93
kd is an effective soil diffusivity and 7d is the reservoir temperature. The t.erms LH and
SH are described in section 2.1.2 (Eqs. (18) and (19)). Eq. (8) is non-linear in Tg , with
non-linearities entering the long wave radiative cooling term EO BTi and the temperature
dependency of the saturation vapor pressure in the evaporatioIl (or latent heat) term
(see Eq. (19)). Eq. (8) can be solved numerically using iterative methods, and in
more advanced schemes it is coupled to similar equations for the deep soil and vegetative
canopy temperatures. In the presence of snow and/or vegetation, energy balance equations
similar to (8) can be solved for the snow skin temperature and foliage (or canopy "skin")
temperature. The long wave surface emission term is then averaged over the fractional
bare soil, vegetation and snow areas.
2.1.2. Turbulent processes
The atmospheric region where surface-atmosphere turbulent exchange processes take
place is generally referred to as the atmospheric Planetary Boundary Layer (PBL). The
depth of the PBL can vary widely in response to surface heating, from a few tens of meters
in thermally very stable conditions (cooling from below), to a few km in conditions where
strong heating of the surface occurs.
Many models of the PBL are today available (e.g. Stull 1989), and although PBL
modeling is not the topic of this paper, it is useful for the following discussions to briefly
describe the PBL structure. This is schematically represented in Fig. 2 (from Brutsaert
1978). Under neutral conditions, the structure of the atmospheric PBL presents distinct
inner and outer regions. In the outer region the flow is nearly independent of the nature
of the surface and is mainly determined by the pressure gradient and Coriolis forces.
Conversely, in the inner region (also called surface layer) the flow is strongly affected by
the nature of the surface. Under strongly unstable conditions, the effects of the pressure
and Coriolis terms are small and the outer region is dominated by thermal convective
turbulence characterized by eddies which can span the whole depth of the PBL. In such
conditions, the outer region can be referred to as mixed or free convection layer. The
surface sublayer may be defined as a fully turbulent region where the vertical fluxes
do not change substantially from thcir valucs at the surface. In the lower regions of
the surface sublayer, called the dynamic sublayer, the effects of density stratification
are small and wind speed, temperature and water vapor generally follow logarithmic
vertical profiles. Under neutral conditions, the dynamic sublayer occupies the whole
surface sublayer. Finally, in the immediate vicinity of the surface (the interfacial, or
transfer, layer) turbulence is strongly affected by the structure of the roughness elements
and viscous effects may become important. The interfacial layer may be a few em in
depth over smooth surfaces (e.g. over water in low wind conditions) or can occupy the
whole canopy layer over vegetated surfaces.
kd is an effective soil diffusivity and 7d is the reservoir temperature. The t.erms LH and
SH are described in section 2.1.2 (Eqs. (18) and (19)). Eq. (8) is non-linear in Tg , with
non-linearities entering the long wave radiative cooling term EO BTi and the temperature
dependency of the saturation vapor pressure in the evaporatioIl (or latent heat) term
(see Eq. (19)). Eq. (8) can be solved numerically using iterative methods, and in
more advanced schemes it is coupled to similar equations for the deep soil and vegetative
canopy temperatures. In the presence of snow and/or vegetation, energy balance equations
similar to (8) can be solved for the snow skin temperature and foliage (or canopy "skin")
temperature. The long wave surface emission term is then averaged over the fractional
bare soil, vegetation and snow areas.
2.1.2. Turbulent processes
The atmospheric region where surface-atmosphere turbulent exchange processes take
place is generally referred to as the atmospheric Planetary Boundary Layer (PBL). The
depth of the PBL can vary widely in response to surface heating, from a few tens of meters
in thermally very stable conditions (cooling from below), to a few km in conditions where
strong heating of the surface occurs.
Many models of the PBL are today available (e.g. Stull 1989), and although PBL
modeling is not the topic of this paper, it is useful for the following discussions to briefly
describe the PBL structure. This is schematically represented in Fig. 2 (from Brutsaert
1978). Under neutral conditions, the structure of the atmospheric PBL presents distinct
inner and outer regions. In the outer region the flow is nearly independent of the nature
of the surface and is mainly determined by the pressure gradient and Coriolis forces.
Conversely, in the inner region (also called surface layer) the flow is strongly affected by
the nature of the surface. Under strongly unstable conditions, the effects of the pressure
and Coriolis terms are small and the outer region is dominated by thermal convective
turbulence characterized by eddies which can span the whole depth of the PBL. In such
conditions, the outer region can be referred to as mixed or free convection layer. The
surface sublayer may be defined as a fully turbulent region where the vertical fluxes
do not change substantially from thcir valucs at the surface. In the lower regions of
the surface sublayer, called the dynamic sublayer, the effects of density stratification
are small and wind speed, temperature and water vapor generally follow logarithmic
vertical profiles. Under neutral conditions, the dynamic sublayer occupies the whole
surface sublayer. Finally, in the immediate vicinity of the surface (the interfacial, or
transfer, layer) turbulence is strongly affected by the structure of the roughness elements
and viscous effects may become important. The interfacial layer may be a few em in
depth over smooth surfaces (e.g. over water in low wind conditions) or can occupy the
whole canopy layer over vegetated surfaces.