A Comparison of Vertical Velocity in Cirrus Obtained from Aircraft and Lidar Divergence Measurements during FIRE

Abstract

Techniques are presented to obtain vertical velocity in cirrus clouds from in situ aircraft lateral wind measurements and from ground-based remote Doppler lidar measurements. In general, direct measurements of absolute vertical velocity w from aircraft are currently not feasible because of offsets in the air velocity sensors. An alternative to direct measurement is to calculate w from the integral of the divergence of the horizontal velocity around a closed path. We discuss divergence measurements from both aircraft and Doppler lidar. The principal errors in the calculation of w from aircraft lateral wind measurements are bias in the lateral wind, ground speed errors, and error due to vertical shear of the horizontal wind. For Doppler lidar measurements the principal errors are in the estimate of mean terminal velocity and the zeroth order coefficient of the Fourier series that is fitted to the data. The technique is applied to a cirrus cloud investigated during the FIRE (First International Satellite Cloud Climatology Regional Experiment) Cirrus Intensive Field Observation Program. The results indicate that the error in w is about ±14 cm s?1 from the aircraft technique. We show that this can be reduced to about ±2 to 3 cm s?1 with technical improvements in both ground speed and lateral velocity measurements. The error in w from Doppler lidar measurements, which is about ±8 cm s?1, can be reduced to about ±5 cm s?1 by improvements in the Doppler velocity measurements with technology that is currently available.