A Comparison of Techniques to Estimate Vertical Air Motions and Raindrop Size Distributions

Doppler velocity spectra collected at vertical incidence contain information on vertical air motions and drop-size distributions with high spatial and temporal resolution. In the past, the computational interdependence between vertical air velocities and drop-size distributions has severely limited the accuracy with which they could be estimated. A dual-wavelength technique is applied in which vertical air motion is determined independently of the drop-size distribution. The Rogers reflectivity method and an extended version of the Hauser-Amayenc method are also applied. The latter technique fits Doppler spectra in a nonlinear least-squares sense, using two exponential drop-size distribution models. Results of applying each method to Oklahoma squall line data are compared, and the strengths and weaknesses of the three techniques are assessed. For the methods tested, there is a trade-off between potential accuracy and potential for successful application; e.g., the dual wavelength method is theoretically accurate but is extremely sensitive to poor data quality.