Retrieval of Air Vertical Velocity and Droplet Size Distribution in Squall Line Precipitation Using C-FMCW Radar

Abstract

The zenith C-band frequency modulation continuous wave(C-FMCW) radar has good detection capability with high temporary-spatial resolution and large dynamic range. The Doppler spectral density data of two squall lines precipitation cases at Longmen of Guangdong are utilized to retrieve the air vertical velocity (Va) in clouds and droplet size distribution (DSD). The empirical relation method (checking relationship between mean particle falling velocity (Vt) and reflectivity factor) and the small-particle-trace method are explored to retrieve the air vertical velocity in clouds. And then the droplet size distribution is retrieved from the translated Doppler spectral density by a velocity-diameter relation. The retrieved DSD of two squall lines are then compared and validated with the observation of K-band micro rain radar and second-generation Parsivel disdrometer. The retrieved Va by the empirical relation method is slightly smaller for strong monomer than that by the small-particle-trace method and slightly larger for weak convective precipitation, but Vt is the opposite. The absolute value of Vt negative velocity by the empirical relation method and the small-particle-trace method corresponds to the moment of large particles and heavy rain observed by Parsivel disdrometer, indicating that Va of two methods is basically reliable. The comparison in DSD retrieval show that the number of small droplets observed by radar is higher, but Parsivel disdrometer may underestimate it. The results of the empirical relation method are closer to micro rain radar and Parsivel disdrometer when rain rate is below 1 mm·h-1. The medium droplets obtained by radar retrieval are consistent with Parsivel disdrometer measurements, but the concentration of large droplets is low when rain rate is stronger than 10 mm·h-1. The retrieval results of both methods are close to Parsivel disdrometer and micro rain radar when rain rate is between 1 mm·h-1 and 10 mm·h-1. The strong convection makes droplets rupture severer in the peak area of heavy precipitation in the squall line, resulting in smaller mass-weighted mean diameter (Dm) and larger generalized intercept parameter Nw of the empirical relation method and the small-particle-trace method retrieval. For weak convective precipitation at the back of the squall line, the value of empirical relation method is quite close to Parsivel disdrometer. Under different rain rate, μ value of C-FMCW radar is less than 10 and the fluctuation is smaller, indicating that the results of C-FMCW radar is even more reliable than Parsivel disdrometer and micro rain radar.

Keywords

• spectral density
• air vertical velocity
• droplet size distribution