Using Differential Doppler Velocity from Vertically Pointing S- and Ka-band Radars to Estimate Vertical Air Motion and Raindrop Size Distributions

When Doppler radars are pointed vertically, the measured mean Doppler motion is the sum of two components: the mean reflectivity-weighted raindrop fallspeed plus the vertical air motion. Interestingly, the reflectivity-weighted raindrop fallspeed is dependent on the radar operating frequency with the precipitation backscattering regime changing from Rayleigh to non-Rayleigh scattering as the operating frequency increases. Thus, two vertically pointing radars operating side-by-side but operating in the Rayleigh and non-Rayleigh regimes will have different mean Doppler velocities. This differential Doppler velocity (DDV) is dependent on the shape of the raindrop size distribution (DSD) and is independent of DSD number concentration, reflectivity, and attenuation. Also, since both radars are observing the same hydrometeor distribution, coordinated variations in Doppler velocity observed by both radars are due to changes in vertical air motion. After accounting for wet radome attenuation effects, the difference in measured reflectivities at both frequencies provides another constraint on the DSD shape and provides a gate-by-gate attenuation estimate through the rain. Thus, profiles of Doppler velocities and measured reflectivities from two side-by-side radars operating in the Rayleigh and non-Rayleigh precipitation scattering regimes provides enough information to estimate the vertical air motion and three DSD parameters (intensity, mean diameter, and breadth) throughout the rain column.

A retrieval technique was developed using 2.8 GHz (S-band) and 35-GHz (Ka-band) radar observations collected during the Mid-latitude Continental Convective Cloud Experiment (MC3E) held in Northern Oklahoma in April-June 2011. The retrieved DSDs and vertical air motions are within the retrieval errors estimated from retrievals using the co-located 449-MHz vertically pointing radar, but have a temporal resolution of 7-second versus the 45-second 449-MHz radar resolution. One benefit of this retrieval technique is that it provides a vertical air motion estimate without directly observing the air motion using Bragg scattering signals which are used with lower frequency wind profiling radars (e.g. 50, 449, and 915 MHz).