Sub-grid variability of the drop size distribution in orographic Mediterranean rainfall

Weather radars use the distribution of drop sizes in precipitation to convert the measured reflectivity into rain rate. The drop size distribution (DSD), measured in number of drops per drop diameter per unit volume of air, also allows for the calculation of many other precipitation variables of interest.

The DSD is often assumed to be uniform across a radar or numerical weather prediction model pixel. In reality, however, the DSD is highly variable across even small regions of space, and in time. Our aim is to investigate and characterise the small-scale variability of the ground-level DSD.

We deployed seven Parsivel 1 disdrometers for the Hydrological Cycle in the Mediterranean Experiment (HYMEX) Special Observation Period 1 (SOP1), in the Cévennes region in France during Fall 2012. This region is known to experience heavy precipitation events through orographic Mediterranean rainfall. The disdrometers were deployed across an area of about 5x5 km^2, which corresponds to a GPM radar pixel. Using geostatistical techniques we investigated the variability of the DSD over this region. We characterised the variability using sample variograms for convective and stratiform rain events. These variograms allowed us to interpolate fields for both the DSD and precipitation properties of interest across the pixel-sized area. While the decorrelation distance was not reached over our focus area, there was significant spatial variability in properties of interest such as rain rate.

The results of this study have implications for the estimation of the DSD within radar or numerical weather prediction model pixels, and will contribute to greater accuracy in estimation of ground precipitation variables.