The evolution of AVHRR-derived water temperatures over boreal lakes

Higher spatial resolution in Earth–atmosphere models requires improved information on surface fields with very different thermal characteristics, such as land and water. Remote sensing is a useful tool for this purpose. A method is presented here to determine and compare the temperature evolution of water bodies. Thermal IR observations were extracted from NOAA's advanced very high-resolution radiometer (AVHRR) satellite data over the period from April 1 to August 31, 1994. The IR temperatures were calibrated and adjusted to account for the intervening atmosphere. For each day, 1-km resolution temperature scenes were generated for a 1000×1000-km rectangular region extending from about 60°N, 111°W over the northern part of the Canadian Province of Alberta to about 50°N, 97°W in southern Manitoba. There are 132 water bodies of area larger than 100 km 2 in this study area. The temperature cycle over these water bodies can be decomposed into a straight line with a small positive slope for spring ice break-up, followed by a fitted quadratic curve for the summer temperature variations. The curve fit parameters give information on the transition between ice and ice-free conditions. Data stratification confirms a strong latitudinal influence on the shape of the curves. It was not possible to quantify the effect of water depth, as the depth of most boreal lakes is not known. Instead, a list of fit coefficients for all the water bodies is provided. Once detrended from the quadratic curves, the amplitude of the diurnal cycle and major variations in temperatures of the water bodies can be determined. The detrended water temperature variations are about one-third the magnitude of the detrended air temperature variations from nearby weather stations.