Ground based passive remote sensing of ice clouds with scattered solar radiation in the near infrared

Abstract

The feasibility of ground based passive remote sensing in the near infrared for the estimation of optical properties of thin ice clouds is examined and confirmed. The radiative transfer through ice clouds is simulated as a function of crystal size, crystal shape and optical depth under the assumption of single scattering. A method is derived to estimate the effective radius r(eff) for optically thin ice clouds from radiances at wavelengths of 1046 and 1550 nm. The results are not dramatically different if one assumes hexagonal crystals or surface equivalent spheres. The estimation of the ice cloud optical depth depends strongly on particle shape, leading to a less reliable estimate. Ground based measurements of scattered near infrared radiances with the array spectrometer OVID have been used to verify and to apply the derivation technique. The evaluation of measurements for different ice cloud types and contrails mostly yields particle sizes between 30 and 200 μm. Occasionally they are as small as 1.6 μm or as large as 600 μm. The known increase of particle size with cloud base temperature was confirmed. Contrails prove to contain small crystal sizes that increase with age to the values found in the surrounding cirrus clouds. A systematic size reduction in contrail clouds older than 30 minutes is indicated but cannot be conclusively proven by the present data.