A network of disdrometers to quantify the small scale variability of the microstructure of precipitation

The rain drop size distribution (DSD) is of primary importance for the quantitative interpretation of weather radar measurements, in particular for the parameterization of the relationship between the radar reflectivity Z and the rain rate R. The spatial variability of the DSD at small scales is poorly documented and understood. This is an important issue with respect to radar rainfall estimation because the DSD is generally assumed to be uniform within the radar sampling volume and because the parameters of the Z-R relationship are usually derived from point measurements.

To investigate this question, we have designed and deployed a wireless network of 16 disdrometers over a typical pixel of an operational weather radar (i.e. 1x1 km2). The disdrometers are autonomous (solar panel and battery). The DSD spectra, measured every 20 s, are transmitted to data loggers via radio modems and then transmitted to a central server using GPRS in real-time. The 16 optical disdrometers (PARSIVEL) have been deployed over EPFL campus, Switzerland. The network is running for several months and different types of precipitation have been collected. The analysis of the data shows that the spatial variability of the DSD is significant even at such a small scale. For instance, the coefficient of variation of the 16 simultaneously measured values of the 3 parameters of a gamma DSD modelfitted to observed spectra is about 30% with peaks above 100%. In terms of the Z-R relationship, the prefactor and the exponent exhibit a strong variability with a deviation from a mean value of about ±10% and ±2% respectively.