SATELLITE REMOTE-SENSING OF THE OPTICAL DEPTH AND MEAN CRYSTAL SIZE OF THIN CIRRUS AND CONTRAILS

Crystal size and optical depth of optically thin cirrus clouds and contrails over the North Sea and Adriatic Sea on the 18(th) of October 1989 are retrieved by comparison of NOAA AVHRR/2 brightness temperatures of channel 4 (9.97 mu m-11.56 mu m) and channel 5 (11.075 mu m-12.76 mu m) with one dimensional radiative transfer calculations. Measured brightness temperatures in all three infrared channels and their differences show higher values for contrails than for cirrus. The radiative properties of young contrails are consistent only, if smaller crystal size than those given for natural cirrus are adopted for the calculations. However, there is a continuous transition in radiative parameters between clouds classified as natural cirrus or contrails. For the test areas ice clouds are classified with respect to optical depth and mean crystal size. Finally infrared fluxes and heating rates in the spectral range 4 mu m-40 mu m are calculated for an atmosphere with a 500 m thick contrail or cirrus uncinus. At given ice content a far stronger atmospheric warming is found for a contrail with relatively small ice crystals: up to 80 K/day at cloud base for an ice content of 0.05 gm(-3) compared to 10 K/day for a cirrus uncinus with large crystals.