Raindrop Breakup and Coalescence Diagnosed from Dual-Wavelength Vertically Pointing Radar Observations

This study uses three activities to bridge the research areas of radar remote sensing and precipitation process modeling. The first activity exploits Rayleigh and Mie scattering processes to retrieve vertical air motions and raindrop size distributions (DSDs) from observations made by 0.915-GHz (UHF band) and 35-GHz (Ka-band) vertically pointing radars deployed side-by-side at the Southern Great Plains (SGP) central facility. When both radars are observing the same raindrops, they measure different radial velocities. This velocity difference is due to Mie scattering from large raindrops influencing Ka-band observations such that the Ka-band measured velocity is less than the UHF band measured velocity. An outcome of this activity is a Rayleigh/Mie scattering retrieval methodology that estimates vertical air motion and three parameters of a gamma shaped DSD in the vertical column.

The second activity of this study applies the Rayleigh/Mie scattering retrieval method to over 100 hours of stratiform rain passing over the Department of Energy (DOE) Southern Great Plains (SGP) central facility UHF and Ka-band radars from 2011 through 2016. By aggregating observations to 1-minute intervals and assuming 10 m/s horizontal advection speed, retrievals and their uncertainties represent air motions and DSD parameters at approximately 600 m horizontal resolution and can be used to assess the representativeness of model parameterizations.

To bridge the radar remote sensing work with precipitation process modeling work, the third activity quantifies the vertical evolution of the retrieved falling raindrops in terms of liquid water content, total number concentration, and mass-weighted effective shape that incorporates both distribution size and breadth. Using vertical decomposition diagrams, changes in liquid water content with height quantify evaporation and accretion. When the raindrops are not evaporating, net raindrop breakup and coalescence are identified by changes in the total number of raindrops and changes in the DSD effective shape. Analysis of non-evaporating SGP stratiform rain events suggest that raindrops tend to coalescence as they fell with smaller raindrops decreasing in number and larger raindrops increasing in number. These remote sensing results pose an opportunity for the radar observational and modeling communities work together to determine if similar raindrop coalescence features occur in model simulations.

The proposed presentation will be based on the work published since the 2015 AMS Radar Conference:

Williams, C.R., 2016: Reflectivity and liquid water content vertical decomposition diagrams to diagnose vertical evolution of raindrop size distributions. J. Atmos. Oceanic Technol., 33, 579-595, doi: 10.1175/JTECH-D-15-0208.1.

Williams, C.R., R.M. Beauchamp, and V. Chandrasekar, 2016: Vertical air motions and raindrop size distributions estimated using mean Doppler velocity different from 3- and 35-GHz vertically pointing radars. IEEE Trans. Geosci. Remote Sens., 54, 6048-6060, doi: 10.1109/TGRS.2016.2580526.