Near concurrent MIR, SSM/T-2, and SSM/I observations over snow-covered surfaces

Abstract

The airborne Millimeter-wave Imaging Radiometer (MIR) and MODIS Airborne Simulator (MAS) measurements over the Arctic region and the Midwest region of the US are used to derive surface emissivities ξ( ν) for three frequencies, ν=89, 150, and 220 GHz, as well as Normalized Difference Snow Index (NDSI) and R87 (0.87-μm reflectance). These derived parameters are compared with parameters estimated from near concurrent measurements made by the SSMI and SSM/T-2 over snow-covered areas. It is shown that the MIR-estimated ξ( ν) values at ν=89 and 150 GHz agree well with those estimated from the SSM/T-2 at ν=91 and 150 GHz, respectively. Low MIR-estimated ξ( ν) values are generally associated with high NDSI and R87 over the snow-covered areas. Over forested areas, more fluctuations in the values of MIR-estimated ξ( ν), NDSI and R87, as well as a reduction in polarization index (PI) at 37 and 85 GHz are observed. Both observations and results from radiative transfer calculations show a change in the difference between brightness temperatures ( T b) at 19 and 37 GHz, as well as PI at 37 and 85 GHz, when measured at satellite altitudes and at the surface. The amplitude of the T b difference and PI is reduced by about 10–15% from surface to high altitudes when integrated water vapor is ≤1.5 g/cm 2. This effect is readily correctable and requires consideration when validating satellite retrieval algorithms based on surface and low-elevation aircraft measurements.