Document Type


Publication Date



Past studies of the variability of drop size distributions (DSDs) have used moments of the distribution such as the mass-weighted mean drop size as proxies for the entire size distribution. In this study, however, the authors separate the total number of drops Nt from the DSD leaving the probability size distributions (PSDs); that is, DSD = Nt × PSD. The variability of the PSDs are then considered using the frequencies of size [P(D)] values at each different drop diameter P(PD | D) over an ensemble of observations collected using a network of 21 optical disdrometers. The relative dispersions RD of P(PD | D) over all the drop diameters are used as a measure of PSD variability. An intrinsic PSD is defined as an average over one or more instruments excluding zero drop counts. It is found that variability associated with an intrinsic PSD fails to characterize its true variability over an area. It is also shown that this variability is not due to sampling limitations but rather originates for physical reasons. Furthermore, this variability increases with the expansion of the network size and with increasing drop diameter.

A physical explanation is that the network acts to integrate the Fourier transform of the spatial correlation function from smaller toward larger wavelengths as the network size increases so that the contributions to the variance by all spatial wavelengths being sampled also increases. Consequently, RDand, hence, PSD variability will increase as the size of the area increases.

Publisher's Statement

© 2015 American Meteorological Society. Article deposited here in compliance with publisher's policy. Publisher's version of record:

Publication Title

Journal of the Atmospheric Sciences


Publisher's PDF

Included in

Physics Commons



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.