41 Retrieval of Raindrop Size Distribution Parameters from Dual-Polarization Radar Measurements

Monday, 28 August 2017
Zurich DEFG (Swissotel Chicago)
Leo Pio D'Adderio, Univ. of Ferrara, Ferrara, Italy; and A. Tokay, D. A. Marks, J. L. Pippitt, D. B. Wolff, and W. A. Petersen

One of the main goals of the National Aeronautics and Space Administration (NASA) Global Precipitation Measurement (GPM) mission is to retrieve the parametric form of the raindrop size distribution (DSD) through its Dual-frequency Precipitation Radar (DPR) on board GPM core satellite. The GPM mission adopted a specific form of the three-parameter gamma model DSD but there is a lack of information to obtain the three unknown parameters of the gamma DSD with the dual frequency reflectivity measurements. At this stage, the shape parameter is set to 2 in combined radar-radiometer algorithm and set to 3 in DPR algorithm. The normalized intercept parameter, Nw, and the mean mass diameter, Dmass, are the other two parameters of the gamma model DSD and are the standard outputs from the DPR algorithm. The retrieval of Dmass within ±0.5 mm is one of the Level One (L1) requirements of the GPM mission. The accuracy of Nw and Dmass retrieval is determined through a comparative study of ground based and space borne products. While the ground based products are often considered as a reference, they inherit their have their own errors. Both Nw and Dmass rely on empirical relationships that are function of horizontal (ZH) and differential (ZDR) reflectivity. The Dmass-ZDR and Nw-ZH-Dmass relationships were derived using the sequential intensity filtering technique (SIFT, Lee and Zawadzki, 2005) utilizing the two-dimensional Video Disdrometer (2DVD) and Parsivel2 disdrometer (P2) database collected during GPM Ground Validation (GV) field campaigns. These relationships were then applied to the polarimetric observables of selected operational and research radar across the United States and elsewhere including NASA’s S-band dual-polarization radar (NPOL).

This study investigates the accuracy of the Dmass-ZDR and Nw-ZH-Dmass relationships and their sensitivity to the climate regime and the choice of disdrometer, as well as the accuracy of the DSD parameters estimation from radar measurements. The accuracy of Dmass-ZDR and NW-ZH-Dmass relationship was evaluated by comparing Dmass and logNw that was either directly calculated from 2DVD or retrieved from 2DVD based-ZDR and ZH-Dmass measurements. Good agreement was evident for both parameters with absolute bias of 0.12-13 mm and 0.06-0.07 m-3 mm-1 for Dmass and logNw, respectively. The Dmass and Nw estimated from NPOL-based ZDR and ZH-Dmass measurements were in a reasonable agreement with those that were directly measured by 2DVDs. The Dmass estimation as compared to the 2DVD generally satisfied the L1 requirements with an absolute bias ranging between 0.30 and 0.40 mm for the most sites. A number of sites exhibited larger error due to ground clutter and time-height ambiguity. While not defined in terms of L1 requirement limits, LogNw had a wider range of values with respect to the 2DVD measurements (2-4.5 m-3 mm-1), but was within acceptable range of 0.5-6 m-3 mm-1. This was partly due to the fact that NPOL measured ZH-ZDR values outside the observational ZH-ZDR envelope of the 2DVD. These outliers were found at high ZDR-low ZH and low ZDR-high ZH regimes and did not necessarily occur in a particular storm or segment of the storm. A sensitivity study was performed by both including and excluding these samples showed that the high ZDR samples generally produce an overestimation Dmass, and underestimation of Nw, while vice versa was true for the low ZDR samples.

Considering the sensitivity of Dmass-ZDR relationship to the climate regime, three years of P2 data from a tropical site (Kwajalein - 8.8˚N, 163.7˚E) were analyzed and compared to two mid-latitude GV field campaigns, IFloodS and OLYMPEx representing mid-continent springtime rainfall and coastal orographic effect through pre- and post-frontal rainfall, respectively. Orographic lifting resulted in an abundance of small drops shifting the probability distribution of logNw to higher values with respect to both the Kwajalein and IFloodS datasets. The use of OLYMPEx Dmass-ZDR relationship resulted in absolute bias of 0.17 mm in Dmass when it was applied to the Kwajalein dataset. This was a higher absolute bias with respect to the use of Kwajalein or IFloodS Dmass-ZDR relationships for the other datasets.

Considering the sensitivity of Dmass estimation to the instrument type the co-located 2DVD and P2 measurements from IFloodS and OLYMPEx were employed to derive Dmass-ZDR relationships from coincident datasets. The difference in Dmass based on 2DVD and P2 Dmass-ZDR relationships remained within ± 0.2 mm until ZDR of 1.6 dB and 2.6 dB for IFloodS and OLYMPEx, respectively. The absolute bias was 0.10 mm or lower in Dmass indicating a low sensitivity to the instrument type.


Lee, G.W. and Zawadzki, I.: Variability of Drop Size Distributions: Noise and Noise Filtering in Disdrometric Data, 2005, J. Appl. Meteor., 44, 634-652.

- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner