Application of the Generalized Gamma Model to represent the full DSD spectra

Drop size distribution data from two collocated disdrometers (Meteorological Particle Spectrometer and 2D-video disdrometer) from recent measurement campaigns (Thurai et al., 2017), in Greeley, CO, and Huntsville, AL, have revealed that the full DSD spectra (at hourly and 5-min averages) can be represented by a combination of (i) a drizzle mode for drop diameters less than around 0.7 mm and (ii) a precipitation mode starting around 0.7-1 mm region, i.e. the ‘shoulder’ region, and extending to larger sizes. The two modes tended to be more prominent (at the hourly averaging) for the Huntsville cases relative to the Colorado events reflecting the climatological differences between the two locations.

Our new results point to the potential need for additional work on modeling the full DSD spectra. In this paper, we consider the formulation given in Lee et al. (2004), in particular the scaling form of the generalized gamma DSD with the 4 parameters namely: generalized characteristic number density, N₀’, the generalized characteristic diameter, D_m’ and two ‘shape’ parameters c and μ.

Such formulation is being tested using our measurements of the full DSD spectra. Initial results appear promising. For example, a 5 minute sample DSD measurements from a Huntsville event which occurred on 11 April 2016 provided a very good fit to the data simultaneously at both the small and large drop ends. The standard 3-parameter gamma DSD was unable to provide a good fit at both the small and large drop ends. Additionally, sample DSD measurements from a Greeley event on 17 April 2015 have also shown better representation by the generalized gamma formulation.

We will present illustrative case examples and compare the characteristics of the two early spring time (April) events in both locations in terms of variability of the 4 generalized gamma DSD parameters. Implications for rain microphysics, polarimetric radar-based retrievals of DSD and for mm-wave propagation impairment will be discussed.

References:

Lee, G., I. Zawadzki, W. Szyrmer, D. Sempere-Torres, and R. Uijlenhoet, 2004: ‘A general approach to double-moment normalization of drop size distributions’, J. Appl. Meteor., Volume 43, pp. 264–281

Thurai, M. P. Gatlin, V. N. Bringi, W. Petersen, P. Kennedy, B. Notaroš, and L. Carey, 2017: ‘Toward Completing the Raindrop Size Spectrum: Case Studies Involving 2D-Video Disdrometer, Droplet Spectrometer, and Polarimetric Radar Measurements’, Journal of Applied Meteorology and Climatology, Volume 56 (4), pp. 877–896.