Effect of atmospheric radiance errors in radiometric sea-surface skin temperature measurements.

Errors in measurements of sea-surface skin temperature (SSST) caused by inappropriate measurements of sky radiance are discussed; both model simulations and in situ data obtained in the Atlantic Ocean are used. These errors are typically caused by incorrect radiometer view geometry (pointing), temporal mismatches between the sea surface and atmospheric views, and the effect of wind on the sea surface. For clear-sky, overcast, or high-humidity atmospheric conditions, SSST is relatively insensitive (<0.1 K) to sky-pointing errors of ?10 degrees and to temporal mismatches between the sea and sky views. In mixed-cloud conditions, SSST errors greater than ?0.25 K are possible as a result either of poor radiometer pointing or of a temporal mismatch between the sea and sky views. Sea-surface emissivity also changes with sea view pointing angle. Sea view pointing errors should remain below 5 degrees for SSST errors of <0.1 K. We conclude that the clear-sky requirement of satellite infrared SSST observations means that sky-pointing errors are small when one is obtaining in situ SSST validation data at zenith angles of <40 degrees . At zenith angles greater than this, large errors are possible in high-wind-speed conditions. We recommend that high-resolution inclinometer measurements always be used, together with regular alternating sea and sky views, and that the temporal mismatch between sea and sky views be as small as possible. These results have important implications for the development of operational autonomous instruments for determining SSST for the long-term validation of satellite SSST.