Retrieving the Median Volume Diameter with a Polarimetric Cloud Radar

Abstract

The knowledge of the raindrop size distribution is key for characterizing precipitation. It is however still a challenge to retrieve it with radars. Several polarimetric and spectral techniques are proposed for cm-wavelength radars (weather radars). What about the mm-wavelength radars (cloud radars), which have a better spatial and time resolution and can still measure light and moderate rain? Knowing that 90% of the rain volume in Europe comes from rainfall rates between 0.1 mm/h and 10 mm/h, this is worthwhile to investigate. The goal of this thesis is to retrieve 1 of the 3 parameters of the modelled gamma raindrop size distribution, the median volume diameter (D₀), during stratiform rainfall events using a slantwise profiling dual-frequency polarimetric cloud radar. Focus is given to phase measurements, which are not affected by attenuation. Simulations show that the differential backscatter phase (δ_co) strongly depends on D₀. At mm-wavelength, backscattering and propagation effects need to be disentangled first. To achieve this, an algorithm to detect and characterize Rayleigh plateaus is proposed and implemented. After the application of this algorithm, a methodology to estimate the differential backscatter phase and its error is given. The 95% confidence interval of δ_co is estimated with the re-sampling method bootstrapping. 
Using simulation results, an attempt is made to find combinations of D₀ and the raindrop size distribution shape parameter μ that match with the confidence interval of δ_co. The confidence interval of δ_co restricts D₀, but not μ in most cases. This proposed technique is applied for both the 35 and 94 GHz frequency band of the new cloud radar at Cabauw (Ruisdael Observatory site near Utrecht). The resulting 95% confidence intervals of D₀ with 35 and 94 GHz and their overlap are compared with in-situ disdrometer measurements of the mass-weighted mean diameter (D_m) which is closely related to D₀. The median volume diameter retrieved with the 35 and 94 GHz frequency bands both shows a normalized cross correlation coefficient of 0.845 with the measured D_m of the disdrometer. Therefore, the cloud radar seems to have the capability to provide the detailed variations of the raindrops mean/median diameter like a local disdrometer, but at different heights. Nonetheless, the values differ. The disdrometer provides higher values than the cloud radar. One possible explanation is the inability of the disdrometer to measure raindrops smaller than 0.25 mm and the expected underestimation of the number of raindrops with sizes between 0.25 and 0.375 mm. However, because D₀ values retrieved from 35 GHz data are also higher than the ones at 94 GHz, further research, which can use all the methodologies proposed in this master thesis work, is needed to examine the quantitative values of the median volume diameter retrieval. 
These techniques can be implemented for all the single-frequency cloud radars (94 GHz) of the national Ruisdael Observatory (cloud and precipitation profiling mobile station and Lutjewad site above Groningen).