Measurement Principles

Infrared radiometers operate at wavebands around 3.7, 10.5 and 11.5 ^m where the atmosphere is almost transparent. The brightness temperature measured from infrared radiometers differs from the actual temperature of the observed surface because of non-unit emissivity and the effect of the atmosphere. Emissivity at IR

frequencies is between 0.98 and 0.99 (close to a black body). Atmospheric correction is based on multispectral approach, when the differences between brightness temperatures measured at different wavelengths are used to estimate the contribution of the atmosphere to the signal. At 10 ^m, the solar irradiance reaching the top of the atmosphere is about 1/300 of the sea surface emittance. At 3.7 ^m, the incoming solar irradiance is the same order as the surface emittance. As a result, this wavelength can be used during nighttime only. Different algorithms are thus used for nighttime and daytime.

There is no IR way of measuring SST below cloud. The first priority is thus to detect cloud through a variety of methods. For cloud detection, the thermal and near-infrared waveband thresholds are used, as well as different spatial coherency tests. Consequences of poor cloud detection are low biases in SST climatic averages and "false hits" of cloud that can hide frontal and other dynamical structures. Geostationary infra-red sensors can see whenever the cloud breaks.

Microwave sensors operate at several frequencies. Retrieval of SST is done at 7 and/or 11 GHz. Higher frequency channels (19-37 GHz) are used to precisely estimate the attenuation due to oxygen, water vapor, and clouds. The polarization ratio (horizontal versus vertical) of the measurements is used to correct for sea surface roughness effects. The great advantage of microwave measurements compared to infra-red ones is that SST can be retrieved even through non-precipitating clouds, which is very beneficial in terms of geographical coverage.

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