Derivation and validation of the seasonal thermal structure of Lake Malawiusing 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 o f the lake are principally controlled by the lake's thermal structure, whic h modulates the mixing of nutrient-rich deep water with that of the phytopl ankton-rich near-surface layer. Satellites potentially offer an efficient, low cost method of providing information on the lakes thermal structure ove r the longer term via remote sensing observations of lake surface temperatu re. Here we investigate the accuracy of remotely sensed lake surface temper atures derived using data from the NOAA-11 AVHRR over a two-year period (19 92-1993). Optimised triple window atmospheric correction algorithms are sho wn to provide an accuracy of around 0.5 degreesC when compared to in situ w ater temperatures. The effect of the 1994 switch in operational night-time satellite from NOAA-11 to NOAA-14 is assessed using modelling of the transf er of radiation through the Malawian atmosphere, combined with detail on th e differences in the satellite spectral response functions. These simulatio ns indicate that lake surface temperatures derived from NOAA-14 are warmer than those that would be derived from NOAA-11 under the same conditions. Th e 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 inf ormation relevant to the lakes 3-D thermal structure. Evaluations of the an nual mixing regime of the lake can be based on this information, this mixin g being directly relevant to the seasonal variations in lake primary produc tion.