Observation of particle fall velocity in cirriform cloud by VHF and millimeter‐wave Doppler radars

Abstract

In this study, it is demonstrated that a combination of VHF and millimeter‐wave Doppler radars is a key tool for observing particle fall velocity in cirriform clouds. VHF (47‐MHz) and millimeter‐wave (95‐GHz) Doppler radars observed cirriform clouds at West Sumatra, Indonesia (0.2°S, 100.32°E) from 2000 LT 14 to 0800 LT 15 November 2005. Radar reflectivity factor (Ze) observed by the 95‐GHz radar showed that echoes from cloud particles had tops around 12–14 km and bottoms around 8–10 km. Doppler velocity observed by the vertically pointed beam of the 95‐GHz radar (Vair+Z) was compared with vertical air velocity (Vair) observed by the 47‐MHz radar to confirm that Vair+Z, a sum of Vair and reflectivity‐weighted particle fall velocity (VZ), showed consistent changes with Vair and hence VZ is able to be retrieved by subtracting Vair from Vair+Z. The correlation coefficient between VZ and Ze in the middle part of clouds (10.5–12.2 km) was −0.81, which was higher than that (−0.47) in the bottom part (7.2–10.5 km). The change of VZ for Ze in the middle part was larger (Ze = −31.9 VZ − 32.2) than that in the bottom part (Ze = −90.2 VZ − 71.8). These results suggest that particle size was a dominant factor that determined Ze in the middle part. Using VZ, median volume diameter (D0) was estimated to suggest that D0 was larger than ∼70 μm in the bottom part and ranged from ∼40 μm to larger than ∼106 μm in the middle part.