Imagine that you are at a barn fire on a cool autumn evening. As you face the fire, your front warms because you are receiving more energy from the fire than you are losing. Your back cools down and gets cold because you are losing more energy than you receive from the cooler air around you. You experience a local energy imbalance, your front warms and your back gets cold. If you get too uncomfortable, you can turn around, allowing your back to warm-up and your front to cool-down. As we look at the radiation balance of the planet, just because a given region may be receiving more radiative energy than it is losing does not mean the region's temperature is increasing. The atmosphere and ocean may move this excess energy to a different region of the globe, perhaps one that is losing more energy than it is receiving.
The determination of the Earth's radiation budget is essential to atmospheric modeling and climate studies. Radiation budget experiments have used satellites to measure the fundamental radiation parameters -- the amount of solar energy received by the planet, the planetary albedo (the portion of incoming solar radiation that is reflected back to space), the emitted terrestrial radiation (also referred to as the outgoing longwave radiation -- OLR), and the net planetary energy balance (the difference between the absorbed solar energy and the OLR). The most recent experiment to measure these parameters is the NASA Earth Radiation Budget Experiment or ERBE. Results from ERBE are presented below, you can find more information about ERBE in the papers cited below. As you look at these maps, the month is printed in the upper left hand corner and the legend is given along the bottom of the figure. Albedo values are given in units of percent reflectance, all others are in terms of energy per unit time per unit area (Watts per square meter).
The planetary averaged albedo is a key climate variable as it, combined with the solar insolation determines the radiative energy input to the planet. The global annual averaged albedo is approximately 0.30. The annual average albedo of the northern and southern hemispheres is nearly the same, demonstrating the important influence of clouds. The albedo varies quite markedly with geographic region and time of year. You can view a loop of the monthly mean planetary albedo or view each month individually. There are many interesting features in these maps. For instance, notice the high albedos off the west coast of South America. This is a region of persistent low level clouds -- stratus clouds. Can you find other regions of oceanic stratus? Notice the strong dependence of albedo on season, the annual cycle of the albedo follows the annual cycle of the position of the sun. Also notice that cloud free ocean regions have low albedos while deserts generally have high albedos. In the tropical regions the albedo variation is influenced primarily by weather disturbances and their associated cloud distributions. In the polar regions, seasonal variations in albedo are due to the distribution of major ice sheets and the decreasing mean solar elevation angle with latitude.
The minimum in OLR, or the longwave emitted flux near the equator is due to the high cloud tops associated with the inter-tropical convergence zone (ITCZ), a region of persistent thunderstorms. This minimum migrates about the equator as seen in the monthly mean maps, and is also seen as a maximum in albedo. Notice how it is difficult to observe the oceanic stratus regions we observed in the albedo maps. This is because the temperature of the clouds is similar to the surrounding oceans, making it difficult to observe.
Observe how the major deserts have their largest OLR during their summer. This results from the annual temperature cycle -- as the desert surface heats up it emits more longwave radiation. Note also the large emission in the vicinity of the oceanic subtropical highs (30N and 30S).
You can even see the onset of the southwest summer monsoon over the Indian subcontinent in June and July.
The difference between the absorbed solar energy and the OLR is referred to as the net radiation. 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.
A major research problem addressed with the ERBE program was how clouds affect the radiative energy balance of the planet, and thereby climate change. The cloud-radiative forcing is simply the difference between the clear-sky and cloudy-sky radiative energy gains. To make this determination, ERBE had to separate clear-sky scenes from all others.
The solar and terrestrial properties of clouds have offsetting effects in terms of the energy balance of the planet. In the longwave, clouds generally reduce the radiation emission to space and thus result in a heating of the planet. While in the solar (or shortwave), clouds reduce the absorbed solar radiation, due to a generally higher albedo than the underlying surface, and thus result in a cooling of the planet. View the maps of cloud forcing given above. Does the presence of low level clouds over oceans heat or cool the planet? What about the convective clouds over the oceans?
The latest results from ERBE indicate that in the global mean, clouds reduce the radiative heating of the planet. This cooling is a function of season and ranges from approximately -13 to -21 Wm-2. While these values may seem small, they should be compared with the 4 Wm-2 heating predicted by a doubling of carbon dioxide concentration.
In terms of hemispheric averages, the longwave and shortwave cloud forcing tend to balance each other in the winter hemisphere. In the summer hemisphere, the negative shortwave cloud forcing dominates the positive longwave cloud forcing, and the clouds result in a cooling. For deep convection the solar and longwave effects also cancel.
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For more information on the Earth's radiation budget see:
Meteorology Today, An Introduction to Weather, Climate and the Environment, by C. Donald Ahrens, West Publishing Company
Meteorology, The Atmosphere and the Science of Weather, by Joseph M. Moran and Michael D. Morgan, MacMillan College Publishing Company
Barkstrom, B. R., The Earth Radiation Budget Experiment (ERBE), 1984: Bull Am. Meteorol. Soc., 65, 1170-1185.
Barkstrom, B. R., E. Harrison, G. Smith, R. Green, J. Kibler, R. Cess and the ERBE Science Team, 1989: Earth Radiation Budget Experiment (ERBE) archival and April 1985 results. Bull. Am. Meteorol., Soc., 70., 1254-1262.
Harrison, E. F., D. R. Brooks, P. Minnis, B. A. Wielicki, W. F. Staylor, G. G. Gibson, D. F. Young, F. M. Denn, and the ERBE Science Team, 1988: First estimates of the diurnal variation of longwave radiation from the multiple-satellite Earth Radiation Budget Experiment (ERBE), Bull. AM. Meteorol, Soc., 69, 1144-1151.