Aircraft Observations of the Radiative and Microphysical Properties of Stratocumulus and Cumulus Cloud Fields

Abstract

This paper reports on a series of flights that were conducted off the east coast of Australia through and over stratocumulus and fair-weather cumulus cloud fields. The CSIRO Fokker F-27 research aircraft was used to obtain radiation and in situ cloud microphysical and thermodynamical measurements. Central to the analyses presented in this paper were the measurements obtained by a spectrally scanning visible near-infrared radiometer (SPERAD) which was designed specifically for the experiments reported herein. Analyses of the data obtained during the flights that are reported in this paper showed that the clouds sampled were warm and mainly maritime in character, with both low droplet concentrations and liquid water contents. The stratiform clouds were shallow, with optical depths of about 10. Despite the lack of cloud vertical development significant concentrations of large droplets were recorded by the Knollenberg 2D probe. Variance analyses of the cloud optical properties indicated that the sampled cloud layers possessed highly variable volume extinction coefficients with fractional deviations exceeding 0.5 at most levels, whereas the single-scattering albedo and the asymmetry parameter were more uniform along any given level. Variance analyses of the bidirectional reflected radiation from Sc clouds indicated a variability of cloud reflectance on two distinct horizontal scales, which could in turn be related to the scale of the relevant mixing processes. It was also found that the reflected radiances from cumulus clouds were far more anisotropic in character than thou reflected from stratocumulus clouds. The spectral variation of cloud reflectance with wavelength also exhibited features that, on the basis of the comparisons reported, could not be fully explained by existing theory.