Measurements and Modeling of the Full Rain Drop Size Distribution

Created by: Merhala Thurai

The following is the abstract for the article in:

The raindrop size distribution (DSD) is fundamental for quantitative precipitation estimation (QPE), in numerical modeling of microphysical processes, as well as for evaluating radiowave propagation effects for communication links operating in the microwave and millimeter wave frequency bands. Conventional disdrometers cannot capture the small drop end, in particular the drizzle mode which controls collisional processes as well as evaporation. To overcome this limitation, the DSD measurements were made using (i) a high-resolution (50 microns) meteorological particle spectrometer (MPS) to capture the small drop end, and (ii) a 2D video disdrometer (2DVD) for larger drops. Measurements were made in two climatically different regions, namely Greeley, Colorado, and Huntsville, Alabama, both in the United States. To model the DSDs, a previously published formulation is utilized, which is based on (a) two reference moments and (b) the generalized gamma (GG) model to describe the generic or underlying shape with two shape parameters to yield sufficient flexibility including double curvature, a feature often observed in equilibrium DSDs.

A total of 4550 3-minute DSDs were used to assess the suitability of our chosen model. Nearly 49% of these DSDs were from Greeley and the remaining 51% from Huntsville. The latter includes outer bands of hurricane Irma as well as tropical storm Nate. We show illustrative examples of the data and the fitted models and assess the goodness of fits by examining the histograms of relative bias corresponding to individual drop diameter intervals. For a few chosen Greeley events, the 3-minute rain rates derived from the fitted GG model show excellent agreement with collocated Pluvio gauge measurements in term of rain accumulation. Finally, for the tropical storm Nate, the co-variabilities of the two main DSD parameters (normalized intercept and mass-weighted mean diameter) were compared with those derived from the dual-frequency precipitation radar onboard the global precipitation mission satellite.

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Examples of accurate measurements and modelling of the full rain drop size distribution spectra can be found in (a) presentation at the 2017 Radar Meteorology Conference, Chicago; (b) a web article containing animations, together with polarimetric radar images; (c) A journal article.The links are given below.







  1. Merhala Thurai, Patrick Gatlin, V. N. Bringi, Walter Petersen, Patrick Kennedy, Branislav Notaroš, and Lawrence Carey; Toward Completing the Raindrop Size Spectrum: Case Studies Involving 2D-Video Disdrometer, Droplet Spectrometer, and Polarimetric Radar Measurements. Journal of Applied Meteorology and Climatolology 2017, 56, 877-896, 10.1175/JAMC-D-16-0304.1.