The Relationship Between Microwave Brightness Temperature, Salinity, and Thickness of Sea Ice Acquired With a Tank Experiment

There is a strong need for measuring sea ice thickness with passive microwave radiometers onboard satellites. However, since the footprint size of passive microwave radiometers is 5–50 km, it is not easy to verify the possibility using satellite data. To study how the microwave brightness temperature and emissivity change with sea ice thickness and salinity, the authors have been performing a sea ice tank experiment for the past few years. The tank, which has a diameter of 2.6 m, was filled with seawater having a salinity of 32.5 ppt to a depth of 0.85 m. The tank was installed outdoors and covered with a hood to keep the surface from being covered by snow. The continuous measurement of microwave brightness temperature, salinity, and thickness of artificially grown sea ice in a tank was performed under subfreezing temperatures. The brightness temperature measurements were made at 7, 18, and 36 GHz for both vertical (V) and horizontal (H) polarizations using a portable microwave radiometer MMRS2 developed by MELOS. During the experiment, the sea ice thickness went up to 54.5 cm. The brightness temperature and emissivity measured by the potable microwave radiometer saturated at sea ice thickness of 3.0 cm for 36 GHz, 5.0 cm for 18 GHz, and 13.5 cm for 7 GHz. The relationship between PR and ice thickness as well as the relationship between GR and ice thickness were also examined. The result suggested the limitation and possibility of estimating sea ice thickness with passive microwave radiometers.