Values

Typical radiation conditions are shown in Table 2.5 for a forest and meadow. Even though the solar radiation onto the meadow happened to be more (i.e. 336 instead of 292 W/m2), the net radiation onto the forest was greater, on account of a lower albedo. The radiation efficiency (i.e. the ratio Rn/Rs) was 64 per cent for the forest and

51 per cent for the meadow at the time of the measurements. The ratio is quite different at the South Pole, where Rn is negative (Table 2.6).

The net radiation at the surface varies primarily according to latitude (Figure 2.17), as do the components of the net radiation at the top of the atmosphere (Figure 2.18). The latter shows the variation with latitude of (i) the longwave-radiation loss to space, and (ii) the net solar-radiation income Ra.(1-a), where Ra is the extra-terrestrial radiation (Note 2.F) and a is the planetary albedo according to latitude (Figure 2.15). The shortwave income varies greatly with latitude because of latitudinal differences of albedo (due to ice and cloud) and of Ra. But the longwave loss is remarkably unaffected by latitude, despite the variation of surface temperature from equator to pole. This is because much of the loss is from the tops of clouds, which tend to be warmer than the ground at the poles, but cold in the tropics because clouds there are so tall, on account of the strong

Table 2.5 Typical radiation budgets for a forest and a meadow at 44°N in Oregon, averaged over different entire clear days

Radiation *

Symbol

Forest

Meadow

Shortwave

Incoming

Rs f

100

100

Reflected

a Rs t

-10

-24

Longwave

Incoming

Rid

108

86*

Outgoing

Rh

-135

-112

net

Rn

63*

50*

* Radiation components are expressed as percentages of the incoming solar radiation, i.e. of 292 W/m2 for the forest and 336 W/m2 for the meadow t Note that the figures for Rs and a.Rs imply an albedo of 10% for the forest (i.e. 10/100), and 24% for the meadow

* Radiation components are expressed as percentages of the incoming solar radiation, i.e. of 292 W/m2 for the forest and 336 W/m2 for the meadow t Note that the figures for Rs and a.Rs imply an albedo of 10% for the forest (i.e. 10/100), and 24% for the meadow

Table 2.6 Components of radiation fluxes on the ground at the South Pole in units of W/m2

Direction

Shortwave SW

Longwave LW

All-wave

Downwards

+ 137

+ 113

+250

Upwards

-116*

-143t

-259

Net

+21

-30

-9t

* Note the high albedo, inferred as 85% (i.e. 116/137).

t This was estimated by means of the Stefan-Boltzmann equation (Note 2.C), from the mean temperature of -49°C. t The net radiation is negative at the South Pole even in January (i.e. -1 W/m2). Different values have been measured at Vostok (78°S, 3,450 m elevation), i.e. an annual mean of -5 W/m2, ranging from a net income of + 11 W/m2 in January to a net loss of -22 W/m2 in July.

* Note the high albedo, inferred as 85% (i.e. 116/137).

t This was estimated by means of the Stefan-Boltzmann equation (Note 2.C), from the mean temperature of -49°C. t The net radiation is negative at the South Pole even in January (i.e. -1 W/m2). Different values have been measured at Vostok (78°S, 3,450 m elevation), i.e. an annual mean of -5 W/m2, ranging from a net income of + 11 W/m2 in January to a net loss of -22 W/m2 in July.

Figure 2.17The annual mean net radiation at the Earth's surface in W/m2. The dashed line is the annual mean boundary of sea-ice; net radiation to the surface can be negative poleward of that line, i.e. upwards.

convection. Even in cloud-free parts of the humid tropics the longwave radiation loss is less than might be expected, since it comes not from the hot surface but from the atmospheric moisture at cooler levels above.

The comparison shows that the global income is exceeded by the loss over the 43 per cent of the Earth's area which is above about 35 degrees latitude, i.e. there is an overall inflow of radiation from space (i.e. the Sun) equatorward of 35 degrees latitude, but an equal outflow from higher latitudes. That would mean a tendency for the equator to become ever warmer and the poles to cool, except for the flow of energy from the equator to the poles in winds and ocean currents (Chapters 5, 11 and 12).

There is a notable contrast between the net radiation onto oceans near the Tropics, and onto adjacent lands. A high intensity over the oceans (Figure 2.17) arises from the low albedo (Table 2.3) and a reduced upward LW due to a

1 100

1 100

-

absorbed

SW

-

f

/

y

X

\

«

/ f

outgoing

LW

11

1 1

1 1

1 1

1 1

11

60 40 20 0 20 40 60 South North latitude: degrees

60 40 20 0 20 40 60 South North latitude: degrees

Figure 2.18 Effect of latitude on the extra-terrestrial fluxes of (i) the net shortwave radiation absorbed by the Earth, i.e. [1-a] R, where a is the average planetary albedo and Ra is the average extra-terrestrial radiation at the particular latitude, and of (ii) the longwave radiation emitted by the surface and atmosphere of the Earth.

Figure 2.19 Net radiation onto Australia in units of W/m2

combination of high humidity, cloudiness and a surface cooled by evaporation (Chapter 4).

However, adjacent land areas near the Tropics tend to be dry, so that net radiation is reduced by a high albedo (Section 2.5) and an extreme LW loss, due to low humidity, an absence of clouds and high daytime surface temperatures.

Cloud alters the net radiation at the surface less than might be expected, despite the reduced solar radiation Rs. That reduction tends to be offset by (i) less upward reflected solar radiation

(a.R), (ii) an extra downward flux due to some reflection of the upwards a.RS back down by the cloud, (iii) increased sky radiation downwards, and (iv) less terrestrial radiation up on account of the cooling of areas shaded by cloud. A similar compensation applies with an increase of altitude; greater solar radiation (due to less attenuation by the atmosphere) is offset by less longwave radiation from the colder and drier sky.

Surface net radiation in Australia is low in the south of the continent in winter but fairly uniform in summer (Figure 2.19). Part of the reason is that summers in northern Australia are cloudy, offsetting the reduced insolation at higher latitudes. Such values of the net radiation explain the patterns of surface temperature in time and space, which are the topic of the next chapter.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment