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

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 (V_air+Z) was compared with vertical air velocity (V_air) observed by the 47-MHz radar to confirm that V_air+Z, a sum of V_air and reflectivity-weighted particle fall velocity (V_Z), showed consistent changes with V_air and hence V_Z is able to be retrieved by subtracting V_air from V_air+Z. The correlation coefficient between V_Z and Z_e 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 V_Z for Z_e in the middle part was larger (Z_e = -31.9 V_Z - 32.2) than that in the bottom part (Z_e, = -90.2 V_Z - 71.8). These results suggest that particle size was a dominant factor that determined Z_e in the middle part. Using V_Z, 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.