Observations, Evaporation and Preliminary Modelling of Over-Lake Meteorology on Large African Lakes

Abstract

Water quality models of lakes require accurate specification of the advective and turbulent transport fields. These are usually obtained from lake hydrodynamic models. In turn, hydrodynamic models require accurate specification of meteorological forcing. Uncertain specification of meteorological forcing over large lakes is one of the main reasons for the lack of correspondence between three-dimensional hydrodynamic models and observations of lake currents, temperatures and water levels. This is especially the case for intermontane lakes where sheltering effects of the surrounding topography disturb the air flow and generate such other mesoscale meteorological features as slope winds which can reinforce lake breezes. Direct observations of meteorological variables on lakes are sparse in the tropics. We present here the results of such observations for Lakes Malawi/Nyasa and Tanganyika. During 1998–1999 a roving meteorological station was mounted aboard the research vessel, R/V Usipa, on Lake Malawi/Nyasa. Ship velocity and position were recorded, thus allowing winds to be measured aboard the moving platform. On Lake Tanganyika similar data were recorded at two moored meteorological buoys for substantial periods over a period of four years. An examination of the longest running series of winds and air temperatures over Lake Malawi/Nyasa showed no obvious interannual differences in wind speed although air temperatures in the second half of 1999 were cooler than in the same period in 1998. On Lake Tanganyika wind speeds decreased between 1993 and 1996 but air temperatures were highest in 1995. Based on spectral analysis, both lakes illustrate a strong diurnal signal of wind components and air temperatures. Calculations of an average evaporation rate for Lake Malawi/Nyasa based on observed meteorological data from all temporal scales and three different calculation methods resulted in a mean of 6.4 ± mm/d. Diurnally fluctuating meteorological conditions accounted for 36% of the total evaporation. Wet and dry season evaporation rates were compared for the two extremities of Lake Tanganyika and found to be higher in south and during the dry season. Preliminary results of an application of a three-dimensional mesoscale meteorological model to Lake Malawi/Nyasa are compared to direct over-lake observations of a number of forcing parameters required by hydrodynamic models. Comparisons of over-lake winds show that modeled winds are superior by three statistical measures to those interpolated from a limited number of shore-based stations.