Tuesday, 11 February 2003
On the enhancement of evaporation from a large northern lake by the entrainment of warm, dry air
The turbulent exchange of water vapor and heat were measured above Great Slave Lake, Northwest Territories, Canada, using the eddy covariance method for most of the ice-free period in 1997, 1998, and 1999. In all years, evaporation tended to occur in episodic pulses lasting 50-60 hours, between which quiescent periods dominated. The contributions of these evaporation pulses to the measured total evaporation were 45%, 65%, and 47%, 1997 through 1999, respectively. Despite the suppression of turbulent mixing due the stable atmospheric conditions that dominated much of the ice-free periods, analyses of high-frequency wind, air temperature and humidity data revealed that evaporation was enhanced by the mixing of warm, dry air down to the lake surface. The turbulent structures associated with this entrainment typically had vertical and horizontal dimensions of 7 m and 18 m, respectively. They retained this structure over a vertical distance of 1.2 m for 0.24 s, and over a horizontal distance of 650 m for 125 seconds with a horizontal wind speed of 5.2 m s-1. Conditional sampling of turbulent measurements showed that these sweeps of warm, dry air were infrequent yet the dominant turbulent transfer mechanism. We propose that since the 50-60 hour evaporation pulses are comprised of an integration of the high-frequency sweeps, measurements of air-lake turbulent heat exchange need to be made at a high frequency in order to capture these significant events. Implications of climate variability on the mechanisms that control short- and long-term evaporation rates are discussed, in terms of the positive feedback that develops between entrainment and lake ice conditions.