Sea Ice Emissivities and Effective Temperatures at MHS Frequencies: An Analysis of Airborne Microwave Data Measured During Two Arctic Campaigns

Satellite-based sounding of the temperature and humidity of the lower troposphere is only carried out over open sea surfaces because of large uncertainties in the surface emissivity and effective emitting temperature of other surfaces. The study of sea ice and snow surface emissivities at Microwave Humidity Sounder (MHS) frequencies has been the focus of two airborne campaigns carried out by the Met Office over the past decade to promote the assimilation of lower tropospheric sounding radiances over the polar regions. The Polar Experiment (POLEX) campaign occurred during March 2001. Five flights were carried out over the Arctic Ocean north of Svalbard. The Cold Land Processes II (CLPX-II) campaign took place during February 2008. The sea ice portion of CLPX-II consisted of four flights sampling the emissivities and effective temperatures in the Chukchi and Beaufort Seas and frozen wetlands near Barrow, Alaska. This paper summarizes the findings of a recent analysis from the CLPX-II and POLEX campaigns. First, time series of Lambertian emissivities and effective temperatures at MHS frequencies are retrieved for each flight. These time series and the satellite imagery are then used to classify the surfaces over flown. The behaviors of the emissivity spectra with changing surface type are discussed in terms of changes in the ice and snow conditions on the surface. The difference between 89- and 157-GHz emissivities is found to be related to both the snow depth and the relative amounts of depth hoar and wind slab within the snowpack.