Possibility to discriminate snow types using brightness temperatures in the thermal infrared wavelength region

Spectral emissivity of snow surface in the thermal infrared (TIR) wavelength region is an important parameter for monitoring snow surface temperature in cold climate regions and also for discriminating clouds and underlying snow surfaces in polar nights using satellite observed brightness temperature data. Past in-situ observations of snow emissivity revealed that the emissivity of snow surfaces varies depending on snow type [1]. Fine dendrite snow exhibits high emissivity over 0.98 in TIR at all exiting angles (θ). As ice granules of snow surface become large, the snow emissivity in TIR decreases and exhibits a wavelength dependence due to enhanced Fresnel reflectance at a wavelength around 12μm. Reduced snow emissivity is further enhanced as exiting angle increases. For example, emissivities of coarse grain snow at wavelengths of 11μm and 12μm are 0.99 and 0.975 for the zenith direction (θ=0°) but 0.965 and 0.93 for the slant direction of θ=75°. For sun crust snow, wavelength and directional dependences of snow emissivity are further enhanced. As the extreme case, emissivity of smooth bare ice can be approximated using the Fresnel reflectance theory. This snow type dependence of TIR emissivity as a function of wavelength and exiting angle is expected to make snow type discrimination possible using TIR brightness temperatures remotely sensed from space. In this study the possibility of snow type discrimination using TIR brightness temperatures is examined. Typical channels employed for satellite TIR image sensors are at wavelengths of 11μm and 12μm. Brightness temperature differences (BTD) at these two TIR channels (11μm-12μm) are calculated using the in-situ measured emissivities. The results showed that at the zenith direction the calculated BTD ranges from 0.5K for fine snow to 1.5K for bare ice, whereas the BTD ranges from 0.5K to over 2.3K at the slant direction of θ=60°. Thus, remotely sensed BTD ranges of around 1.0K at the zenith direction and 1.8K for the slant direction could be attributed to differences in snow surface type. Considering weak water vapor absorption in the atmosphere in polar regions, the BTD range is considered large enough to roughly discriminate snow surface type into three or four categories such as fine dendrite snow, middle to coarse grain snow, sun crust, and smooth bare ice. In this paper, TIR emissive properties of several snow types are also visualized using images taken with a portable thermography camera.