Outgoing solar radiation at TOA

We present global distribution results of the outgoing solar radiation at TOA based on a deterministic radiative-transfer model on a mean monthly and 2.5° x 2.5° longitude-latitude resolution, spanning the 14-year period from January 1984 through December 1997. The model uses data from the ISCCP D2-series supplemented by water-vapour and temperature data taken from NCEP/NCAR. Model input data were also taken from other global databases, such as TIROS-TOVS, ISLSCP, and GADS. The model computations were validated at the pixel level, against 5-year accurate scanner data from ERBE, which ensures the quality of the long-term model results. The incoming solar radiation was computed as described in Chapter 5, where the zonal-seasonal distribution is also given.

8.4.1 Planetary albedo

The net incoming solar radiation at TOA (planetary absorption), F^ TOa, the outgoing SW radiation (OSR) at TOA, FTOA, and the planetary albedo Rp, are related through the following expressions where ap is the planetary absorptivity. The scattering and absorption of the incoming SW radiation at TOA, depends strongly on the presence and type of

flG. 8.7. Long-term (1984-1997) mean planetary albedo for January. (Hatzianas-tassiou et al. 2004b)

flG. 8.7. Long-term (1984-1997) mean planetary albedo for January. (Hatzianas-tassiou et al. 2004b)

clouds in the atmosphere, the composition of the atmosphere (gases plus aerosols) and the reflectivity of the Earth's surface. It is weakly dependent on the thermal structure of the atmosphere. The cloudy-sky component can be subdivided into three components covered by low-, middle-, and high-level clouds, according to ISCCP. The planetary absorptivity, and hence the OSR, has a clear-sky component, as, and three cloudy-sky components, aci, and is expressed as ap = (1 - Ac) as + Aciaci. (8.5)

In Fig. 8.7 we show the long-term (1984-1997) mean planetary albedo for January. The planetary albedo, Rp, is highest over polar and high-altitude areas, which are characterized by large surface albedos, throughout the year. There, Rp values are as high as 75%. Secondary maxima of planetary albedo values occur over cloudy tropical and sub-tropical areas and over highly reflecting surfaces such as the Sahara. The lowest Rp values are found over tropical and sub-tropical oceanic areas (with surface albedo lower than 10%), especially those with small cloudiness. When clouds are present above such oceanic areas, e.g. marine stratus clouds off the coasts of western United States, South Africa and South America, then Rp increases up to 40%. The systematically large values of Rp over mid-to-high latitude and especially polar regions, indicates the importance of these regions in terms of their sensitivity to possible climatic changes. In Fig. 8.8 is shown the high-level cloud cover albedo for January 1988. We see that tropical

flG. 8.8. High-level cloud albedo for January 1988. White squares and lines are missing data whilst the white area of the Arctic has nighttime. (Data from NASA-Lan-gley)

flG. 8.8. High-level cloud albedo for January 1988. White squares and lines are missing data whilst the white area of the Arctic has nighttime. (Data from NASA-Lan-gley)

high-level clouds over oceans tend to have low albedo, especially in the Eastern Pacific and off the west coast of Africa.

8.4.2 Global distribution

The outgoing solar radiation (OSR) fluxes are determined by the incoming solar flux (ISR) and the planetary albedo. The latitudinal gradient (Fig. 8.9) is determined by the incoming solar flux, while the main longitudinal patterns are primarily due to features of the global distribution of surface albedo and cloud cover. Thus, maximum OSR values reaching 350 W m~2 are found in polar regions during local summer, i.e. in January and July in the Southern and Northern hemispheres, respectively, when the incoming solar radiation reaches maximum values of 500 W m~2, especially during perihelion. The OSR decreases gradually from the summer towards the winter pole, in January and July, where it switches off poleward of mid-to-high latitudes. Such a strong latitudinal OSR gradient is not found during spring and autumn. Large OSR fluxes are found over areas characterized by large surface albedo. For example, as large as 350 and 300 W m~2 are computed over Antarctica in January, and over Greenland and Arctic Ocean areas in July, respectively, i.e. over areas with surface albedo values larger than 70%. Also, large OSR values are found over regions having large cloud amounts; thus, OSR values as high as 220 W m~2 are found over the

Outgoing SW Radiation at TOA

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flG. 8.9. Long-term (1984-1997) average global distribution of outgoing shortwave radiation (in W m~2) at the top of the atmosphere for January. (Hatzianastassiou et al. 2004)

50 100 150 200 250 300 350

flG. 8.9. Long-term (1984-1997) average global distribution of outgoing shortwave radiation (in W m~2) at the top of the atmosphere for January. (Hatzianastassiou et al. 2004)

storm track zone of the Southern Hemisphere, around 60S, in January, whereas values between 130-200 W m~2 are computed in October (Fig. 8.9). A corresponding feature, with OSR up to 200 W m~2 in April and July, appears over the northern Pacific oceanic areas with large cloud amounts.

8.4.3 Zonal-seasonal variation

Monthly mean 10-degree latitude zonal fluxes were computed by averaging along latitude, by considering the fraction of surface area contained in each 2.5-degree zone. To a very good approximation the surface area can be taken to vary as cos 0, where 0 is the latitude and so the surface area fraction is given by Ej = sin 10j — sin 10(j — 1) and equal to 0.1736 for j = 1 corresponding to zone 10-0°, and sequentially equal to 0.1684, 0.1580, 0.1428, 0.1232, 0.10, 0.0737, 0.0451, 0.0152, through to zone 90-80°. The lowest seasonal variation of OSR occurs in the tropics, where values remain near 100 W m~2 throughout the year, as can be seen in Fig. 8.10. The seasonal variability increases towards the poles, with the largest values of about 300 W m~2 occurring during polar summer, and is primarily associated with the solar zenith angle. The latitudinal variation of OSR depends also on scattering and absorption processes and on atmospheric conditions such as the variation in amounts and types of clouds with latitude.

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Month Month Month flG. 8.10. Long-term (1984-1997) zonal-seasonal variation of outgoing shortwave radiation (W m~2) at TOA, for both hemispheres, NH (dotted lines) and SH (solid lines).

8.4.4 Mean annual latitudinal variation

Monthly mean 10-degree latitude zonal fluxes were computed by averaging along latitude, by considering the fraction of surface area contained in each 2.5-degree zone. Subsequently, annual mean quantities were computed by summing the corresponding monthly means for each 10-degree latitudinal zone over the 12 months of the year. The reflected OSR flux (Fig. 8.11) is maximum near the equator, has a minimum of about 93 W m~2 in the sub-tropics (10-20° N and S), and increases towards the poles, with secondary minima of 100 and 90 W m~2 near 60-70° S and N, respectively. In the tropics, about 25% of the incoming mean annual solar radiation is scattered back to space (Rp « 0.25, Fig. 8.12). The planetary albedo increases from the equator to the poles, with a small minimum between 10 and 20° N and S, reaching values of 65% and 50% at the south and the north pole, respectively. The general equator-to-pole increase in Rp is caused by increasing surface albedo values, associated with larger land-to-sea fractions and snow and ice-covered areas, increasing solar zenith angle, and high cloud amounts. The secondary Rp minimum near the equator is due to large cloud amounts along the intertropical convergence zone (ITCZ).

130-

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flG. 8.11. Long-term (1984-1997) annual average latitudinal variation of outgoing shortwave radiation (W m~2) at the top of the atmosphere. (Hatzianastassiou et al. 2004b)

20 40 60 80 100

Latitude flG. 8.12. Long-term (1984-1997) annual average latitudinal variation of planetary albedo at the top of the atmosphere. (Hatzianastassiou et al. 2004b)

8.4.5 Seasonal variation

The seasonal range of OSR for the Southern Hemisphere is slightly larger than for the Northern Hemisphere. It ranges from about 60 to about 140 W m~2 for the Northern Hemisphere, and between 55 and 155 W m~2 for the Southern Hemisphere. The mean global OSR (Fig. 8.13) has a very small seasonal variability of about 15 W m~2, i.e. between 95 and 110 W m~2, with a minimum in late summer-early autumn. The mean hemispherical planetary albedo (Fig. 8.14) does not show great seasonality; it varies between 27.5% (minimum value in October) and 30.2% (maximum value in May) for the Northern Hemisphere, and between 26.6% (minimum in June) and 31.5% (maximum in December) for the Southern Hemisphere. The global mean planetary albedo ranges from about 28% in September and 30.5% in December.

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flG. 8.13. Long-term (1984-1997) latitudinal average seasonal variation of outgoing shortwave radiation (W m~2) at the top of the atmosphere. (Hatzianastassiou et al. 2004b)

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South Hemisphere

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South Hemisphere

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flG. 8.14. Long-term (1984-1997) latitudinal average seasonal variation of planetary albedo at the top of the atmosphere. (Hatzianastassiou et al. 2004b)

8.4.6 Mean annual hemispherical variation

Hemispherical means are computed by weighting the latitudinal variations of the fluxes F with the fraction of surface area, Ej (§8.4.3), contained in each latitudinal zone j

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