Radiation regime

Some aspects of Greenland's radiation were examined in Chapter 5. To summarize and expand, during winter, net allwave radiation is strongly negative. This is a function of the high elevation of the ice sheet and the relative lack of cloud cover compared to other areas of the Arctic, which helps to keep the downwelling longwave flux low. Solar radiation is of course essentially absent in winter. It follows that, compared to other Arctic regions, the summer downwelling solar radiation flux over the ice sheet is quite high (Figure 5.1), again due to more limited cloud cover and (from the high elevation) less attenuation of the incident flux by aerosols and water vapor. But even in July, net allwave radiation over the higher elevation parts of the ice sheet is less than about 40 W m-2 (Figure 5.6). In part, this manifests the fairly small flux of downwelling longwave radiation (Figure 5.4). There is hence strong compensation between the two incoming radiation terms. However, the much larger contributor to the low net allwave radiation in summer is the high albedo - most of the incident solar flux is lost to space. From Figure 5.3 we see that, even in July, the mean surface albedo over most of the ice sheet is around 0.80 although much lower values can be found in the warmer parts subject to summer melt.

The role of albedo is highlighted in radiation measurements collected by Konzelmann and Ohmura (1995) in 1990 and 1991 at the Swiss Camp (69.6° N, 49.3° W, 1149 m). Swiss Camp is located in West Greenland, near the mean equilibrium line. These data show that the decrease in incoming solar radiation through the summer melt season associated with increased cloud cover (principally observed in August as a result of stratus) is essentially offset by the tendency for clouds to increase the downwelling longwave radiation. Because of the melting surface, the outgoing longwave radiation flux is steady. Hence, variations in the net surface (allwave) radiation are determined largely by variations in albedo. Albedo decreased from 0.86 in May to 0.72 in July. Overall, a positive net radiation balance can be expected from late May to mid August. But even in July, net radiation averaged for the two years was only 37 W m 2. This is consistent with the satellite-derived estimates given in Figure 5.6.

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