Polarimetric Radar Relations for Quantification of Snow Based on Disdrometer Data

Abstract

Accurate measurements of snow amounts by radar are very difficult to achieve. The inherent uncertainty in radar snow estimates that are based on the radar reflectivity factor Z is caused by the variability of snow particle size distributions and snow particle density as well as the large diversity among snow growth habits. In this study, a novel method for snow quantification that is based on the joint use of radar reflectivity Z and specific differential phase KDP is introduced. An extensive dataset of 2D-video-disdrometer measurements of snow in central Oklahoma is used to derive polarimetric relations for liquid-equivalent snowfall rate S and ice water content IWC in the forms of bivariate power-law relations S = and along with similar relations for the intercept N0s and slope Λs of the exponential snow size distribution. The physical basis of these relations is explained. Their multipliers are sensitive to variations in the width of the canting angle distribution and to a lesser extent the particles’ aspect ratios and densities, whereas the exponents are practically invariant. This novel approach is tested against the S(Z) relation using snow disdrometer measurements in three geographical regions (Oklahoma, Colorado, and Canada). Significant improvement in snow estimates relative to the traditional Z-based methods is demonstrated.

10th European Conference on Radar in Meteorology and Hydrology (ERAD 2018) : 1-6 July 2018, Ede-Wageningen, The Netherlands

“…Following derivations by Hogan et al (2006) and Bukovcic et al (2018), one can obtain analytical expressions for the IWC(Z) parameterized either by the intercept N 0s of the size distributions …”

“…The disdrometer measurements of snow during 16 winter events in Oklahoma (Bukovcic et al 2018) have been used to test the procedure for estimating the mean volume diameter of snowflakes D m from the ratio Z/K DP . A scatterplot of D m measured by the disdrometer versus the retrieved value is in Fig.…”

“…The inherent uncertainty in the IWC(Z) estimate is caused by the variability of the size distribution (SD) of ice / snow particles and (to lesser extent) by the variations of density. The IWC(Z) relations are parameterized by either D m or N t (e.g., Hogan et al 2006;Bukovcic et al 2018) which have to be estimated independently.…”

“…Therefore, their ratio is approximately proportional to the cube of D m . Indeed, the expressions for Z and K DP in the Rayleigh approximation are (Bukovcic et al 2018) 494 In (5) - (6), K i = (ε i -1)/(ε i + 2), K w = (ε w -1)/(ε w + 2), ε i and ε w are the dielectric constants of solid ice and water, and ρ i = 0.917 g cm -3 is the density of solid ice, λ is the radar wavelength, F shape is the "shape factor" determined by the aspect ratio of the particle, and F orient is the "orientation factor" depending on the width of the canting angle distribution σ (Ryzhkov 2001):The dependence of F shape on the aspect ratio of oblate spheroids is illustrated in Fig. 1.…”

Dual-wavelength radar technique development for snow rate estimation: a case study from GCPEx

“…Huang et al: Dual-wavelength radar technique development for snow rate estimation (MASC; Garrett et al, 2012). When these instruments are used in conjunction with a well-shielded GEONOR or PLU-VIO gauge, it is shown that a physically consistent representation of the geometric, microphysical, and scattering properties needed for radar-based QPE can be achieved (Szyrmer and Zawadzki, 2010;Huang et al, 2015;von Lerber et al, 2017;Bukovčić et al, 2018). In this study, we use the 2DVD and PLUVIO gauge located within a double fence international reference (DFIR) wind shield to reduce wind effects.…”