(See also Earth Surface Temperature retrieval from ATSR data)

The ATSR SST retrieval scheme has been implemented at the Rutherford Appleton Laboratory (UK). (Zavody et al., 1995). The scheme is based on the 11 and 12 micron spectral channels in day time whereas at night the 3.7 micron spectral channel is also used in addition to them.

In order to retrieve the SST from ATSR Brightness Temperature (BT) a radiative transfer model has to be used, to take into account for absorption/emission/scattering processes of the radiation, taking place in the atmosphere.

The radiative transfer model used in the retrieval scheme has been validated using AVHRR BT data and concomitant in-situ SST measurements. SST accuracy figures appear comparable, if not better, than those obtained by using the algorithm implemented at the NOAA. (Llwellyn-Jones et al., 1984).  

The radiative transfer model includes water vapour, varying with the height, uniformly mixed gases like CO₂, O₃, CH₄, NO, NH₃, etc. as well as marine aerosols, confined to the first kilometre of the troposphere.

Atmospheric gases act as absorbers while in the case of aerosols scattering processes are also taken into account; scattering by aerosols is modelled basically as an extinction process. Clouds are not included in the model because SST retrieval is only feasible over cloud-free pixels.

A selection of atmospheric temperature and humidity profiles has been used to work out BT values at the top of the atmosphere. In order to reduce the computational effort, the computations have been carried out at 8 pressure levels and 3 temperature profiles, and at frequency intervals of 0.04 cm^-1. The absorption is computed at the required pressure/temperature and scaled to the given amount of each absorber. Total absorption per unit path length (including scattering extinction) is then computed and the results are stored in a database. As the atmospheric path varies along the swath, depending on the satellite view angle, 10 angles have been considered for deriving it. The BT values from the database are then used for deriving the coefficients of a linear combination as mentioned below.

SST is expressed as a linear combination of 2-3 BTs (3 spectral channels are considered at night), derived from, both, the nadir view and the nadir/forward views. The coefficients are computed so that the variance of the error is a minimum (the variance of the error is the sum of the squares of the differences between the SSTs, derived from the linear combination, and the SST derived from the atmospheric profiles).

The retrieval procedure presents some limitations due to the simplifications underlying the atmospheric model and to the limited number of the atmospheric profiles used in the computation of the coefficients. Some of the simplifications are: the fixed amount or distribution of the atmospheric constituents and of the aerosols, the constant value of the sea surface emissivity and the accuracy of the water vapour absorption coefficients (in the continuum spectral region). Various sources of error are discussed in Zavody et al. (1995). Several validation studies have been carried out since the beginning of the first ERS mission; they have shown a good accuracy of the ATSR SST product.

(See also ATSR SST validation studies)

Llwellyn-Jones, D.T., P.J. Minnet, R.W. Saunders, and A.M. Zavody, "Satellite multi-channel infrared measurements of the surface temperature of the N.E. Atlantic Ocean using AVHRR/2, Q.J.R. Meteorol. Soc., 110, 613-631, 1984.

Zavody, A.M., Mutlow, C.T., and Llewellyn-Jones, D.T., 1995, "A radiative transfer model for sea surface temperature retrieval for the along-track scanning radiometer", J. Geophys. Res., 100, C1, 937-952.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry