Icefield Breezes: Mesoscale Diurnal Circulation in the Atmospheric Boundary Layer Over an Outlet of the Columbia Icefield, Canadian Rockies.

Atmospheric boundary layer (ABL) dynamics over glaciers mediate the response of glacier mass balance to large‐scale climate forcing. Despite this, very few ABL observations are available over mountain glaciers in complex terrain. An intensive field campaign was conducted in June 2015 at the Athabasca Glacier outlet of Columbia Icefield in the Canadian Rockies. Observations of wind and temperature profiles with novel kite and radio‐acoustic sounding systems showed a well‐defined mesoscale circulation developed between the glacier and snow‐free valley in fair weather. The typical vertical ABL structure above the glacier differed from that expected for "glacier winds"; strong daytime down‐glacier winds extended through the lowest 200 m with no up‐valley return flow aloft. This structure suggests external forcing at mesoscale scales or greater and is provisionally termed an "icefield breeze." A wind speed maximum near the surface, characteristic of a "glacier wind," was only observed during night‐time and one afternoon. Lapse rates of air temperature down the glacier centerline show the interaction of down‐glacier cooling driven by sensible heat loss into the ice, entrainment and periodic disruption and warming. Down‐glacier cooling was weaker in "icefield breeze" conditions, while in "glacier wind" conditions, stronger down‐glacier cooling enabled large increases in near‐surface temperature on the lower glacier during periods of surface boundary layer (SBL) disruption. These results raise several questions, including the impact of Columbia Icefield on the ABL and melt of Athabasca Glacier. Future work should use these observations as a testbed for modeling spatio‐temporal variations in the ABL and SBL within complex glaciated terrain. Plain Language Summary: Mountain glaciers are experiencing rapid change in response to climate change. But to confidently attribute past change and predict future change, our mathematical models need to account for how glaciers modify the climate they experience. New measurements of wind and temperature have been made over an outlet glacier of the Columbia Icefield, Canadian Rockies using a novel kite‐based measurement system along with an extensive set of meteorological measurements on the glacier surface and in the valley below the glacier. They show that the persistent down‐glacier winds observed at the glacier surface have a different vertical structure than expected from previous observations and theory. Down‐glacier winds typically cool the air next to the surface as it travels down‐glacier. Here it is shown that this cooling process is more limited when the wind is strong throughout the lower valley over the glacier, compared with times when wind speed is strongest next to the glacier and weaker above. The data set presented in this paper provides an opportunity to test the models used to link glaciers and climate. Key Points: A well‐developed mesoscale wind develops over a Columbia Icefield outlet glacier but differs from previously documented "glacier winds"Novel kite and radio‐acoustic observations indicate strong down‐glacier winds typically extend through the lowest 200 m of above‐glacier ABLKatabatic cooling in the glacier surface boundary layer is reduced during periods with "icefield breezes" compared to periods with typical "glacier winds" [ABSTRACT FROM AUTHOR]