2002 Annual

Monday, 14 January 2002: 2:29 PM
Seasonal Cycles of Surface Radiation Budget and Climate Classes
Anne Wilber, AS&M, Hampton, VA; and G. L. Smith and P. W. Stackhouse
The Langley Research Center Surface Radiation Budget database is used to examine the seasonal cycles of surface radiation budget (SRB) in terms of regional climate class. An increased understanding of these cycles will improve our insight into their governing processes. The net shortwave (NSW) and the net longwave (NLW) radiant fluxes at the surface are considered. First the seasonal cycles are defined in terms of Empirical Orthogonal Function (EOF) maps whose Principal Components (PC) are the associated time histories.

The first 3 EOF’s account for 92.1, 3.9 and 2.2 % of the variance of NSW and the first 4 EOFs account for 70.2, 13.6, 7.9 and 1.4 % of the variance of NLW. NSW EOF—1 is due simply to the variation of solar declination through the year, with effects of the atmosphere included. The PC—1 is very near a sine wave with minimum of – 60 W m2 and maximum of 60 W m2. Histograms are examined of the EOF values for different climate classes. Boreal forest, temperate and subtropical regions all have NSW EOF—1 values between .75 and 1.75, varying by 45 to 105 W m2. Tropical wet (rainforests) and Tropical wet/dry (savanna) regions have NSW EOF—1 values between ±0.25. Semi-arid (steppe) regions cover a broad range from -. 25 to 1.75, due to variety of conditions which result in steppe: adjacency to deserts, high altitude lands, or location in rain shadow of mountains.

NLW PC—1 is an annual cycle which lags the NSW PC—1 by about a month and varies from –10 to 12 W m2. Over the southern oceans, the North Polar Sea and much of the northern oceans, the NLW has a very small annual cycle, indicating close coupling of the temperatures of the ocean surface and the atmospheres. The movement of the Intertropical Convergence Zones causes a large variation near the Equator and there is a large variation also around the boundaries of the ocean basins in the Northern Hemisphere. The largest response of NLW to the annual cycle is in the region of the Indian Subcontinent and Southeast Asia (tropical wet/dry), where EOF—1 exceeds 3. Also, the tropical wet/dry region of Sub-Saharan North Africa has a variation of 3 or greater. The Saharan Desert and the deserts of the Middle East have a negative EOF—1, showing that as NSW increase the LW radiation from the atmosphere to the surface increases faster than the LW from the surface.

The variations in the other EOFs are also explained in terms of the region’s climate class.

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