Passive microwave signature of sea ice

From OceanWiki_EN

The electromagnetic properties of sea ice are a function of the physical properties of the ice (e.g., crystal structure, salinity, temperature, snow cover). The emissive character also changes depending on the microwave frequency, but in general, the passive microwave sea ice signal at a given frequency and polarization is distinct from that of open water. The unfrozen water surface is highly reflective in the microwave regime and thus is typically radiometrically cold; additionally, microwave emission from water is strongly polarized . When salt water freezes, the emissive character of the surface changes substantially. First-year sea ice (ice that has formed since the last melt season) is strongly emissive, but with the emission weakly polarized. As ice ages, saline brine trapped within the ice drains, particularly during summer melt, and the emissive character of the ice changes. Thus multiyear ice (ice that has survived at least one melt season) has a more complex radiative signature, generally with an emissivity and polarity between first-year ice and water. Snow cover on top of the sea ice can substantially modify the microwave signal by scattering of the emission from the underlying ice as well as through direct emission from the snow, particularly under melt conditions.

The atmosphere also emits microwave energy and at some frequencies this emission can substantially modify the signal received by the satellite under certain conditions. At frequencies used for sea ice retrievals, the atmospheric emission is primarily due to water vapor and liquid water. Thus high humidity, clouds, and especially rainfall, can result in emission that yields an opaque atmosphere at SSM/I frequencies. Fortunately, the polar regions are generally dry and, at least during the winter, water in the atmosphere is often in a frozen state (though supercooled liquid, which has a more substantial effect on the signal from the surface emission, is also common). Under melt conditions, significant emission will come from the liquid water (meltponds, wet snow) on the surface instead of the underlying ice. These effects can limit the accuracy of satellite-retrieved passive microwave ice products. While the summer melt effects can be substantial, on a basin-scale the surface and atmospheric effects are generally small during winter, allowing for an accurate representation of the overall ice conditions during the cold seasons In summer, such effects can substantially degrade sea ice concentration estimates, though estimates of sea ice extent are less affected.

Source: Meier, W. N. Evaluation of operational ssm/i ice-concentration algorithms 102-108.

URL http://www.ingentaconnect.com/content/igsoc/agl/2001/00000033/00000001/art00016.