Impact of the atmospheric thickness on the atmospheric downwelling longwave radiation and snowmelt under clear-sky conditions in the Arctic and Subarctic

Studies show that the energy available to melt snow at high latitudes is almost exclusively provided by radiation. Solar energy determines the period of possible snowmelt, while downwelling atmospheric longwave radiation modifies the timing and triggers the onset of snowmelt. Atmospheric thickness, defined as the vertical distance between the 500- and 1000- mb pressure surfaces, is directly related to the mean temperature and water vapor path of an atmospheric layer and thus has a direct influence on the downwelling longwave radiation and snowmelt. A comprehensive radiative transfer model was applied to calculate the downwelling longwave radiation to the snow surface over the period of snowmelt from 1980 through 1991 using radiosonde data obtained at Barrow and McGrath, Alaska, under clear- sky conditions. The results indicate that the atmospheric thickness has a positive impact on downwelling longwave radiation, which ranges from about 130 W m(-2) for an atmospheric thickness of 4850 m to about 280 W m(-2) for an atmospheric thickness of 5450 m. This study demonstrates that atmospheric water vapor path has a greater impact on atmospheric downwelling longwave radiation to the snow surface than the mean atmospheric temperature. This study also indicates that when the near- surface air temperature is used to infer downwelling longwave radiation, significant errors can occur. Thus, compared with the results obtained from the atmospheric radiative transfer model, the empirical formula due to Parkinson and Washington underestimates the downwelling longwave radiation when the near- surface air temperature is relatively high and overestimates it when the near- surface air temperature is relatively low. Investigations of the relationship between the atmospheric thickness and the snowmelt onset were conducted. Results indicate that for the period from 1980 through 1991, an atmospheric thickness of 5250 m at Barrow and 5200 m at McGrath in Alaska was sufficient to trigger the onset of snowmelt. The difference in the threshold values of the atmospheric thickness may be due to differences in the atmospheric structure and different contributions of other energy sources such as sensible and latent heat to melt snow. This study also demonstrates that snow cover disappears earlier during warm and wet (higher atmospheric temperature and precipitable water path, and greater atmospheric thickness) springs and later during cold and dry (lower atmospheric temperature and precipitable water path, smaller atmospheric thickness) springs. Atmospheric precipitable water path has a greater impact on snowmelt than the mean atmospheric temperature. Generally, higher atmospheric temperature is correlated with higher atmospheric water vapor path and since atmospheric temperature is closely coupled to the atmospheric water vapor path in the Arctic and Subarctic and since it can be obtained through routine numerical weather prediction models, the atmospheric thickness may be used as a reliable indicator of regional- scale snowmelt in the Arctic and subarctic.