Effect of Irregularities in the Diameter Classification of Raindrops by the Joss-Waldvogel Disdrometer

Abstract

The Joss-Waldvogel Disdrometer (JWD) is an electromechanical counter designed to measure raindrop size distributions for the purpose of calculating radar reflectivities. It has performed this function successfully for 20 years. Recently, JWD measurements have been used by cloud physicists to assess the predictions of models based on raindrop coalescence—breakup processes. The models show that the cloud-base drop size distribution (DSD) evolves with time and fall distance to an equilibrium distribution with secondary maxima. A number of researchers using the JWD have noted peaks in DSDs measured in a large range of rainfall rates. The JWD signal processing electronics sizes the raindrops into 20 channels of diameter intervals. Laboratory calibration of the transfer function of the electronics (model RD-69) indicates that the boundaries of these channels differ from their specified diameter values. The effect of this discrepancy is demonstrated by applying the measured channel boundaries to a Marshall-Palmer distribution. The resultant DSD shows secondary maxima occurring in the channels with diameter boundaries 0.6–0.7, 1.0–1.2, 1.8–2.1 mm. It is recommended that conclusions drawn from secondary maxima in DSDs measured by the JWD consider this artifact.