Brightness temperature of snow on lake ice in the 1.4 to 36.5 GHz range

We have investigated the brightness temperature behavior of snow on lake ice by conducting airborne radiometer measurements over two lakes in southern Finland since 2004 and, with a more intense schedule, since 2011. The flight path includes two lakes of different sizes and, additionally, a forested area and an open, mostly agricultural area next to the lakes. This allows us to compare the behavior of the brightness temperature of snow on lake ice with that of snow on terrain. The main airborne instrument is our HUTRAD system, which operates at 6.9, 10.65, 18.7, 23.8, and 36.5 GHz, providing data for both vertical and horizontal polarization at an incidence angle of 50 degrees off nadir. The antenna beamwidth is 3 to 5 degrees depending on frequency. Additional data have been collected with our HUT-2D interferometric imaging radiometer, which operates at 1.4 GHz and looks straight down covering an incidence angle range up to 25 degrees with a swath of 95 per cent of the flight altitude. The radiometers are accommodated on our Skyvan research aircraft. Insitu measurements have been made simultaneously with airborne data collection. We have covered a variety of conditions including dry snow/ice, occasional water on top of ice, moist snow surface, and the melting season. In order to study the effect of diurnal temperature variations during the melting season we have collected data both in the morning (dry refrozen snow) and afternoon (moist topmost snow layer).