Vertical Air Motions and Raindrop Size Distributions Estimated Using Mean Doppler Velocity Differences

Scientists developed a new technique for combining measurements from two vertically pointing radars for cloud and precipitation studies.

The Science

How raindrops in a cloud break apart and coalesce affects the total amount and intensity of precipitation from the cloud, but these processes are not well represented in models. To improve modeling of raindrop breakup and coalescence processes, innovative retrieval techniques are needed to convert radar observations from the Department of Energy’s (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility into vertical columns of air motion and raindrop size distributions. A new retrieval technique developed by scientists supported under DOE’s Atmospheric System Research program exploits the Mie scattering signatures in the Ka-band ARM zenith radar (KAZR) to estimate air motion and raindrop size distributions.

The Impact

Vertically pointing radars observe the net raindrop radial motion, which is a combination of falling raindrops embedded in updrafts and downdrafts. It is difficult to isolate air motion from raindrop motion using a single radar. By using two radars operating at different frequencies (which have difference sensitivities to Rayleigh and Mie scattering), differences in radial velocities are due to differences in the shape of the raindrop size distributions, while similar variations contained in both radial velocities are due to vertical air motion variations.


A new retrieval technique estimating air motion and raindrop size distributions and associated uncertainties was developed and verified using 3- and 35-GHz radar observations collected during the Midlatitude Continental Convective Clouds Experiment (MC3E) field campaign at ARM’s Southern Great Plains atmospheric observatory. The retrieval technique can be applied to different pairs of ARM radar frequencies, including radar wind profiler (RWP) and KAZR (0.915 and 35 GHz), RWP and W-band (0.915 and 95 GHz), and KAZR and W-band (35 and 95 GHz). The ability to retrieve air motion and raindrop size distributions from multiple sites over many seasons will enable better understanding of the processes of raindrop breakup and droplet coalescence and improved model representations.

Principal Investigator(s)

Christopher Williams
Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder


This work was supported in part by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Atmospheric System Research program under grants DE-SC0007080 and DE-SC0014294. Further support came from the National Aeronautics and Space Administration Precipitation Measurement Mission under grants NNX13AF89G and NNX16AE42G.


Williams, C. R., R. M. Beauchamp, and V. Chandrasekar. 2016. “Vertical Air Motions and Raindrop Size Distributions Estimated Using Mean Doppler Velocity Difference from 3- and 35-GHz Vertically Pointing Radars,” IEEE Transactions on Geoscience and Remote Sensing 54(10). DOI: 10.1109/TGRS.2016.2580526.