The shortwave radiation budget at surface

8.5.1 Global distribution

Figure 8.16 shows the geographical distribution, on a 2.5x2.5 degree resolution, of the 17-year (1984-2000) average downwelling shortwave radiation (DSR) at

flg. 8.16. Long-term (1984-2000) average global distribution of downward shortwave radiation (W m~2) at the Earth's surface for the month of January. (Hatzianastas-siou et al. 2005)

the Earth's surface for the month of January. The latitudinal gradient of DSR is primarily determined by the incoming solar flux at TOA, and secondarily by clouds, while the patterns of longitudinal variation are mostly determined by cloud and surface properties. There is a gradual DSR decrease from the summer to the winter pole in January and July. Overall, the DSR has maximum values over the subsidence regions associated with anticyclonic conditions and small cloud amounts, such as oceanic areas in low latitudes of the summer hemisphere, as well as over the polar areas of the summer hemisphere. In contrast, small DSR values are found over regions with large cloud amounts, such as the middle latitudes of the summer hemisphere. Note that there are small DSR values off the western coasts of South America and South Africa in July, being smaller than corresponding DSR fluxes in adjacent regions of the same latitude, which are attributed to large cloud amounts (of about 80%). Also, there are relatively small DSR values over south-eastern Asia in July (150 W m~2), where large cloud cover (80%) occurs, associated with the monsoons. Apart from oceanic areas, there are some extended continental regions with large DSR values, such as the United States, Southern Europe, North Africa and Middle-East in July, or correspondingly South America, South Africa and Australia in January.

a oi

400 350 300 250 200 150 100 50 0

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Month Month Month

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Month Month Month flG. 8.17. Long-term (1984-1997) zonal-seasonal variation of downward shortwave radiation (DSR in W m~2) at the Earth's surface, NH (dotted lines) and SH (solid lines).

8.5.2 Zonal-seasonal variation

The seasonal variation of DSR is similar to that of OSR, except that there is a larger variation in the subtropics and that the Arctic variation is smaller than the Antarctic, as shown in Fig. 8.17. Large DSR fluxes are found in polar regions during local summer, equal to about 300-350 W m~2 over Antarctica in January, and 200-250 W m~2 over the Arctic in July. The larger values over Antarctica, compared with the Arctic, are due to the larger incoming solar radiation at TOA during perihelion (> 500 W m~2), but also to the smaller ISCCP-D2 summer cloudiness in Antarctica than in the Arctic (20% against 60%, respectively) and drier atmosphere which affects the near-infra-red. In both hemispheres, the maximum of DSR values outside the polar regions occur in subtropical areas (between 10° and 35° latitude) rather than in the tropics. This is due to the fact that total cloudiness has minimum values (about 50%) over the subtropical areas, and not along the equator, where the ITCZ involves total cloud-cover values of about 60-70%. In the equatorial region (10°S-10°N), the double peaks in DSR occur during spring and autumn when the Sun is overhead the equator.

200-

Latitude

flG. 8.18. Mean annual downward shortwave radiation (DSR in W m-2) at the Earth's surface, for the period January 1984 through to December 2000. (Hatzianas-tassiou et al. 2005)

8.5.3 Latitudinal and seasonal variation

Monthly mean 10-degree latitude zonal fluxes were computed by averaging first along 2.5°-width longitudinal circles, then 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 latitudinal variation of mean annual DSR flux has maximum values of about 230 W m~2 in subtropical areas decreasing rapidly to about 80 W m~2 towards the poles (Fig. 8.18). There is a small local minimum in mean annual DSR at the equator (equal to about 220 W m~2), due to clouds associated with the ITCZ. There is also a local maximum of 230 W m~2 in DSR around 15°N. Note that in the Southern Hemisphere there is a local minimum due to the persistent and extended cloudiness occurring over the oceanic zone (storm track zone) surrounding Antarctica. The mean hemispherical DSR fluxes have opposite seasonalities, varying within the range 110-230 W m~2, thus resulting in a mean global DSR flux ranging from 165 to 176 W m~2 throughout the year (Fig. 8.19). The seasonal variability is slightly larger in the Southern than in the Northern hemisphere.

8.5.4 Validation with observations

Model computed DSR fluxes need to be validated through comparison with corresponding extensive measurements from ground-based stations. A comparison of observed versus calculated monthly mean DSR fluxes within each 2.5x2.5

260

240

220

200

CVI

E

180 -

S

/

cc

160 -

/

S

/

Q

/

140 _

/

/

/

120 -

/

100 -

80

■ North hemisphere South hemisphere

1 2 3 4 5 6 7 8 9 10 11 12 Month flG. 8.19. Long-term (1984-2000) average seasonal distributions of downward shortwave radiation (W m~2) at the Earth's surface for the Northern Hemisphere, Southern Hemisphere and the Globe. (Hatzianastassiou et al. 2005)

degree cell containing a GEBA station is shown in the scatterplot of Fig. 8.20. The scatterplot shows a comparison of all measured monthly values against the model values within each cell containing a GEBA station. The overall bias is equal to -6.5 W m~2, the root-mean-square error (RMS) is 23.4 W m~2, the slope of the one-to-one line is 0.974, the correlation coefficient is 0.99 and the number of measurements is N = 27858. The model underestimates DSR in the polar areas, but we note that the detection of clouds is problematic over highly reflecting surfaces.

8.5.5 Mean annual hemispherical variation

Downward and net downward (absorbed) SW radiation at the Earth's surface, for the four midseason months of the year, are given in Table 8.10, for the 17-year period from 1984 to 2000.

The long-term model results indicate that the Earth's surface receives an annual average of 171.6 W m~2 and absorbs 149.4 W m~2 (Table 8.11), resulting in a long-term surface albedo equal to 12.9%. The interhemispherical differences are equal to 4.6 W m~2 for DSR and only 0.3 W m~2 for net DSR, implying slightly larger Northern than Southern hemisphere surface solar radiative fluxes. Overall, according to model results using ISCCP-D2 data, on a mean annual basis and at the global scale, the Earth's surface receives 50.2%, while it absorbs 43.7% of the incoming SW radiation entering the Earth-atmosphere system. Model estimates have varied over the years, with the more recent estimates being closer to those

500 -|

Bias = -6.49

450 -

RMS = 23.43

-

Slope = 0.974

400 -

R = 0.992

350 -

N = 27858

300 -

250 -

200 -

150 -

100 -

50

0

0 50 100 150 200 250 300 350 400 450 500 GEBA DSR (W m-2)

flg. 8.20. Scatterplot comparison between model monthly DSR, against GEBA surface measurements. (Hatzianastassiou et al. 2005)

Table 8.10 Model mean annual hemispherical (NH is Northern Hemisphere, SH is Southern Hemisphere) and global averages of downward shortwave radiation (DSR) and, net downward shortwave radiation (net DSR) at the Earth's surface for January, April, July, October, and the whole year, for the period 1984-2000. The radiative fluxes are expressed in W m-2. Numbers in parentheses are standard deviations and represent interannual variabilities of the means. (Hatzianastassiou et al. 2005)

Table 8.10 Model mean annual hemispherical (NH is Northern Hemisphere, SH is Southern Hemisphere) and global averages of downward shortwave radiation (DSR) and, net downward shortwave radiation (net DSR) at the Earth's surface for January, April, July, October, and the whole year, for the period 1984-2000. The radiative fluxes are expressed in W m-2. Numbers in parentheses are standard deviations and represent interannual variabilities of the means. (Hatzianastassiou et al. 2005)

NH

SH

Global

Downward shortwave radiation

January

113.8(9.8)

234.8(20.7)

174.3(15.3)

April

209.8(16.5)

140.4(11.5)

175.1(14.0)

July

224.8(17.2)

105.6(7.7)

165.2(12.5)

October

148.6(12.0)

197.0(16.6)

172.8(14.3)

Annual

173.9(14.0)

169.3(14.3)

171.6(14.1)

Net downward shortwave radiation

January

98.6(2.7)

204.5(6.5)

151.6(4.6)

April

177.0(4.6)

127.5(3.4)

152.2(4.0)

Jul

193.6(5.1)

94.9(2.1)

144.2(3.6)

October

130.9(3.5)

171.6(4.3)

151.2(3.9)

Annual

149.6(3.9)

149.3(4.0)

149.4(4.0)

given by the GEBA surface observations, as can be seen in Table 8.11. 8.5.6 Long-term anomaly

The computed anomaly (defined as the DSR per cent differences from the long-term mean value) in mean hemispherical and global DSR flux over the 17-year

Table 8.11 Model long-term mean annual hemispherical and global averages of downward shortwave radiation (W m-2) at the Earth's surface (DSR), and net downward shortwave radiation (absorbed) at the Earth's surface (Net DSR) for the Northern Hemisphere, Southern Hemisphere and the Globe. (Hatzianastassiou et al. 2005)

Table 8.11 Model long-term mean annual hemispherical and global averages of downward shortwave radiation (W m-2) at the Earth's surface (DSR), and net downward shortwave radiation (absorbed) at the Earth's surface (Net DSR) for the Northern Hemisphere, Southern Hemisphere and the Globe. (Hatzianastassiou et al. 2005)

Study

DSR

Net DSR

NH

SH

Globe

NH

SH

Globe

Hatzianastassiou et al. (2005)

173.9

169.3

171.6

149.6

149.3

149.4

Wild et al. (1998 - ECHAM4)

170.0

147.0

Liou (2002)

189.0

161.0

Global Energy Balance Archive

169.0

151.0

NCAR/GCM3 (Gupta et al. 1999)

194.4

194.4

194.4

169.0

173.4

171.2

Garratt et al. (1998)

195.5

167.0

Kiehl and Trenberth (1997)

168.0

Fowler and Randall (1996)

172.0

Li et al. (1995)

157.0

Rossow and Zhang (1995)

193.4

165.1

Hartmann (1994)

171.0

Li and Leighton (1993)

155.0

159.0

157.0

Darnell et al. (1992)

173.0

151.0

period, is given in Fig. 8.21. The hemispherical mean DSR anomalies vary from -9 to 9 W m~2, while the mean global DSR anomaly has variations of up to 6 W m~2, about the 17-year mean value. Large negative anomalies, i.e. reduced SW radiation reaching the Earth's surface, were found for the period 1991-1993, which can be attributed to the Mount Pinatubo eruption in June 1991, and to the 1991/1992 El Niño event. During the same period, the outgoing SW radiation increased by about 4 W m~2. The induced rapid decrease in DSR is followed by a recovery period, with positive DSR anomalies through 1994. In the figure, there are other interesting features corresponding to climatic events, such as El Nino and La Ninña, associated with negative and positive DSR anomalies, respectively. For example, negative DSR anomalies as large as 6 W m~2 follow the 1986/1987 El Niño event. The large negative DSR anomalies of about -5 W m~2 during 1984 can be attributed to the influence of the El Chichon eruption that took place in 1983. Also, note that in 1998 the DSR anomalies pass from large positive values (of about 4 W m~2) to negative ones (equal to -2 W m~2), attributed to the 1997/1998 El Niño event. In general, the variation of DSR anomalies is similar but opposite in magnitude to that of outgoing shortwave radiation anomalies.

Also given is a 4th-order polynomial fit to 17-year time-series of global averages of DSR anomalies, which shows significant positive DSR anomalies (increasing DSR) in the period starting from year 1992. In contrast, in the period 1984-1992, negative DSR anomalies mostly occur, indicating a decreasing DSR flux at the Earth's surface. The situation was reversed after the early 1990s. This, according to an analysis based on ISCCP-D2 data (we note that the use of such data for trend analysis has been questioned, but here we are more concerned with the use

-Globe

---North hemisphere

-Globe

---North hemisphere

Pinatubo eruption

South hemisphere Pinatubo recovery -Polynomial fit of global data

Pinatubo eruption r r -i£

flG. 8.21. Time-series of global and hemispherical averages of downward shortwave radiation (DSR) flux anomalies (W m~2) for the period 1984-2000. (Hatzianastas-siou et al. 2005)

flG. 8.21. Time-series of global and hemispherical averages of downward shortwave radiation (DSR) flux anomalies (W m~2) for the period 1984-2000. (Hatzianastas-siou et al. 2005)

of anomalies), is due to a decrease in cloudiness, especially low-level, primarily in tropical and subtropical regions.

8.5.7 Sensitivity analysis

The assessment and quantification of the role of various key climatic parameters that determine the downwelling shortwave radiation (DSR) at the surface is very important for SW radiation budget studies. A series of sensitivity tests are given in Table 8.12. In each test, the relevant parameter, X, was modified by a certain amount, AX in relative percentage terms, and the resultant radiation change A DSR, with respect to defined reference cases, was computed in absolute terms (W m~2). The year 1988 was chosen to represent the reference case. The tests were performed on a monthly basis and at the grid-cell (2.5-degree latitude-longitude) level, and results are given in terms of the maximum A(DSR) values encountered. Clearly, cloud cover plays an important role in determining changes to DSR, and also the cloud asymmetry factor. Aerosol single scattering albedo and surface reflection also have a significant effect on DSR.

Table 8.12 Sensitivity analysis for maximum change in DSR observed globally within a grid-cell. Climatic parameters X are: Ac, cloud cover; t£ , cloud scattering optical depth; t£, cloud absorption optical depth; gc, cloud asymmetry parameter; WH2O, total precipitable water; WO3, total ozone column abundance; WCO2, total column carbon dioxide; Rg, surface albedo; ISR, incoming SW radiation at TOA; AOT, aerosol extinction optical depth; <waer, aerosol single scattering albedo; gaer, aerosol asymmetry parameter. (Hatzianastassiou et al. 2005)

Table 8.12 Sensitivity analysis for maximum change in DSR observed globally within a grid-cell. Climatic parameters X are: Ac, cloud cover; t£ , cloud scattering optical depth; t£, cloud absorption optical depth; gc, cloud asymmetry parameter; WH2O, total precipitable water; WO3, total ozone column abundance; WCO2, total column carbon dioxide; Rg, surface albedo; ISR, incoming SW radiation at TOA; AOT, aerosol extinction optical depth; <waer, aerosol single scattering albedo; gaer, aerosol asymmetry parameter. (Hatzianastassiou et al. 2005)

X

AX (%)

A(DSR) (W m-'2)

Low-Ac

10

-22.05

Middle-Ac

10

-17.51

High-Ac

10

-15.30

Low-t?

10

-5.92

Middle-^?

10

-5.21

High-tcs

10

-5.84

Low-tca

10

-1.64

Middle-tca

10

-1.52

High-tca

10

-1.37

g

5

20.64

WH2O

10

-6.65

WO3

10

-3.37

WCO2

10

-0.51

Rg

10

13.41

ISR

1

4.63

AOT

10

-2.62

^aer

10

11.81

gaer

10

4.79

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  • goldilocks
    How average shortwave radiation is related to averagevtemperature?
    8 months ago

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