Raindrop Size Distribution Parameters of Distrometer Data With Different Bin Sizes

A 2D video distrometer (2DVD) provides raindrop size distribution (DSD) at nominal drop diameters that correspond to the mean of the bin sizes. Selection of bin width may influence the shape of DSD. Therefore, we investigated the effect of binning on the DSD parameter estimates. First, we studied the effect of binning by examining their ability to recover known parameters of simulated DSD. Second, real DSD data collected in the equatorial region by 2DVD were analyzed. We compared the DSD parameters calculated from binned DSD with those calculated from a drop-by-drop data basis. Both simulated and real DSDs were binned at 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, and 0.50 mm. In general, the DSD parameters increased with increasing bin width. With very large number of raindrop which should be accompanied by heavy rain, the bias due to bin width selection is small. However, the bias is significant in the opposite case. The average fractional error between a mass-weighted mean diameter (Dm) calculated from DSD and that derived from drop-by-drop data was relatively small for all rainfall rates. A rather high error was observed in the median volume diameter (D₀) which may be due to moment method and interpolation error. Finally, using small bin widths (0.20-0.30 mm) may be the best choice because the DSD parameters of these bin widths were very close to those obtained from drop-by-drop data.