Solar Radiation at the Earth's Surface

Solar Global Radiation. Summing up the discussions in Section 3, it is most likely that half of the solar radiation coming from the Sun is removed through backscattering and absorption. The remaining 170 W m^-2 reaches the Earth’s surface as solar global radiation, or simply global radiation. Global radiation consists of direct solar radiation and diffuse sky radiation.

On a cloudless day in a clear atmosphere, up to 90% of global radiation is made of direct radiation, while on an overcast day 100% of the global radiation can be due to diffuse sky radiation. Globally averaged, direct and diffuse radiation divide global radiation at about 50-50%. Global radiation is presently the best-known component among all energy fluxes at the Earth’s surface.

Further, global radiation has large spatial and seasonal variations. The following discussions on geographical distribution of the fluxes will be made on the basis of ECHAM4/T106 simulations. Despite many later appearing models, ECHAM4 is considered to reproduce most accurately the seasonal distribution of energy fluxes in the global scale.

ECHAM4 is a general circulation model (GCM) developed at the European Centre for Medium-range Weather Forecasts (ECMWF) and adapted for climate simulations at the Max Planck Institute for Meteorology in Hamburg. This model is equipped with one of the most advanced radiation codes. The model was used to simulate the present climate in T106 resolution, a grid equivalent of 120 km using the climatological sea-surface temperature during the period 1979-1988.

In many regards, the fluxes computed in this simulation offer one of the most realistic distributions of surface fluxes.

The annual mean global radiation at the Earth’s surface, Figure 4, ranges from the smallest value of 60 W m^-2 over the Barents Sea in the North Atlantic to the largest value of nearly 280 W m^-2 in the subtropical East Pacific. Generally, the largest global radiation is not found under the equator, despite the world’s largest TOA irradiance, because of the cloud development associated with the Inter-tropical Convergence Zone (ITCZ).

Figure 4. The annual mean global solar radiation simulated with ECHAM4T106

The largest global radiation appears in the subtropical horse latitudes where potential solar radiation at TOA is still large and the atmosphere is relatively dry and cloud-free. On land, this consists of the major desert regions of the world, such as the Sahara, Arabian, Kalahari, Australian, Mojave, and Atacama. In these regions, the annual global radiation exceeds 250 Wm^-2.

The Gobi and Taklimakan deserts are excluded from this category because they are located at higher latitudes. The largest instrumentally measured annual global radiation on land was observed at Riyadh, on the Arabian Peninsula and at Tamanrasset in the Sahara, where it reaches 260 W m^-2. Yet, the world largest global radiation is probably not found on land, but over an ocean.

The area of the tropical East Pacific between the ITCZ and the Southern Pacific Convergence Zone (SPCZ) is considered to be the sunniest spot in the world. In this area, the annual global radiation exceeds 270 Wm^-2. The largest annual global radiation of 298 W m^-2 is reported at Canton, on Canton Island in Kiribati. This is basically a westward extension of the South American desert climate to the ocean, but the atmosphere contains much less aerosol in comparison with the sites in the continental desert. The middle latitudes are the regions of a large gradient of global radiation with a rate of -3 W m^-2 per one degree of latitude.

The smallest global radiation does not appear right at the pole but in the subpolar regions from latitudes 60°-70° in both Hemispheres. These are regions of larger cloud amount owing to the subpolar cyclones. From here toward the poles, global radiation increases. The higher surface albedo in the Arctic Ocean and the Antarctic continent also enhances global radiation through multiple reflections between the surface and the atmosphere.

At the North Pole, the annual global radiation is estimated at about 90 W m^-2, while at the South Pole, the long-term mean of the observed annual global radiation is found to be 130 W m^-2. The difference in the annual global radiation between the North and South Poles is caused by the cloud conditions, altitudes, and the surface albedo.

Globally averaged, the Earth’s surface receives 7% more global radiation in January than in July because of the occurrence of the perihelion in January and the present eccentricity of the Earth’s orbit around the Sun amounting to 0.0167.

The seasonal fluctuation of global radiation is large, except for the equatorial zone as is witnessed in the example of Singapore shown in Figure 5 and Ducke Reserve, Brazil, in Figure 6. In the regions poleward of 10°N and S, the global radiation experiences considerably large seasonal fluctuations.

Figure 5. The seasonal course of monthly mean solar global radiation at selected sites in the Northern Hemisphere. The figure represents the meridional variation of the seasonal global radiation based on the long-term measurements at 10 stations. These stations are spaced approximately by 10° in latitude, starting at the equator for Singapore and ending at the North Pole

Figure 6. The seasonal course of monthly mean solar global radiation at selected sites in the Southern Hemisphere. The figure represents the meridional variation of the seasonal global radiation based on the long-term measurements at 10 stations. These stations are spaced approximately by 10° in latitude, starting at the equator for Ducke Reserve in Brazil and ending at the South Pole

The distribution of the global radiation in winter is strongly affected by the solar elevation, hence latitude, while in summer, it is greatly influenced by the clouds. Thus, in the months of extreme solar declinations of June (Northern Hemisphere) and December (Southern Hemisphere), the latitudinal distribution of global radiation forms a maximum near the pole and a secondary maximum at around 30^° of latitude in the summer hemisphere (Figures 5 and 6).

During these months, the summer hemisphere receives 70% of the total global solar radiation. In December, the world’s largest monthly global radiation of 460 W m^-2 is observed on the plateau of East Antarctica and the lowest in the regions poleward of 70°N, where the monthly global radiation is zero due to the polar night. In June, the largest monthly global radiation is found in the central region of Greenland (380 W m^-2), while in the region of the North Pole it remains smaller at around 300 W m^-2 because of the larger optical air mass, the larger cloud amount, and lower albedo of the melting sea ice.

Across the equator in June, the global radiation declines in the Southern Hemisphere at a rate of 3Wm^-2/degree of latitude, reaching zero at 70°S, which closely touches the Antarctic coast of the Eastern Hemisphere.