Derivation and validation of the seasonal thermal structure of Lake Malawi using multi-satellite AVHRR observations

Lake Malawi is the second largest lake in Africa by volume and an important regional source of food. Seasonal fluctuations in the primary production of the lake are principally controlled by the lake's thermal structure, which modulates the mixing of nutrient-rich deep water with that of the phytoplankton-rich near-surface layer. Satellites potentially offer an efficient, low cost method of providing information on the lakes thermal structure over the longer term via remote sensing observations of lake surface temperature. Here we investigate the accuracy of remotely sensed lake surface temperatures derived using data from the NOAA-11 AVHRR over a two-year period (1992-1993). Optimised triple window atmospheric correction algorithms are shown to provide an accuracy of around 0.5 degreesC when compared to in situ water temperatures. The effect of the 1994 switch in operational night-time satellite from NOAA-11 to NOAA-14 is assessed using modelling of the transfer of radiation through the Malawian atmosphere, combined with detail on the differences in the satellite spectral response functions. These simulations indicate that lake surface temperatures derived from NOAA-14 are warmer than those that would be derived from NOAA-11 under the same conditions. The magnitude of the temperature difference is estimated at 0.27 degrees+/-0.07 degreesC, depending on the viewing zenith angle. Finally, we illustrate the ability of the remotely derived surface temperature maps to provide information relevant to the lakes 3-D thermal structure. Evaluations of the annual mixing regime of the lake can be based on this information, this mixing being directly relevant to the seasonal variations in lake primary production.