Infrared emissivity of cirrus ‐ simultaneous satellite, lidar and radiometric observations

The infrared emissivity pattern of a large‐scale jet‐stream cirrus system has been mapped from Nimbus 4 satellite THIR (10·5‐12·5μm) data using simultaneous groundbased lidar (0·694μm) observations. The average cirrus emissivity was 0·28. Dense cells in the cirrus canopy were evident in both the infrared map and lidar backscatter time‐height profiles. The cells were about 10km to 50km in horizontal diameter and 1km deep and had liquid water contents of 0·2gm m−3 or greater. On occasion, the cells were ‘black’ to infrared radiation. Temporal variations of infrared emissivity (10–12μm) measured from the lidar site were in good agreement with spatial variations in the Nimbus THIR emissivity map. The frequency of occurrence, N, of a particular THIR cirrus emissivity ϵ followed the approximate distribution N = Nmax(1 − 1·1ϵ).