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 cumuluscloud is presented. Measurements were made for 36 min near actual cloud top. The small andshallow cloud observed over the lower Alps in the summer of 1982 was intensively mixing withenvironmental air during its growth. A contour presentation of measured cloud droplet spectrashows their variability as related to the dynamical and thermodynamical properties. Twodistinctly different structures were observed, i.e., parts being rather homogeneous with respectto microphysics, and microphysically inhomogeneous parts. Microphysical homogeneity wasobserved in regions characterized by strong upward motion, water vapour saturation andpositive buoyancy. Microphysical inhomogeneity was connected with undersaturation andnegative buoyancies within upward moving parcels. An air mass budget estimation shows thatby the end of the observation time, about 85% of the cloud air near the top consisted of airentrained from the environment during growth. A time constant for turbulent mixing,estimated from observed maximal LWC values and vertical velocities, is of the order ofmagnitude of reported values.