Again there are insufficient direct measurements of the downwelling longwave flux to compile maps for the Arctic. While a number of empirical formulae have been derived that employ the surface air temperature we rely on the ISCCP-D fields. Figure 5.4 and
Figure 5.5 provide mean fields for the four mid-season months of the downwelling longwave and net longwave fluxes, respectively.
The spatial pattern of the mean downwelling longwave flux largely follows the distribution of cloud cover and SAT (see Chapter 2). During January the highest values are in the Atlantic sector where cloud cover is extensive and optically thick and where air temperatures and specific humidity are highest. Note for all months the relatively low values over central Greenland, which is above much of the cloud cover and where air temperatures and water vapor content are low. Comparisons between the January, April and July maps reveal the effects of the seasonal increase in air temperature and humidity. For all months, however, the largest fluxes are over the Atlantic sector. The net longwave flux is negative in all months. Typical values over the Arctic Ocean are -20 to -40 W m-2. Larger deficits are found over Greenland and land areas, for the latter especially during summer. What this implies is that, over land, summer surface temperatures are fairly high while the lower troposphere remains fairly cool
and dry. The large deficits over Greenland in all months point to the high elevation of the ice sheet. The smaller July deficits over sea ice of about -40 W m-2 relative to land areas are understood in that the melting sea fixes the skin temperature, limiting the magnitude of the upward longwave flux.
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