A simulation approach to sampling effects in raindrop size distribution measurements in non-stationary rain

The raindrop size distribution (DSD) is fundamental for the interpretation of radar measurements, especially with respect to the parameterization of the power laws used to describe the relations between the bulk variables (e.g. radar reflecvity Z, rainfall intensity R). The different types of sensors used to measure the DSD (e.g. Joss-Waldvogel disdrometer, spectropluviometer and video-disdrometer) present a limited sampling volume or a limited sampling surface. The issue of uncertainty in DSD characterization due to sampling errors (independently of measurement errors) is crucial and has been recognized for a long time (e.g. Joss and Waldvogel, 1969). That question has been investigated through (1) empirical approaches, based on the analysis of data from collocated sensors (e.g. Tokay et al., 2005); and (2) theoretical approaches, where one of the main limitations is the assumption of stationary rain (e.g. Gertzman and Atlas, 1977). A recently developed stochastic simulator of time series of DSDs (Berne and Uijlenhoet, 2005) enables to combine the two types of approaches and to relax some of the limitations of previous work. In particular, non-stationary rain profiles are generated and the sampling error of a DSD measurement sensor is quantified using a Monte Carlo technique, as well as its influence on the deduced Z-R relations. the simulation approach allows a separation of the influence of the natural rainfall variability and that of the sampling fluctuations.

References:

Berne, A. and R. Uijlenhoet, 2005: A stochastic model of range profiles of raindrop size distributions: Application to radar attenuation correction. Geophys. Res. Lett., 32, L10803, doi:10.1029/2004GL021899.

Gertzman, H.S. and D. Atlas, 1977: Sampling errors in the measurement of rain and hail parameters. J. Geophys. Res. 82(31), 4955-4966.

Joss, J. and A. Waldvogel, 1969: Raindrop size distribution and sampling size errors, J. atmos. Sci., 26, 566-569.

Tokay, A., P.G. Bashor and K.R., Wolff, 2005: Error characteristics of rainfall measurements by collocated Joss-Waldvogel disdrometers. J. Oceanic Atmos. Technol., 22, 513-527.