Heat exchange through latent and sensible heat

2.2.1 Latent heat

Radiation dominates the exchange of heat between the atmosphere and ocean. Other physical mechanisms do, however, also contribute to the net heat flux. The most important of these is latent heat transfer. When water is evaporated from the ocean surface energy is supplied to the molecules to free them from the strong inter-molecular bonds within liquid water. This process was discussed in §1.3.1. When the water molecules condense to form water droplets, usually in clouds, this energy is released to heat the surrounding air. This internal source of heat adds to a cloud's buoyancy, allowing it to penetrate higher into the troposphere. Such additions of heat can eventually assist in the creation of new pressure gradients within the atmosphere, thus driving large-scale atmospheric motion.

Latent heat transfer is therefore an important means of re-cycling energy through the ocean-atmosphere system. A peculiar property of the

Fig. 2.5. Variation of saturated specific humidity with temperature at a pressure of 1000 mb. [From Bigg, 1992b.]

Fig. 2.5. Variation of saturated specific humidity with temperature at a pressure of 1000 mb. [From Bigg, 1992b.]

atmosphere - possible saturation with respect to water vapour - makes this form of heat exchange highly temperature-dependent. A parcel of air said to be saturated with respect to water vapour is one where its constituent water vapour is in equilibrium with an underlying flat surface of water. This means that the same number of water molecules enter the body of water from the air, per second, as are evaporated from the surface. We have already implicitly assumed knowledge of this property in discussing condensation; air containing more water vapour than its saturated level tends to form clouds, with the water vapour condensing to form droplets (the climatically important process of cloud formation, including the influence of the ocean, will be discussed in more detail in §2.8.3, §4.4, and Chapters 5 and 7).

The variation with temperature of the saturation level is shown in Fig. 2.5, in terms of the saturated specific humidity. At temperatures near 0°C only a few grammes of water vapour, per kilogramme of air, need to be present in the atmosphere before saturation is reached, while at 30°C, 30 grammes are required. Air in the tropics, therefore, can contain much more vapour than air in polar latitudes. Tropical weather systems can thus be much more energetic, because of the greater potential source of latent heat. In §2.11.1 we will see that hurricanes are the most dramatic consequences of this tropical latent heat excess, but Chapter 5 will show that this tropical energy release can significantly influence the climate of the extratropics as well. The latent heat, Q E (Wm-2), added to the atmosphere by the ocean is

where E is the evaporation (in kgm 2s 1) and L v is the latent heat of vaporization. The latter is the energy required to vaporize one kilogramme of water. It depends slightly on temperature, but for our purposes can be taken as Lv = 2.5 x 106 Jkg-1.

The determination of E is more difficult. In the first few millimetres above the ocean surface the air is in equilibrium with the underlying water surface: it is saturated with respect to water vapour. The evaporation rate will therefore depend on the humidity gradient between this saturated microlayer and the planetary boundary layer as a whole. Conventionally, the specific humidity at 10 m is assumed to represent that of the lower boundary layer atmosphere. The evaporation rate will also depend on both the degree of turbulence and strength, u, of the wind, as the wind transports fresh supplies of air to the location being studied. Once again it is extremely difficult to measure evaporation at sea (it is more complex than measuring the depth of water lost from a pan over land as well!), so an empirical formula must be used for its estimation:

In (2.4) cE is a non-dimensional number approximately equal to 1.5 x 10-3, pa is the density of air, qs is the saturated specific humidity at the sea surface (where the air is at the sea surface temperature, TS), and qa is the specific humidity at 10 m above the sea surface (qs and qa are measured in g/kg). This latter variable is unlikely to be at saturation for the air temperature, TA.

Latent heat transfer is extremely variable, both in space and time. It can be near zero in still, foggy conditions or comparable with the radiation terms in dry, warm, and windy weather. Average evaporation rates for the Atlantic Ocean in January and July are shown in Fig. 2.6. These reveal very different patterns to the net radiation of Fig. 2.4. Peaks in evaporation occur throughout the year in the sub-tropics because of the warm dry air in these zones of atmospheric subsidence (see §1.2). The strong evaporation off South Africa all year, off the eastern United States in January, and in the northeast Atlantic throughout the year occurs because of warm upper ocean currents. These are the Agulhas Current, the Gulf Stream, and the North Atlantic Drift respectively (see Fig. 1.15). The surface water in these currents is at generally higher temperatures than the overlying air, particularly in winter, so that the humidity gradient in the lower atmospheric boundary layer is enhanced. The frequent occurrence of strong winds adds to this intensification of evaporation, as does the supply of dry air off the neighbouring land masses. In Chapter 5 we will see that such strong forcing of the atmospheric heat budget by the ocean is important in both regional and global climate.

2.2.2 Sensible heat

The direct physical contact of the atmosphere and ocean enables energy to be exchanged between them by conduction. Such energy exchange is known as sensible heat. This occurs due to collisions between the molecules of the two fluids at their interface, with energy being transferred to the cooler, and therefore, slower, molecules. It should be noted that this process is a statistical one: the molecular speeds cover a range of values with their mean being the important quantity for determining temperature and energy transfer through conduction.

Fig. 2.6. Latent heat flux (in Wm-2) at the ocean surface over the Atlantic in (a) January, and (b) July. Dotted contours indicate net loss of heat from the surface. Note that the contour interval changes between months, due to the greatly enhanced fluxes in winter. [Data from Oberhuber, 1988.]

Sensible heat transfer therefore depends on the temperature difference between the near-surface air and the sea surface. As with the latent heat flux calculation, the air's temperature at 10 m is assumed representative of the near-surface region. Turbulence, and high wind speeds, encourage conduction by mixing air from higher in the atmosphere with that at the surface, allowing the ocean to interact with more air than that in the shallow surface layer.

The sensible heat transferred to the atmosphere, QS (measured in Wm-2), is again difficult to measure, so the empirical formula

is often used. The Dalton number, cH, is usually taken to be a function of the degree of turbulence in the atmosphere. A typical range of values is from 1.10 x 10-3 in an atmosphere with much vertical mixing to 0.83 x 10-3 in stratified air. The sensible heat transfer is generally much smaller than the other components of the heat balance at the air-sea interface because the temperature difference between the ocean and atmosphere is often less than 2°C. This is shown for the Atlantic Ocean in Fig. 2.7. Almost everywhere equatorwards of 30° there is a sensible heat flux to the atmosphere smaller than 10 Wm-2, in some places even negative, or towards the ocean. Higher values occur in some

similar places to latent heat flux peaks, such as over warm ocean currents. This is particularly true in winter when the air-sea temperature difference is greatest and the winds tend to be stronger. Thus, these two processes reinforce in such areas to contribute substantially to the atmospheric heat supply.

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Responses

  • maciej
    Why is sensible heat important for oceans?
    8 years ago
  • Mareta
    Why is more sensible heat exchange with more wind?
    5 months ago

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