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The role of the land surface in
the climate system
April 2002
European Centre for Medium-Range Weather
Forecasts
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2 . Surface energy and water budget
Fig. 1 represents in a schematic form the energy and water balance at the land surface. The surface albedo, controlling the fraction of the incident shortwave radiation absorbed by the surface, depends on the soil and vegetation type and state and on the amount of snow present. The net longwave radiation, LW, depends also on properties of the land surface, namely the surface emissivity and the surface skin temperature. Since the net radiation flux (the sum of solar and longwave radiation) is downward, and because the land surface has a small thermal inertia, the sum of latent and sensible heat fluxes must be an upwards flux. Note that the surface latent heat flux, LE, in the energy budget (left panel) is equal to the latent heat, L, times the evaporation flux, E, in the water budget (right panel), indicating that the water is available at the surface in a condensed phase and is passed to the atmosphere in the vapour phase. In that process, the surface undergoes evaporative cooling.
As mentioned in the introduction, the partitioning of the energy available at the surface into latent and sensible heat depends crucially on the soil moisture. Vegetated covered surfaces have the ability to draw water from a depth of order 1 m (the root layer), while for bare ground only the water in the top few cm of soil contributes for evaporation. The latent and sensible heat fluxes (LE and H, respectively) play a different role for the atmosphere. Sensible heat at the bottom means energy immediately available to the atmosphere, and contributes to the heating and/or deepening of the planetary boundary layer (BL), that shallow portion of the atmosphere directly affected by the surface. The surface evaporation flux does not directly heat the atmosphere, but provides moisture to the BL or, in the case of deep convection, to the whole troposphere. In favourable conditions, that contributes to precipitation generation mechanisms, with the associated release of latent heat into the whole troposphere. It is clear that, when compared to the sensible heat flux, evaporation can indirectly lead to a very efficient transfer of energy affecting a much deeper atmospheric layer. For an entire atmospheric column, the net radiative cooling is balanced by energy involved in phase changes inside the column (condensation of water vapour and evaporation of rain) and sensible heat flux at the surface. Land surface processes affect directly or indirectly this balance.
Table 1 summarises the surface annual mean fluxes for the 1979-1993 period covered by the 15 year European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA15, Gibson et al. 1997). Values presented are global averages over land and sea separately, in W m-2, and downward fluxes are positive. The net heat flux, G0, is the sum of all the surface fluxes. The contrasts between land and sea are clear. Even for such a large time period, the net flux is non-zero over sea, emphasising the larger thermal inertia of the oceans. The continents have a fast responsive surface and adjust their surface temperature in order to maintain a zero-heat flux at the surface, while the oceans have a much larger thermal inertia, with relatively small variations in surface temperature and flux imbalances allowed in much longer time scales. The last column, the Bowen ratio, Bo, is the ratio of sensible and latent surface heat fluxes. The larger values over land are indicative of the relatively difficulty of accessing the water at the surface. Over vegetated surfaces, this corresponds to the physiological mechanisms controlling transpiration while over bare ground the water directly accessible for evaporation is limited to the top few soil cm.
Surface energy fluxes for an entire season (not shown in the table above) still balance out, indicating that the energy associated to the seasonal changes in soil temperature are negligible when compared to the individual surface energy fluxes. However, for the surface water balance on a seasonal time scale, the storage term, or change in total soil water content, can be of similar magnitude to the precipitation or evaporation, which is self-evident in any extended drought period. In the next section, we will discuss the different time scales regulating the surface and regulated by the surface.
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12.06.2002 |
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