WATER-SURFACE TEMPERATURE CHARACTERISTICS AND THERMAL BAR EVOLUTION DURING SPRING IN LAKE LADOGA

The present article is a study of the surface temperature distribution and its evolution during spring in Lake Ladoga, using NOAA-AVHRR Chan nel 4/Channel 5 (CH4/CH5) data, with special focus on a thermohydrodyn amic phenomenon, called thermal bar. The split-window technique, where both constant and atmospheric-mass-dependent retrieval coefficients a re tested, is used for calculating the atmosphere effects in the tempe rature derivation. The satellite-derived temperature data are compared with field data from six different years. The results show that on av erage, the satellite-derived temperatures exceed the in situ data by a pproximately 0.5-degrees-C. The relative advantages of images from day or night passes for the study of the thermal bar is also discussed. S urface temperature differences in the convectively mixed region (tempe ratures less than 4-degrees-C) show that day temperatures are on the o rder of 0.5-degrees-C higher than the night temperatures. However, the surface temperatures from day images seem to be more representative o f the bulk temperature in a water column. A comparison of the thermal bar progression in Lake Ladoga during spring 1988 as obtained from sat ellite images and as predicted by a simple theoretical model shows goo d agreement. This result and results from an earlier study indicate th at the model is a good prediction tool for thermal bar movement in lar ge temperate lakes.