Meteorology: Understanding the Atmosphere            Ackerman and Knox



 The difference between the absorbed solar energy and the emitted OLR of the planet is referred to as the net radiation budget. The annual variation in net radiative energy follows that of the solar declination due to the annual variation of the incoming solar energy being greater than the annual variation of the albedo.

In general, the absorbed solar radiation exceeds the outgoing longwave radiation in the tropical and subtropical regions, resulting in a net radiative heating of the planet, while in the middle to polar latitudes there is a net cooling. This equator-to-pole difference, or gradient, in radiative heating is the primary mechanism that drives the atmospheric and oceanic circulations. On an annual and long-term basis in which no energy storage and no change in the global mean temperature occurs, this radiative imbalance between the tropics and polar regions must be balanced by meridional heat transport by the atmosphere and oceans.

The measured outgoing longwave radiation and albedo also indicate regional forcing mechanisms. For example, in the tropics east-west variations can be as large as the north-south averages and are associated with east-west circulations. Tropical regions, in general, display a net radiative heating, the Sahara is often experiencing a net radiatively cooling. This is due to the high surface albedo, the warm surface temperatures and the dry and cloud free atmosphere. The radiative cooling is maintained by subsidence warming, which also has a drying effect and therefore helps maintain the desert.

The albedo, OLR and net radiation are closely related to surface type and the weather regime. For example, look at the Sahara Desert and the Amazon basin in the summer and winter. The incoming solar radiation is a function of latitude and time of year. The desert is approximate 20 degrees north latitude while the Amazon basin is approximately 20 degrees south of the equator. So the incoming solar radiation in the Amazon in January (Southern Hemisphere Summer) is nearly the same as the incoming solar radiation over the Sahara in July (Northern Hemisphere Summer). The two regions also have very high albedos during their respective summers -- but for two different reasons. The high albedos of the Amazon are the result of highly reflecting deep-convective cloud systems. Over the desert, there are few clouds, but the surface, which is mostly dry soil, is highly reflective. The OLR is very different for these two regions. The amount of terrestrial radiation is a function of temperature, the tops of the convective clouds are very cold, and so the outgoing energy is small. In contrast the desert surface is very warm, and so the OLR is large. What is the difference in the net radiation for these two regions?

Notice also that in the middle and high latitudes of the southern hemisphere, the radiation budget is zonally symmetric -- lines of constant albedo (or OLR) are parallel to the lines of constant latitude. This is not the case in the middle and high latitudes of the northern hemisphere, where contrasts between land and ocean are obvious. It is also interesting to contrast the summer and winter season in the northern middle latitudes. During the summer the OLR is greater over land than the oceans, because the temperatures are warmer, while the albedo is greater over the oceanic regions where there are more clouds. In the net radiation balance, the land surfaces receive more radiation than the oceans. The opposite occurs in the winter -- the oceans gain more radiative energy than the land regions.