Direct and indirect measurement of rain drop size distributions using an acoustic water tank disdrometer

Abstract

Several rain drop size distribution (DSD) point measurement technologies exist, but all are unable to sample either short timescales or the large drop tail of the DSD due to inherent instrumental limitations. The development of an acoustic water tank disdrometer (AWTD) is described, which improves the sampling statistics by increasing the catchment area. This is achieved by distinguishing individual drops, locating them on the surface of the tank then converting the impact pressure into a drop size. Wavelet decomposition is used to distinguish the broadband, short duration impact events and a fast multilateration method is used to position the drop. Issues relating to the different types of noise are also investigated and mitigated. Also, further work on inverting the measured acoustic intensity into a DSD, by fitting sampling distributions, is presented. Six months of data were collected in the Eastern UK. The AWTD then converted the data into DSDs and the results were compared to a commercially available co-located laser precipitation monitor. The sampling errors are far lower due to the increased catchment size, and hence the large drop sized tail of the DSD is greatly improved. DSD results compare favourably to other disdrometers for drop diameters greater than 1.8 mm. Below this size individual drops become increasingly difficult to detect and are underestimated.