AN OBSERVATIONAL AND THEORETICAL-STUDY OF THE RADIATIVE PROPERTIES OFCIRRUS - SOME RESULTS FROM ICE89

Observations of the microphysical and radiative properties of two cirr us clouds are reported. The measurements were taken from the C-130 air craft of the UK Meteorological Research Flight as part of the field ca mpaign of the International Cirrus Experiment, ICE'89. Cirrus microphy sical properties are analysed in terms of ice water content and a suit ably defined ice crystal effective radius'. Estimates of the ice water content are derived from both a standard Particle Measuring Systems 2 D-C cloud probe and a Lyman-alpha 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 conten ts derived from the TWC probe being consistently greater than those fr om the 2D-C probe. Evidence is presented which suggests that the diffe rences near the cloud base are due to a significant number of large (> 800 mum) crystals, which the 2D-C probe is unable to sample. Near the cloud top the differences are assumed to be due to small (< 100 mum) c rystals, which the 2D-C probe is again unable to sample correctly. Thu s more weight has been given to the TWC probe measurements of ice wate r content, with the measurements of ice crystal effective radius being adjusted accordingly. Model clouds, constructed from the microphysica l measurements, are used as input to a number of radiation parametriza tions, both in the long-wave and short-wave regions of the spectrum. I n 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 measur ed effective radii from one of the flights are too large to explain th e radiative characteristics of the cloud. The effect of long-wave scat tering is shown to be small. The inferred long-wave broad-band mass-ab sorption coefficient varies between 0.002 m2g-1 and 0.087 m2g-1, and i t shows a marked inverse dependence on the ice crystal effective radiu s, as anticipated by simple theoretical calculations. In the short wav e, agreement between the measured irradiances and those computed using several parametrizations is again reasonably good, although limited b y the natural variability of the clouds. The net radiation budgets of both cloud layers are analysed, and the possibility of the existence o f a relationship between ice water content and effective radius is con sidered.