Dm, and the normalized intercept parameter (NW). One built-in assumption is that the drops are oblate and that the minor-to-major axis ratios reduce with increasing size in accordance with theoretically- based approximations. For light rain, however, the Zdr approaches 0 dB because of the dominance of the small drops (D<0.8 mm) which are spherical in shape. Our scattering calculations using measured DSDs in light rain and drizzle show that for DSDs with Dm < 0.8 mm, the S-band Zdr tends to be < 0.2 dB and that using Zdr to retrieve Dm will have large uncertainties due to measurement errors as well as parameterization errors. On the other hand, Dm shows a more gradual variation with Zh for light rain and drizzle DSDs. Simulations of Zh and Zdr using measured DSDs with optical array probe and 2D-video disdrometer located inside a Double Fence International Reference (DFIR) wind shield were used to develop an algorithm to estimate Dm=f(Zh) for light rain. For drizzle, the fitted equation was derived using aircraft-based data in stratocumulus rain. Validation of the Dm retrievals was performed using Zh measured by scanning S-band polarimetric radars (CSU-CHILL in Colorado and NPOL in Delmarva peninsula) over two DFIR locations versus Dm from disdrometer measurements in the two locations. Consistent results were obtained but only for reflectivity less than 18 dBZ for light rain and 5 dBZ for drizzle. Finally, 500m by 500m gridded data from NPOL, are used to identify light rain and drizzle regions and their Dm histograms are compared with those derived from stratiform and convective rain regions. Comparisons are also made for the histograms of the normalized intercept parameter (NW).

Keywords: Light rain; drop size distribution; drizzle; polarimetric radar