Parameterisation of the raindrop size distribution using quality controlled polarimetric Doppler weather radar measurements at C-band

The quantitative areal precipitation estimation by using ground based weather radar measurements is an appropriate method for researchers and end-users in the fields of meteorology, hydrology and water management. Although there are existing single-parameter algorithms, such as the Z-R relations, which are often used to estimate the quantitative areal precipitation when having non-polarimetric weather radar measurements, these algorithms are often of poor accuracy. A more accurate estimation of the quantitative areal precipitation is possible when having a more detailed knowledge of the raindrop size distribution within the observed radar resolution volume. This work will reflect upon the parameterisation of the raindrop size distribution using quality controlled polarimetric weather radar measurements (coherent polarimetric C-band weather radar POLDIRAD, DLR site at Oberpfaffenhofen) collected during the AQUARadar project (Advances in Quantitative Areal Precipitation Estimation by Radar) field campaign in Southern Germany in 2006. The radar measurements consist of polarimetric observables like reflectivity, differential reflectivity, linear depolarisation ratio and differential propagation phase and are time series of PPI volume scans and RHI scans, which cover two test beds where other instruments like meteorological stations, rain-gauges, disdrometer (Joss-Waldvogel, Parsivel) and vertically pointing micro rain radars were located. As a first very important and necessary, but unfortunately often neglected, step a quality control is applied to the radar measurements. Parts of the quality control algorithm are for example a check for fixed ground target (clutter) contamination of the observed resolution volume (due to low elevation angle or refraction), a check for the occurrence and the height of a melting layer over the region of interest and the identification of regions where the electromagnetic signal may be irretrievable modified or even destroyed (e.g. behind strong convective cores). Passing the quality control, which also contains a plausibility control, the reflectivity measurements are corrected for attenuation. Having quality controlled polarimetric weather radar measurements we continue with estimating the parameters of the raindrop size distribution within an observed radar resolution volume. This is accomplished by applying regression and variation techniques to the radar observables. Using these parameterised raindrop size distributions, we now obtain an estimate of the quantitative areal precipitation, which is compared with the results of the other meteorological instruments of the two test beds. The results of this study will be presented and summarised in the form of empirical relationships between the polarimetric weather radar measurements and the parameters of the raindrop size distribution.