Surface meteorological observations over Lake Malawi/Nyasa

Over-lake surface meteorology is essential for the estimation of lake evaporation and for the forcing of lake water quality and circulation models. A roving meteorological station was mounted aboard the research vessel, R/V Usipa, on Lake Malawi/Nyasa. Ship velocity and position were recorded, thus permitting winds to be corrected aboard the moving platform. This type of data is particularly sparse for tropical lakes. Of the eight full-lake cruises over the period from February, 1997 to May 1999, four yielded usable data comprising 3,316 30-min averages but restricted mainly to the wet season. In addition to winds, air temperatures, relative humidities, and water temperatures were recorded. Similar parameters were measured concurrently at up to four fixed locations on the shoreline providing a basis for comparison. An examination of the longest running series of winds and air temperatures showed no obvious inter-annual differences in wind speed and air temperature. Based on wind spectra, winds were divided into a diurnal component and a longer term smoothed component by digital filtering. Analysis of the shipboard winds shows an unexpectedly strong diurnal wind field in the open lake which appears to be dominated by the diurnal atmospheric circulation along the eastern shoreline. This effect is likely due to the interaction between southeast trade winds and the diurnal wind field. The long-term smoothed winds vary along the longitudinal axis of the lake, being weakest at the extremities and strongest in the middle. Calculations of an average evaporation rate based on observed meteorological data from all temporal scales using three methods resulted in a mean of 6.4 +/- 1 mm/d. Diurnal meteorological fluctuations accounted for 36% of the total evaporation.