Longwave radiation budget at TOA

Over the past three decades, considerable effort has been expended in the global measurement of the TOA longwave radiation budget. The Earth Radiation Budget Experiment (ERBE) provides the most comprehensive set of data for the TOA energy budget. The calculated global long-term (1984-2004) average of the outgoing longwave radiation (OLR) at the top of the atmosphere is found to be 239 W m~2, based on model runs with ISCCP climatological data.

90" n

90" n

OLR (W m"2)

100 150 200 250 300

Fig. 8.25. Model long-term (1984-2004) outgoing longwave radiation (OLR) in W m-2 at TOA for January.

8.7.1 Global distribution

An example of the global distribution of the OLR at TOA for the month of January (long-term average 1984-2004) is shown in Fig. 8.25. Tropical regions covered by clouds in January, such as the Amazon, central Africa and Indonesia during the southern wet season exhibit lower OLR (lower emission temperature) than the desert regions of the subtropics and oceanic areas west of Africa, Australia and South America (Eastern Pacific). The lowest OLR is in Greenland during the polar night because of its altitude.

8.7.2 Zonal-seasonal variation

The seasonal variability of the outgoing longwave flux at TOA is relatively small in the tropical and subtropical zones, especially in the zone 20-30°, and increases at higher latitudes poleward of 30°, with the largest variability in the polar zones (Fig. 8.26). Antarctica has the coldest and driest climate resulting in the lowest OLR, especially during the polar night where in July it drops to about 120 W m~2. Similar values are found for Greenland during January.

300 250 200 150

300 250 200 150 100.

7 8 9 10 11 12 ---1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 S 6 7 8 9 10 11 12

_—,—,—,—,— 300—,—,—,—,—,—,—,—,—,—,— 300r

250 200 150

250 200 150

1 2 3 4 5 6 7 8 9 10 11 12 ---1 23456789 10 11 12 123456789 10 11 12

250 200 150

250 200 150

1 2 3 4 5 6 7 8 9 10 11 12 ---1 23456789 10 11 12 123456789 10 11 12

1 2 3 4 5 6 7 8 9 10 11 12 ---1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

Month Month Month

1 2 3 4 5 6 7 8 9 10 11 12 ---1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

Month Month Month flG. 8.26. Model long-term (1984-2004) seasonal-zonal variation of outgoing longwave radiation (W m~2) for the Northern Hemisphere (solid line), and Southern Hemisphere (dotted line).

8.7.3 Latitudinal and seasonal variation

The mean annual OLR increases slightly from the equator up to 25° latitude (where it is equal to about 250 W m~2) and then decreases gradually towards the poles to values between 150-180 W m~2 (Fig. 8.27). We also note that the decreasing OLR towards the polar regions is negatively correlated with the increasing planetary albedo, principally due to the presence of clouds. The small increase in OLR from 0° to 30° is due to the presence of deserts in these latitudes, where cloud cover is at a minimum. The hemispherical and global mean seasonal variation of OLR is shown in Fig. 8.28. The global mean OLR is 239.1 W m~2 with a peak in summer because of the larger OLR in summer of the Northern Hemisphere. The Southern Hemisphere generally exhibits a weaker seasonal variation due to the larger surface fraction of oceans.

8.7.4 Log-term hemispherical and global means

In Table 8.14 is shown the global radiation balance at TOA . The net incoming solar radiation appears to be larger then the outgoing longwave radiation by 4.9 W m~2, according to ERBE measurements and only 1.2 W m~2 based on model calculations using ISCCP climatological data. This imbalance is within the

Latitude

flG. 8.27. Model long-term (1984-2004) average annual mean latitudinal variation of OLR (W m~2).

1 2 3 4 5 6 7 8 9 10 11 12 Month flG. 8.28. Model long-term (1984-2004) average global mean seasonal variation of OLR (W m~2).

errors of the model estimates of each component. However, there appears to be an imbalance between planetary solar heating and planetary thermal cooling to space in the Southern Hemisphere of 2 W m~2, while the Northern Hemisphere appears to be in radiative balance.

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