Cold Surfaces

At noon on a day in February at 4,400 m on the Meren Glacier (near the equator in New Guinea), there was both (i) incoming net radiation of 398 W/m2 to the snow and (ii) a 31 W/m2 flux of sensible heat from the air. The total income of 429 W/m went into sublimation and evaporation from the surface (153 W/m) and into warming and melting the snow (276 W/m2, i.e. 429-153). Such a description of the energy flows shows precisely what processes were dominant. It shows, for instance, that 64 per cent (i.e. 276/429) of the incoming energy was used in warming and melting the snow (Chapter 10).

Values from Antarctica in midwinter are shown in Table 5.2. The net radiation is opposite and equal to the total of the other three factors, and is negative (i.e. upwards, away from the

Table 5. 1 Average energy fluxes in selected regions. The fluxes are of net radiation Rn, latent heat LE, vertical convection of sensible heat into the air H, and horizontal advection A. The signs of the energy fluxes (W/m2) are consistent with Figure 5.1

Average energy fluxes: (W/m2)

Region_Rn_LE_H_

Antarctica -15 0 -15 0

Australia 93 38 54 0

South America 93 67 25 0

Indian Ocean 113 102 9 2

Table 5.2 Typical Antarctic energy balances in midwinter; the signs of the energy fluxes (W/m2) are consistent with Figure 5.1

Place

Rn

L.E

H

Gor A*

South Pole

-20

-1

-18

-2

Coast

-43

+6

-39

-10

Ocean

-12

+24

+29

-65

* Ground heat-flux G for land surfaces, but advection A in ocean currents

* Ground heat-flux G for land surfaces, but advection A in ocean currents surface). It is offset partly by a downward flux of sensible heat from the air, which must therefore be warmer than the ground (Figure 1.9, Note 3.A). Also, there is some sublimation of vapour onto the ground at the pole, and considerable advection of heat in ocean currents from lower latitudes.

The circumpolar oceans are generally frozen in winter, except for temporary gaps in the ice. They occupy only a few per cent of the area, but importantly affect evaporation and the sensible-heat flux.

We can compare the fluxes in Antarctica shown in Figure 5.5 with those in Figure 5.3 for the whole world. There is less attenuation of the incoming solar radiation by cloud and air turbidity, since 78 per cent reaches the ground instead of 56 per cent, and the ice of Antarctica reflects a notable amount of shortwave radiation (65 units instead of 6). The surface flux of sensible heat is downwards towards the surface, and exceeds the upwards latent-heat flux due to the sublimation of snow. The net radiation loss of 27 units (i.e. 100-[65+l6]-46) shows that Antarctica is an energy sink, i.e. it absorbs energy on the whole (Figure 2.18). The absorption of heat is made good by heat-flux convergence in the winds.

Another set of data was obtained over six days in summer, from an ice-free rock valley in Antarctica. The surface at noon was about 10 K warmer than the air 1.6 m above the ground, and simultaneously the upwards flow of heat into the air was 33 W/m2. This was only a small part of the incoming net radiation, which was 130 W/m2. The heat consumed in evaporation was measured as 16 W/m2. So the energy-balance equation shows that the remainder (due to the noontime flux of heat into the ground) was 81 W/m2 (i.e. 130-16-33).

A Lake

Figure 5.6 shows the terms of the energy balance each month at the surface of a reservoir

Figure 5.5 Annual mean fluxes of energy over the Antarctic. Underlined numbers refer to longwave radiation. [D+S] is the sum of direct and diffuse shortwave radiation. [H+LE] is the sum of sensible and latent-heat fluxes.
Figure 5.6 Variation of the components of the energy balance at the surface of Cataract reservoir in New South Wales.

in New South Wales. The highest rate of evaporation (equivalent to a latent-heat flux of about 130 W/m2) represents a lowering of the lake level by 4.6 mm daily (Note 4.D). Incoming net radiation was less than the heat used in evaporation from March-August, but greater in the warmer months. The small values of H indicate that little heat was carried away in the wind, i.e. there was not much difference between lake-surface and surface-air temperatures (Note 3.A).

The term G in the case of a lake is a heat-storage term, and a negative G implies heat flowing upwards towards the surface (Figure 5.1), i.e. the lake is cooling. In the present case, the lake cooled between April and July, allowing the latent-heat flux to exceed the net radiation. The lake was coldest in August and warmest in late March.

Other Examples

(a) The energy balance in Chilton Valley in the South Island of New Zealand shows a strong seasonal variation (Figure 5.7). All fluxes are small in winter. November is a dry month so there is less evaporative cooling of the soil, consequently its surface temperature rises, and therefore there is an increase of the sensible-heat flux to the atmosphere. Data on soil heating G show a small absorption of heat from October to February and a corresponding release in the cooler months.

(b) Figure 5.8 shows the seasonal variations of net radiation, evaporation and convection at four places. There are two net-radiation maxima at the lowest latitudes, when the Sun passes overhead at noon (Section 2.2). Annual fluctuations increase with latitude, in accord with the larger annual range of temperature (Section 3.3). Most of the net radiation is used in evaporation in the cases of the oceans and the moist climate of Manaus in the Amazon basin. The opposite is true of Mocamedes at the northern end of the Namibian desert, where there is practically no water for evaporation.

(c) Apart from average values over a month or more, there can be large changes from day to day, or hour to hour, depending on changes of cloud, rain or wind temperature. Figure 5.9 illustrates the example of a

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