Height-resolved Raindrop Size Distributions Retrieved from Vertically Staring Coherent Doppler Lidar

Coherent Doppler lidars have proven to be useful tools for the study of wind and turbulence in the atmospheric boundary layer under clear-air conditions due to their sensitivity to aerosol backscatter and ability to provide highly accurate line-of-sight velocity information. These instruments are also quite sensitive to backscatter from hydrometeors, which could be exploited to retrieve precipitation properties. However, this capability has not been explored as thoroughly. In this study, we use observations from a vertically staring Doppler lidar (Halo XR+) to retrieve normalized rain Drop Size Distributions (DSDs) from observed lidar Doppler spectra. Results from the lidar’s lowest valid height bin are compared to a pair of surface-based laser disdrometers (OTT Parsivel²). These observations were obtained during the American WAKE experimeNt (AWAKEN) in Oklahoma and include multiple precipitation events from 4 May to 5 July 2023.

When both aerosol and raindrops are present in the lidar’s sample volume the Doppler spectra can exhibit bi- or multi-modal structures due to the velocity differences between the aerosol and the drops. The aerosol “mode” exhibits a very repeatable structure that can be accurately modeled using a modified Gaussian function. Terminal fall speeds are obtained by shifting the Doppler spectrum so that the peak of the aerosol mode occurs at 0 m s^-1. Subtraction of the aerosol mode from the shifted Doppler spectrum provides an estimate of the precipitation backscatter spectrum. An empirical relationship between terminal fall speed and drop diameter is then used to represent the precipitation backscatter spectrum as a function of drop diameter. Normalizing that result by the differential backscatter cross section for a water drop yields a curve that is proportional to the DSD at a given altitude. For this study, Mie scattering theory was used to compute the differential backscatter cross section of spherical water drops (at the laser wavelength) for diameters ranging from 0.2 to 10 mm.

Comparisons between lidar- and disdrometer-derived DSDs show good agreement in the location of the primary modes, but the lidar results exhibit slightly more broadening, as expected. This presentation summarizes the lidar DSD retrieval technique and presents results from several rain events. Differences between the lidar- and disdrometer-derived DSDs are also discussed.