The Microstructure of Cumulus Cloud. Part III. The Nature of the Updraft

Abstract

Results are given of detailed measurements of vertical air velocity in cumulus clouds of depth ranging from 0.7–4 km. Both peak updrafts and downdrafts and rms vertical velocity were found to increase with height above cloud base, the latter at a rate of about 0.7 m sec⁻¹ km⁻¹. The rms velocity also increased as the stability of the clear air environment of the cloud decreased, at a rate of about 0.3 m sec⁻¹ per °C km⁻¹. Maximum, upper decile and median peak updrafts were 12.7, 10.0 and 4.9 m sec⁻¹, respectively; corresponding values for downdrafts were 9.5, 7.5 and 3.5 m sec⁻¹. The structure obtained by combining measurements of horizontal and vertical air velocity at short intervals throughout a traverse showed well-defined overturning motions near the cloud top but not elsewhere. Power spectra are presented which show that most of the energy is at long wavelengths; peaks are also present at wavelengths of 500–600 m within the body of the cloud, but at 100–200 m near cloud base or cloud top.