The effects of anomalous atmospheres on the accuracy of infrared sea-surface temperature retrievals: Dry air layer intrusions over the tropical ocean

The effects of layers in the atmosphere with anomalously low moisture content on the accuracy of the sea-surface temperature (SST) derived from measurements of infrared radiometers on earth observation satellites are quantified using measurements taken from research cruises and numerical simulations. Radiosonde data from areas of the oceans that are seasonally affected by intrusions of dry air masses of continental origin were used with the Line-By-Line Radiative Transfer Model (LBLRTM) to simulate brightness temperatures measured in MODIS bands 31 and 32 (at wavelengths of ~ 11 μm and ~ 12 μm). The radiosonde datasets contained profiles with and without a dry layer, representing the baseline (no dry layer aloft) and the anomalous conditions (dry layer present). MODIS SST retrieval algorithm versions 5 and 6 were applied to the simulated brightness temperatures to obtain the SST and the retrieval errors were examined. Whereas the average errors for the baseline 'no dry layer' conditions range between 0.29 and 0.72 K, in the case of very deep dry layers the errors can be > 1 K. Simulations were also performed for atmospheric profiles that were created from the measured profiles by 'drying' layers at various altitudes. It was found that the retrieval errors 1) depend on the amount of water vapor in the atmosphere, 2) change in a systematic way dependent on the presence and characteristics of a dry layer, 3) dry layers in the lower troposphere make the SST retrieval errors more positive, and dry layers in the upper troposphere tend to introduce SST retrieval errors that are more negative.