A case study of cirrus layers with variable 3.74 mu m reflection properties in the first FIRE experiment, 2 November 1986

Remotely sensed scanning radiometer and lidar data on cirrus clouds were obtained during the cirrus FIRE IFO experiment in November 1986 from the ER-2 aircraft platform. Data were examined particularly on 2 November for an area in the vicinity of Wausau, Wisconsin where unusual effects were noticed in bispectral histograms from various channels in the scanner data. After calibration of the data in spectral channels of both the Scan Cloud Radiometer (SCR) and Multichannel Cloud Radiometer (MCR) instruments, including direct comparison between compatible channels in the two instruments, it was found that the 0.856 mu m SCR channel gave good data, whereas the 0.665 mu m and 0.74 mu m SCR channels gave large offsets, when compared with the MCR 0.754 mu m data. The latter channel was found to compare well in a second comparison with coincident AVHRR channel satellite data. Similarly, the SCR 11.17 mu m data gave consistent results and the SCR 3.74 mu m data were carefully calibrated. Bispectral histograms formed between 0.856 mu m, 3.74 mu m and 11.17 mu m SCR channel data indicated that some coherent layers of cirrus clouds were giving enhanced solar reflectance at 3.74 mu m, indicative of small (similar to <25 mu m radius) particles, whereas other neighbouring layers gave little reflectance. A comparison of 0.856 mu m reflections with 11.17 mu m absorption optical depth indicated that the small particles where probably ice crystals. A comparison of 3.74 mu m solar albedo and 11.17 mu m absorption optical depths of these layers with theoretical calculations for ice spheres indicated a mode radius of about 8 mu m for the cloud particle size distribution. An estimate from similar recent calculations on hexagonal ice crystals indicated that the retrieved effective radius would be increased to 25 mu m. The difference between the two retrieved radii was a measure of the uncertainty in the retrievals, considering also differences in the assumed size distributions. Qualitative comparison with ER-2 lidar data gave a tentative identification of the reflecting layers. The results demonstrate the power of the 3.74 mu m channel for identification of small-particle layers in cirrus.