Rock and sand

The energy exchanges of dry desert surfaces are relatively simple. A representative diurnal pattern of energy exchange over desert surfaces is shown in Figure 12.3. The 2-m air temperature varies between 17 and 29°C, although the surface of the dry lake-bed reaches 57°C at midday. Rn reaches a maximum at about 13:00 hours when most of the heat is transferred to the air by turbulent convection; in the early morning the heating goes into the ground. At night, this soil heat is returned to the surface, offsetting radiative cooling. Over a twenty-four-hour period, about 90 per cent of the net radiation goes into sensible heat, 10 per cent into ground flux. Extreme surface temperatures exceeding 88°C (190°F) have been measured in Death Valley, California, and it seems that an upper limit is about 93°C (200°F).

Figure 12.3 Energy flows involved at a dry-lake surface at El Mirage, California (35°N), on 10 to 11 June 1950. Wind speed due to surface turbulence was measured at a height of 2 m.

Source: After Vehrencamp (1953) and Oke (1978).

Figure 12.4 Diurnal temperatures near, at and below the surface in the Tibesti region, central Sahara, in mid-August 1961. (A) At the surface and at 1 cm, 3 cm and 7 cm below the surface of a basalt. (B) In the surface air layer, at the surface, and at 30 cm and 75 cm below the surface of a sand dune.

Figure 12.5 Average diurnal variation of the energy balance components in and above the tropical Atlantic Ocean during the period 20 June to 2 July 1969.

Figure 12.4 Diurnal temperatures near, at and below the surface in the Tibesti region, central Sahara, in mid-August 1961. (A) At the surface and at 1 cm, 3 cm and 7 cm below the surface of a basalt. (B) In the surface air layer, at the surface, and at 30 cm and 75 cm below the surface of a sand dune.

Source: After Peel (1974).

Surface properties modify the heat penetration, as shown by mid-August measurements in the Sahara (Figure 12.4). Maximum surface temperatures reached on dark-coloured basalt and light-coloured sandstone are almost identical, but the greater thermal conductivity of basalt (3.1 W m-1 K-1) versus sandstone (2.4 W m-1 K-1) gives a larger diurnal range and deeper penetration of the diurnal temperature wave, to about 1 m in the basalt. In sand, the temperature wave is negligible at 30 cm due to the low conductivity of intergranular air. Note that the surface range of temperature is several times that in the air. Sand also has an albedo of 0.35, compared with about 0.2 for a rock surface.

2 Water

For a water body, the energy fluxes are apportioned very differently. Figure 12.5 illustrates the diurnal regime for the tropical Atlantic Ocean averaged for 20 June to 2 July 1969. The simple energy balance is based on the

Figure 12.5 Average diurnal variation of the energy balance components in and above the tropical Atlantic Ocean during the period 20 June to 2 July 1969.

Source: After Holland. From Oke (1987). By permission of Routledge and Methuen & Co, London, and T.R. Oke.

assumption that the horizontal advective term due to heat transfer by currents is zero and that the total energy input is absorbed in the upper 27 m of the ocean. Thus, between 06:00 and 16:00 hours, almost all of the net radiation is absorbed by the water layer (i.e. AW is positive) and at all other times the ocean water is heating the air through the transfer of sensible and latent heat of evaporation. The afternoon maximum is determined by the time of maximum temperature of the surface water.

3 Snow and ice

Surfaces that have snow or ice cover for much of the year present more complex energy budgets. The surface types include ice-covered ocean; glaciers, tundra; boreal forests, steppe, all of which are snow-covered during the long winter. Rather similar energy balances characterize the winter months (Figure 12.6). An exception is the local areas of ocean covered by thin sea ice and open leads in the ice that have 300 W m-2 available -more than the net radiation for boreal forests in summer. The spring transition on land is very rapid (see Figure 10.38). During the summer, when albedo becomes a critical surface parameter, there are important spatial contrasts. In summer, the radiation budget of sea ice more than 3 m thick is quite low and for ablating glaciers is lower still. Melting snow involves the additional energy balance component (AM), which is the net latent heat storage change (positive) due to melting (Figure

Figure 12.6 Energy balances (W m 2) over four terrain types in the polar regions. M = energy used to melt snow. Source: Weller and Wendler (1990). Reprinted from Annals of Glaciolog/, by permission of the International Glaciological Society.

12.7). In this example of snow melt at Bad Lake, Saskatchewan on 10 April 1974, the value of Rn was kept low by the high albedo of the snow (0.65). As the air was always warmer than the melting snow, there was a flow of sensible heat from the air at all times (i.e. H negative). Prior to noon, almost all the net radiation went into snow heat storage, causing melting, which peaked in the afternoon (AM maximum). Net radiation accounted for about 68 per cent of the snow melt and convection (H + LE) for 31 per cent. Snow melts earlier in the boreal forests than on the tundra, and as the albedo of the uncovered spruce forest tends to be lower than that of the tundra, the net radiation of the forest can be significantly greater than for the tundra. Thus, south of the arctic treeline the boreal forest acts as a major heat source.

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