An observational and theoretical study of the radiative properties of cirrus: Some results from ICE'89

Abstract

Observations of the microphysical and radiative properties of two cirrus clouds are reported. The measurements were taken from the C-130 aircraft of the UK Meteorological Research Flight as part of the field campaign of the International Cirrus Experiment, ICE'89. Cirrus microphysical properties are analysed in terms of ice water content and a suitably defined ice crystal ‘effective radius’. Estimates of the ice water content are derived from both a standard Particle Measuring Systems 2D-C cloud probe and a Lyman-α total water content (TWC) hygrometer. With both flights large discrepancies are seen to occur between the two instruments near the cloud base and top, with the ice water contents derived from the TWC probe being consistently greater than those from the 2D-C probe. Evidence is presented which suggests that the differences near the cloud base are due to a significant number of large (>800 μm) crystals, which the 2D-C probe is unable to sample. Near the cloud top the differences are assumed to be due to small (<100μm) crystals, which the 2D-C probe is again unable to sample correctly. Thus more weight has been given to the TWC probe measurements of ice water content, with the measurements of ice crystal effective radius being adjusted accordingly. Model clouds, constructed from the microphysical measurements, are used as input to a number of radiation parametrizations, both in the long-wave and short-wave regions of the spectrum. In the long wave, model agreement with both narrow-band and broad-band measurements is reasonably good, although discrepancies exist near the cloud tops. Some evidence is presented which suggests that the measured effective radii from one of the flights are too large to explain the radiative characteristics of the cloud. The effect of long-wave scattering is shown to be small. The inferred long-wave broad-band mass-absorption coefficient varies between 0.002 m2g-1 and 0.087 m2g−1, and it shows a marked inverse dependence on the ice crystal effective radius, as anticipated by simple theoretical calculations. In the short wave, agreement between the measured irradiances and those computed using several parametrizations is again reasonably good, although limited by the natural variability of the clouds. The net radiation budgets of both cloud layers are analysed, and the possibility of the existence of a relationship between ice water content and effective radius is considered.