Determination of Effective Emittance and a Radiatively Equivalent Microphysical Model of Cirrus from Ground-Based and Satellite Observations during the International Cirrus Experiment: The 18 October 1989 Case Study

Abstract

Ground-based observations and satellite data have been compared for the18 October 1989 case study of the International Cirrus Experiment (ICE)field campaign. They correspond to thin cirrus clouds with infraredemittances in the range 0-0.3. Good correspondence was obtained whencomparing the time variability of the effective downward beam emittanceof the cirrus clouds observed at Nordholz (53.8 degrees N, 8.3 degreesE) to the spatial variability of the effective upward beam emittancederived from NOAA-II Advanced Very High Resolution Radiometer (AVHRR)data acquired at 1225 UTC. A simple model of cirrus cloud particles wasfound to satisfy both the ground-based observations of the angulardependence of the scattered solar radiation at 0.85 mu m and thesatellite observations of the brightness temperatures in channel 4 (11mu m) and channel 5 (12 mu m) of NOAA-II AVHRR. The best fit wasobtained for fully randomly oriented hexagonal ice plates with athickness of 10-20 mu m and a diameter of 200-500 mu m. Although actualcloud ice crystals are probably not all hexagonal plates, our simplemodel of randomly oriented ice plates allows us to appropriatelysimulate the optical properties of the observed cirrus in whichparticles surely present a large variety of shapes. The equivalentradius of the retrieved ice plates (i.e., the radius of spheres of thesame volume) is 50-80 mu m. However, ice spheres do not simulate thehalo of cirrus clouds observed from the aureolemeter measurements.Moreover, assuming spherical particles to explain brightness temperaturemeasurements in AVHRR channels 4 and 5 leads to an effective radius of27 mu m, which is noticeably smaller than the one obtained with thehypothesis of hexagonal plates.On the other hand, analysis of AVHRR data also highlights the importantdifference between natural thin cirrus and jet contrail microphysics.Contrails are revealed td be composed of smaller equivalent sphericalparticles with an effective radius of about 4.5 mu m.