Snow grain size determination from AVIRIS data over Arctic ocean

A snow grain size retrieval algorithm is described here that relies on the use of the data collected by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) at the wavelengths 0.86, 1.05, 1.24 and 1.73 micron. These typical wavelengths will be available on many satellite sensors and can be used to retrieve snow grain size. This algorithm is based on the principle that snow reflectance is mainly sensitive to grain size in the near-infrared (NIR) spectral range. A comprehensive radiative transfer model has been used to calculate the bidirectional snow-reflectance as a function of snow grain size. Comparing the retrieved grain sizes from four different AVIRIS channels, we found that the retrieved mean grain size decreased with increasing channel wavelength. The reason is that the penetration depth of radiation depends on wavelength. A theoretical snow reflectance calculation shows that the penetration depth decreases with inceasing wavelength, so that the grain size retrieved from the longer wavelength channel is smaller than that retrieved from a shorter wavelength channel. The 1.73 micron wavelength channel, available on several future sensors, is unsuitable for snow grain size retrieval because the signal comes from the snow surface and is very sensitive to the surface grain shape. Thus, the assumption of spherical particle becomes invalid, so that it becomes very difficult to estimate grain size.