Thermal Remote Sensing

Advanced Very High Resolution Radiometer (AVHRR)

Water thermal properties in any region can be changed by conservative processes (mixing and advection) and by nonconservative interactions with the atmosphere (insulation, back-radiation, evaporation, condensation, and conduction; Pickard and Emery 1990). If water advects rapidly, less time is allowed for heat to be exchanged with the atmosphere, and conditions are more conservative. If the waters adjacent to a current are of a different temperature, the flow patterns are easily observed in AVHRR data. For this reason, the strong western boundary currents such as the Kuroshio and Gulf Stream currents can be observed by AVHRR. The temperature itself cannot be used directly to indicate flow, but the thermal gradients are indicative of fronts maintained by flow characteristics (see Eddy Separation and Shedding from the Loop Current).

The satellite-measured skin surface temperature (SMSST) is that of the micron-thin skin of water on top of the ocean and may not be representative of the mixed layer beneath it. The skin temperature has been observed to be between 1.2°C cooler and 0.8°C warmer than the bulk temperature ( Wick et al. 1992). Multichannel sea surface temperature (MCSST) uses comparisons between satellite infrared measurements and drifting buoys in order to determine sea surface temperature ( McClain et al. 1985). The mean offset between SMSST and MCSST is approximately 0.45°C, but without more thorough in situ measurements, one cannot say which method is more accurate.

Surface isotherms can be indicative of the flow path of the water, similar to slowly changing streak lines. In addition to depicting general flow characteristics of the Loop Current, warm core rings and smaller eddies, meanders, and filaments can also be clearly observed. Temporal changes in MCSST imagery provide a means of inferring some characteristics of water parcel motion and the evolution of current trajectories (e.g., Maul 1977).

For more information on AVHRR go to http://edc.usgs.gov/products/satellite/avhrr.html

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