| Summary: | In mountain environments, longwave radiation provides large amounts of melt energy for high-albedo snow surfaces and can dominate in the energy balance of snow or glacier surfaces under cloudy skies. This study examines the atmospheric controls of sky longwave radiation at Glaciar Zongo, Bolivia (16 degrees 15' S, 5060 m a.s.l.) over an entire year to derive a parameterization suitable for melt studies. Tropical glaciers are characterized by a pronounced seasonality of longwave radiation, due to cloud emissions during the wet season that strongly enhance the small emissivity of the thin and dry clear-sky atmosphere at very high altitudes. Clear-sky radiation is well simulated as a function of air temperature and humidity, but changes in humidity atmospheric profiles from daytime to night-time entail different optimized coefficients for hourly and daily data. Cloud emission, which enhances clear-sky emissivity by up to 55%, with an average of 20%, is estimated using daily atmospheric transmissivity for solar radiation. Partial correlations show that in high mountains cloud emissions control the variations of longwave radiation, far more than clear-sky emissivity and temperature of the emitting atmosphere. An independent test on Glaciar Antizana in the humid tropics of Ecuador (0 degrees 28' S, 4860 m a.s.l.) indicates that the parameterization is robust for the Central Andes.
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