| description abstract | Ten years of hourly data on radiation, cloud and temperature collected at Resolute, Canada (75°N) show that with respect to clear skies: (i) clouds of all types, heights and extents heat the surface when it is snow-covered; (ii) low clouds certainly cool and high clouds probably warm the surface when it is snow-free; (iii) the transition to scattered then broken then complete cloud cover is accompanied, at least over snow-covered surfaces, by mostly monotonic changes in most radiation-balance quantities, including net radiation; (iv) cirriform overcasts alter the surface radiation climate by relatively strong greenhouse heating offset by relatively modest attenuation of solar radiation, and our empirical results help to substantiate recent model calculations of the cirrus greenhouse effect. There appears to be no difference in the albedo of bare ground between clear-sky and cirriform overcast conditions, but under stratiform overcasts the albedo of bare ground is on average ?3% below the value for clear-sky conditions. The dependence of snow albedo on solar elevation angle is complex, and we stress the importance of considering seasonal and other forms of variability in snow albedo parameterizations. There is evidence for contamination of the snow at Resolute by soot or dust. | |
