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 52–78 h, between which quiescent periods dominated. The contributions of these evaporation pulses to the measured total evaporation were 45%, 65%, and 47% for 1997, 1998, and 1999, respectively, yet occurred on only 24% (1997), 37% (1998), and 25% (1999) of the total number of days observed. Despite the suppression of turbulent mixing, due to 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. Conditional sampling of turbulent measurements showed that these sweeps of warm, dry air were infrequent, yet were the dominant turbulent transfer mechanism. Because the approximately 3-day-long evaporation pulses were composed of an aggregation of sweeps, measurements of air–lake turbulent heat exchange needed to be made at a high frequency in order to capture these significant events. Implications of climate variability on the mechanisms that control shortand long-term evaporation rates were discussed, in terms of the positive feedback that developed between entrainment and evaporation.