Microphysical, thermodynamical and dynamical properties as observed in the upper part of a small growing warm cumulus cloud

Abstract

A detailed aircraft case study of the upper part of a growing, warm, nonprecipitating cumulus cloud is presented. Measurements were made for 36 min near actual cloud top. The small and shallow cloud observed over the lower Alps in the summer of 1982 was intensively mixing with environmental air during its growth. A contour presentation of measured cloud droplet spectra shows their variability as related to the dynamical and thermodynamical properties. Two distinctly different structures were observed, i.e., parts being rather homogeneous with respect to microphysics, and microphysically inhomogeneous parts. Microphysical homogeneity was observed in regions characterized by strong upward motion, water vapour saturation and positive buoyancy. Microphysical inhomogeneity was connected with undersaturation and negative buoyancies within upward moving parcels. An air mass budget estimation shows that by the end of the observation time, about 85% of the cloud air near the top consisted of air entrained from the environment during growth. A time constant for turbulent mixing, estimated from observed maximal LWC values and vertical velocities, is of the order of magnitude of reported values.