92
point is assumed to be covered either by one or by a combination of surface types, each
covering a fraction of the grid cell. Details on how different models treat different surface
types is given in section 3, while approaches to the description of surface heterogeneity
are discussed in section 4. The next two subsections present a more general discussion of
the representation of the surface radiative and turbulent flux terms.
2.1.1. Radiative processes
The net radiation at the surface, RN is given by
(7)
where So is the surface solar incoming radiation, I Rd is the surface downward long wave
radiation, aB is the Stefan-Boltzmann constant, G is the broad-band surface albedo, E
is the surface emissivity and Tg is a surface skin temperature. In Eq. (7), So and I Rd
are produced by the AM radiative transfer scheme given vertical profiles of atmospheric
constituents and clouds, while G, E and Tg are quantities generated in the ESEM.
Generally, the albedoes and cmissivities are either given within the ESEM as a function of surface characteristics or are specified spatially from observational (e.g. remotely
sensed) data. As such, they can be considered as input parameters in an ESEM. Broadband vegetation albedoes vary generally in the range of 0.1 - 0.3. The spectral albedo of
vegetation, however, has a marked dependence on wavelength, since chlorophyll absorbs
most strongly at visible wavelengths. In addition, the overall canopy reflectance depends
on the density, geometry and orientation of leaves, and some surface schemes calculate
the canopy albedo based on models of radiative transfer within the canopy (see section
3.2.2). Soil albedos also show a significant spectral dependency and a wide range of values,
0.1 to 0.4, depending on the soil mineral composition and water content. Furthermore,
the albedoes dramatically increase in the presence of snow and ice (up to O.S). Surface
emissivities are generally assumed to be equal to 1, although it is recognized that canopy
surface emissivities can be less than one (in the range of 0.95-0.9S) and that for some
sandy soils the emissivity can be as low as 0.7.
In the absence of vegetation and snow lice cover, the surface skin temperature is generally calculated from an equation expressing the balance between the net radiative flux,
the turbulent sensible heat flux between the surface and the atmosphere, S H, the latent
heat flux associated with surface evaporation, LH, and the heat exchange between the
surface and a deep soil layer (or reservoir), Ds , i.e.
RN-LH-SH-Ds=O
(S)
where the term D s has the form
(9)
point is assumed to be covered either by one or by a combination of surface types, each
covering a fraction of the grid cell. Details on how different models treat different surface
types is given in section 3, while approaches to the description of surface heterogeneity
are discussed in section 4. The next two subsections present a more general discussion of
the representation of the surface radiative and turbulent flux terms.
2.1.1. Radiative processes
The net radiation at the surface, RN is given by
(7)
where So is the surface solar incoming radiation, I Rd is the surface downward long wave
radiation, aB is the Stefan-Boltzmann constant, G is the broad-band surface albedo, E
is the surface emissivity and Tg is a surface skin temperature. In Eq. (7), So and I Rd
are produced by the AM radiative transfer scheme given vertical profiles of atmospheric
constituents and clouds, while G, E and Tg are quantities generated in the ESEM.
Generally, the albedoes and cmissivities are either given within the ESEM as a function of surface characteristics or are specified spatially from observational (e.g. remotely
sensed) data. As such, they can be considered as input parameters in an ESEM. Broadband vegetation albedoes vary generally in the range of 0.1 - 0.3. The spectral albedo of
vegetation, however, has a marked dependence on wavelength, since chlorophyll absorbs
most strongly at visible wavelengths. In addition, the overall canopy reflectance depends
on the density, geometry and orientation of leaves, and some surface schemes calculate
the canopy albedo based on models of radiative transfer within the canopy (see section
3.2.2). Soil albedos also show a significant spectral dependency and a wide range of values,
0.1 to 0.4, depending on the soil mineral composition and water content. Furthermore,
the albedoes dramatically increase in the presence of snow and ice (up to O.S). Surface
emissivities are generally assumed to be equal to 1, although it is recognized that canopy
surface emissivities can be less than one (in the range of 0.95-0.9S) and that for some
sandy soils the emissivity can be as low as 0.7.
In the absence of vegetation and snow lice cover, the surface skin temperature is generally calculated from an equation expressing the balance between the net radiative flux,
the turbulent sensible heat flux between the surface and the atmosphere, S H, the latent
heat flux associated with surface evaporation, LH, and the heat exchange between the
surface and a deep soil layer (or reservoir), Ds , i.e.
RN-LH-SH-Ds=O
(S)
where the term D s has the form
(9)