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The Sun is the source of energy that sustains life on the Earth. Solar energy enters and is redistributed within the Earth-atmosphere system, which subsequently emits radiation at longer wavelengths. The Earth's radiation budget (ERB) drives the general circulation of the atmosphere and determines the climate of the Earth-atmosphere system, being an indicator of possible climatic changes. The ocean-surface energy budget ultimately influences the major features of ocean circulation, and hence regional climate. Thus, it is very important to be able to determine as accurately as possible its components at the top of the atmosphere (TOA), within the atmosphere and at the Earth's surface for both shortwave (SW) and longwave (LW) radiative fluxes. At TOA, the net energy input is determined by the incident SW radiation from the Sun minus the reflected SW energy. The difference defines the net SW radiative flux at TOA. To balance this inflow of SW energy, the Earth-atmosphere system emits LW radiation to space. For the planet to be in radiative equilibrium, the emitted LW radiation must equal the net inflow of SW radiation at TOA. Radiative processes that perturb this equilibrium can produce global climatic change.

In this chapter we shall examine the role of atmospheric molecules, aerosols, clouds and properties of the Earth's surface in the Earth's radiation budget. The net solar and terrestrial longwave irradiances at the Earth's surface, i.e. the surface radiation budget, (SRB), and at the top of the atmosphere (TOARB), determine the distribution of energy in the Earth-atmosphere-ocean system and hence climate. Any externally imposed perturbation on the Earth's energy budget arising from changes in the concentration of infrared radiatively active molecules or greenhouse gases, H2O, CO2, CH4, N2O, clouds and aerosols, incoming solar irradiance, or changes in surface reflection is called radiative forcing. Such perturbations can lead to changes in climatic parameters, resulting in a new equilibrium state of the climate system (IPCC 2001). Thus, the radiation budget of the Earth-atmosphere system plays a fundamental role in determining the thermal conditions and the circulation of the atmosphere and the ocean, shaping the main characteristics of the Earth's climate.

The ultimate source of energy that drives the climate system is radiation from the Sun. The mean global daily incoming solar flux at the top of the Earth's atmosphere is 342 W m~2. About a third of that is reflected directly back to space

Reflected solar radiation 107 Wm-2

\ Reflected by clouds, aerosol and ^atmosphere 77

Reflected by surface^ 30y

\ Reflected by clouds, aerosol and ^atmosphere 77

Reflected by surface^ 30y

Absorbed by 67 atmosphere

Incoming solar radiation 342 Wm '

Absorbed by 67 atmosphere bsorbed by surface

24 78 Thermals Evapotranspiration

Outgoing longwave radiation 235 Wm-2

Incoming solar radiation 342 Wm '

Greenhouse gases

324 Back radiation

Absorbed by surface

Outgoing longwave radiation 235 Wm-2

Greenhouse gases

324 Back radiation

Absorbed by surface

flG. 8.1. Schematic representation for the partitioning of the Earth's global mean energy balance. For updated values of the various components see text. (Kiehl and Trenberth 1997)

by clouds, by the atmosphere and by the surface of the Earth. The remaining radiation is partly absorbed by the atmosphere and partly by the surface, thus warming it up. This heat is returned to the atmosphere as thermal infrared radiation, sensible heat and latent heat, Fig. 8.1. For a stable climate, the amount of net incoming solar radiation must equal the amount of net thermal infrared radiation emitted back to space. The radiation budget represents this balance of energy.

Attempts to construct a global annual mean ERB date back to the beginning of the twentieth century. There has been a long history of early studies that, however, were limited by the lack of knowledge of the planetary albedo. Satellite observations, such as those from Nimbus-7 and the Earth Radiation Budget Experiment (ERBE) have greatly improved our knowledge of TO ARB. Nevertheless, satellites do not measure directly radiative fluxes at the Earth's surface. Therefore, knowledge of SRB is far less advanced than that of TOARB. The SRB, however, is a major component of the energy exchanges between the atmosphere and the land/ocean surface, and hence affects temperature fields, atmospheric and oceanic circulation, and the hydrological cycle. For example, about half of the solar energy absorbed at the surface is used to evaporate water, which eventually forms clouds. SRB data are also valuable for initializing and testing simple climate models and the more complex general circulation models (GCMs).

The estimation of SRB represents one of the most significant objectives of the World Climate Research Programme (WCRP) as demonstrated by its Global Energy and Water Cycle Experiment (GEWEX), and in particular the GEWEX

SRB project. Due to the inability of directly measuring radiation at the Earth's surface with satellites, the SRB has to be derived, as surface observations cannot solve the problem, because of the sparsity and inhomogeneity of meteorological stations, especially over oceans. Thus, modelling remains as the only alternative, provided that the model results are validated against quality measurements.

One modelling approach is to establish correlations between TOA and surface radiative fluxes to derive surface fluxes directly from satellite measurements of TOA fluxes. There is, however, uncertainty regarding the reliability of this method. Another modelling approach, is to develop algorithms and models to compute SRB fluxes on a global scale on the basis of global observations of atmospheric, cloud, and surface properties, preferably from operational satellite sources, since they provide global coverage.

The amount and quality of satellite data have substantially improved with time, especially in terms of cloud properties. The International Satellite Cloud Climatology Project (ISCCP) has provided a complete and comprehensive global cloud climatology. Thus, in the last two decades, a large number of studies that estimate SRB have been published. These were based on the C-series data. The new D-series include significant improvements, especially in terms of cloud structure and cloud detection, over highly reflecting surfaces as well as covering a longer time period, extending from 1983 to 2004. The ISCCP-D2 data are monthly averaged, on a 2.5x2.5 degree (latitude-longitude) resolution.

Early estimates of the planetary albedo ranged from 40% to 50%. Satellite observations from Nimbus-7 and ERBE have greatly improved estimates of the planetary albedo, giving values near 30%. Although the determination of radiative fluxes at TOA is more advanced than at the surface, due to the recent advances in space-borne measurements that provide good knowledge of the net solar energy absorbed by the global climate system, there is a need to model accurately the ERB at TOA. Model validation at TOA is a pre-requisite for obtaining reliable model computations of ERB at the surface, where direct satellite measurements are not possible and surface measurements cannot provide complete global coverage. Satellite observations of ERB have limitations in their accuracy and do not provide continuous data. An outgoing SW radiation (OSR) data set has been produced from the ERB measurements taken aboard Nimbus satellites. The Nimbus ERB scanner data cover the period from January 1979 through May 1980, while non-scanner data extend from November 1978 till December 1993. The ERBE scanner OSR data are considered to be of very good quality in terms of spatial and temporal coverage and instrument accuracy, but they cover only the 5-year period from November 1984 to February 1990. Therefore, although the ERB data obtained from the Nimbus-7 measurements along with those from ERBE, span over a decade (1978-1990), they do not provide continuity. Beyond this, there is a gap in the time series between those and more recent accurate measurements, such as the Scanner for Radia tion Budget (ScaRaB) and the Clouds and the Earth's Radiant Energy System (CERES), especially during the 1990s. More sophisticated space-borne instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua Satellites and the Geostationary Earth Radiation Budget (GERB) instrument on EUMETSAT's Meteosat Second Generation (MSG) satellite have started high-accuracy and high-resolution measurements since early 2000. Furthermore, apart from the fact that satellite measurements have their own errors, especially the SW retrievals, they do not provide complete spatial (global) coverage, as they often miss areas poleward of 70-degree latitude, where model results can be used as first estimates, even though not very accurate. Reliable model ERB computations at TO A, are very important to the study of the temporal variation of the Earth's radiation budget and help us to understand the physical processes that determine the TO A radiation budget.

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