Raindrop Breakup and Coalescence Diagnosed from Raindrop Size Distributions Retrieved from Vertically Pointing Doppler Radars

The vertical evolution of falling raindrops is a result of evaporation, breakup, and coalescence acting upon those raindrops. Computing these processes using vertically pointing radar observations is a two-step process. First, the raindrop size distribution (DSD) and vertical air motion need to be estimated throughout the rain shaft. Then, the changes in DSD properties need to be quantified as a function of height. The change in liquid water content is a measure of evaporation, and the change in raindrop number concentration and size are indicators of net breakup or coalescence in the vertical column.

The DSD and air motion can be retrieved using observations from two vertically pointing radars operating side-by-side and at two different wavelengths. While both radars are observing the same raindrop distribution, they measure different reflectivity and radial velocities due to Rayleigh and Mie scattering properties. As long as raindrops with diameters greater than approximately 2 mm are in the radar pulse volumes, the Rayleigh and Mie scattering signatures are unique enough to estimate DSD parameters using radars operating at 3- and 35-GHz (Williams et al. 2016).

To explore the processes acting on the raindrops, the retrieved DSD parameters are described using liquid water content, total number concentration, and mass-weighted effective raindrop shape. Vertical decomposition diagrams (Williams 2016) are used to explore the processes acting on the raindrops. Specifically, changes in liquid water content with height quantify evaporation or 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 as the raindrops.

Analysis of DSD profiles retrieved from over 100 hours of non-evaporating stratiform rain passing over the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) central facility in Northern Oklahoma 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 remote sensing community to work with the modeling community to improve our understanding and modeling of evaporation, breakup, and coalescence processes.

References:

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.