Raindrop size distribution retrievals from a VHF boundary layer profiler

Abstract

The retrieval of raindrop size distributions (DSDs) in precipitation using boundary layer wind profiler operating at VHF is described. To make the retrievals, a Fourier transform–based deconvolution technique, optimized to run with little human input, is used. The sensitivities of the technique and its overall accuracy are investigated using simulated spectra. The retrievals have an error that depends on the drop diameter, with relative errors varying between ∼10% and 35%. An overall average negative bias of about ∼20% is also found. The magnitude and direction of this bias depend on the spectral width of the input spectrum. The radar and methodology are applied to a case study of a convective cell. Retrievals are made with ∼300 m resolution between 800 and 4600 m. The temporal resolution is 2 min. Comparisons with a rain gauge show that both the magnitude and timing of the precipitation are well captured by the radar. The relationship between the observed rain rate and exponential fits applied to the DSDs agrees very well with previously published studies. A careful analysis of the characteristics of the DSDs within the descending rainshafts provides direct observations of drop size sorting within the precipitation and the formation of hybrid DSDs formed by the overlapping of consecutive rainshafts. This study highlights the potential of the boundary layer profiler in precipitation studies. Some drawbacks exist, such as the wide beam of the radar, which increases the spectral width of the radar and limits its use in windy conditions. However, when observations are available, they appear to be of high quality and fill a gap in observations unavailable to more conventional wind profilers. In the future, it is hoped that refinements in the technique will allow the temporal resolution of the radar to be increased and the quality of the retrievals to be improved.