Optical and microwave response to thermally dynamic saline ice sheets

During CRRELEX´93, joint active optical and microwave measurements were made on thin saline ice sheets during growth of the ice sheet. One set of experiments was performed in an outdoor facility which was subjected to 25°C of thermal cycling (from 0° to -25°C). A second set was performed in an indoor facility with a controlled temperature of -18°C. The microwave response at 10 GHz (VV,HH,X) shows initial modulation due to frazil transition for the thin plate effect. The optical beam transmission at 532 nm (Nd:YAG) shows 1.8 dB loss per mm of increase in ice thickness. As thickness increased to 50 mm, diurnal thermal cycling caused large changes in the microwave backscatter, but only relatively small changes in optical transmission. Some of the observed impacts of thermal cycling are explained with the predicted joint optical and microwave response as derived from zeroth-order model calculations. Physical changes to the surface microlayer from absorbed solar radiation are suspected to have a dramatic effect on observed backscatter and albedo. For the indoor experiment, a correlation between the optical and microwave responses is observed