A couple of days ago, renowned climatologist Brian Brettschneider posted a chart illustrating the rapid loss of solar radiation at high latitudes by this point in late summer. It's a dramatic difference from lower latitudes and explains the acceleration of the jet stream and a tendency for rainier weather in late summer in much of Alaska (read more on that here).
By this point in August, we're receiving 65% to 75% of possible solar energy potential compared to the summer solstice here in Alaska. In the Lower 48, it's still 82% to 92%. pic.twitter.com/quyigqy0Kx
— Brian Brettschneider (@Climatologist49) August 19, 2022
Brian's graphic uses the Bird Clear Sky Model to estimate the normal solar radiation under idealized clear sky conditions. This is useful, but I thought it would also be interesting to compare the model to real-world data from Alaska's CRN sites. See below: the individual points show the drop-off in normal solar radiation from 19 CRN sites that have at least 5 years of data.
There's a fair bit scatter in the CRN results, partly because the seasonal variation in cloud cover differs among the sites, and partly because my estimated normals are based on different numbers of years. Overall the Bird result is confirmed: solar radiation drops off more rapidly at higher latitude, but it's also clear that the model underestimates the drop-off more significantly for the Arctic sites. In particular, the Utqiaġvik CRN, with nearly 20 years of data, actually sees a 62% loss of solar radiation, compared to a modeled loss of only 42%.
Why the more drastic loss of radiation in the real world for most of the sites? Surely a major reason is that cloud cover increases between June 21 and August 21 for most (perhaps all) of the sites; but I think also the very existence of cloud cover means a more rapid loss of energy as solar angle decreases, because of the geometry of cloud shadowing. For instance, a layer of stratocumulus with, say, 70% sky coverage, would produce cloud shadows over 70% of the land surface when the sun is directly overhead, but nearly 100% when the sun is low on the horizon.
The highly maritime climate of Utqiaġvik then becomes clear when we consider that the seasonal normal temperature has dropped only 2.5°F from its summer peak by August 21. In contrast, Fairbanks has cooled by over 8°F already, despite losing "only" 39% of its daily solar input.
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