A Field Study of Pixel-Scale Variability of Raindrop Size Distribution in the Mid-Atlantic Region

J Hydrometeorol. 2016 Jun;17(6):1855-1868. doi: 10.1175/JHM-D-15-0159.1. Epub 2016 Jun 22.

Abstract

The spatial variability of parameters of raindrop size distribution and its derivatives is investigated through a field study where collocated PARSIVEL2 and two-dimensional video disdrometers are operated at six sites in Wallops Island, Virginia from December 2013 to March 2014. The three-parameter exponential function is employed to determine the spatial variability across the study domain where the maximum separation distance was 2.3 km. The nugget parameter of exponential function is set to 0.99 and the correlation distance (d0) and shape parameter (s0) are retrieved minimizing root-mean-square error, after fitting it to the correlations of physical parameters. Fits were very good for almost all fifteen physical parameters. The retrieved d0 and s0 were about 4.5 km and 1.1, respectively, for rain rate (RR) when all twelve disdrometers were reporting rainfall with a rain rate threshold of 0.1 mm h-1 in one-minute observations. The d0 decreased noticeably when one or more disdrometers were required to report rain. The d0 was considerably different for a number of parameters (e.g. mass weighted diameter) but was about the same for the other parameters (e.g. RR) when rainfall threshold was reset to 12 dB for Ka-band and 18 dB for Ku-band reflectivity following the expected Global Precipitation Measurement mission's space-borne radar minimum detectable signals. The reduction of the database through elimination of a site did not alter do as long as the fit was adequate. The correlations of 5-minute rain accumulations were lower when disdrometer observations were simulated for a rain gauge at different bucket sizes.