Quantitative Parametric Approach to Estimating Snowflake Size Distributions Using an Optical Sensing Disdrometer

抄録

We evaluated a method by which to fit gamma distributions with parameters of snowflake size distributions by measuring three physical quantities using an optical sensing disdrometer. The three physical quantities are the diameters of the snowflakes that have 50 and 99 percentiles of volumes (D₅₀ and D₉₉, respectively) and the sum of the sixth powers of the diameters in a unit volume (Z). Snowflake size distribution was well fitted to a gamma distribution between D₅₀ and D₉₉, inclusive. This method prevented the snowflake volume from being considerably underestimated. Although the mean absolute error based on snowflake volume for this method was large compared with that obtained using a moment method, good estimates of snowflake volume were obtained for some samples using this method, for which the snowflake size distribution was less influenced by snowflakes 1 mm or less in diameter. The correlation coefficient was 0.989, as determined by regression analysis based on the observed and estimated snowflake volumes using this method. The estimation of the snowflake volume using this method depends on the quality control of the optimum shape parameter and requires a continuous probability distribution of snowflakes for diameters above 1 mm.