170
M. Menenti
models may become again an essential tool to obtain daily evaporation from instantaneous radiometric observations.
8.3.6 Integrated SVAT and Planetary Boundary Layer (PBL) models [5]
Integrated land surface - PBL models are attractive since upper boundary conditions can be defined using large-scale variables, i.e. truly constant over large distances These models use some height in the atmospheric boundary layer as a reference. Wieringa (1986), for example, proposed to use a blending height as a reference height. Horizontal gradients in PBL variables vanish at the blending height
due to horizontal mixing. In other words, the effect of spatial heterogeneity on the
PBL vanishes at the blending height. Brutsaert et al. (1992) chose the PBL as a
reference.
The advantage of choosing a higher reference height is two- fold. Coupling with
surface conditions is weaker, so air temperature at the top of the PBL changes over
larger spatial scales in comparison with near surface air temperature.
Carlson et al. (1981) developed a detailed land surface - atmospheric boundary
layer model to study sensible and latent heat fluxes, soil moisture availability
(thermal inertia) in a urban - rural environment. The function of the model was to
partition net radiation into sensible and latent heat fluxes and to determine the
daily amplitude of surface temperature. Effective land surface variables were defined to match the observed surface temperatures with simulated ones through heat
balance modeling. Four different sub-systems were considered: a mixing layer, a
50 m surface turbulent layer, a thin transition layer which contains many surface
obstacles and aim ground layer. The model was applied with observations provided by the imaging radiometer on-board the Heat Capacity Mapping Mission
(HCMM).
The necessity of using observations of the atmospheric boundary layer to study
heat fluxes at the land surface was further demonstrated by Diak and Whipple
(1993). Diak et al. (1994) linked top-of the-atmosphere radiances, simulated to
reproduce the observations of the High resolution Interferometer Sounder (HIS),
to heat fluxes and skin temperature at the surface using a planetary boundary layer
(PBL) model. The interest of this approach is that HIS spectral radiances relate to
both the PBL (at some wavelengths) and the surface (at other wavelenghts). A
further refinement of this approach was presented by Anderson et al. (1997) who
coupled a dual- source model for sensible heat transfer at the surface with a parameterization of the development of the PBL in response to surface heating. A
sequence of observations of radiometric surface temperature T rad gives the rate of
change of T rad from which heat fluxes at the surface are estimated by model inversion. The coupling with PBL growth eliminates the need for observations of near
surface air temperature. Soil and vegetation temperatures within a mixed target are
estimated using a parameterization of the angular dependence ofTrad.
M. Menenti
models may become again an essential tool to obtain daily evaporation from instantaneous radiometric observations.
8.3.6 Integrated SVAT and Planetary Boundary Layer (PBL) models [5]
Integrated land surface - PBL models are attractive since upper boundary conditions can be defined using large-scale variables, i.e. truly constant over large distances These models use some height in the atmospheric boundary layer as a reference. Wieringa (1986), for example, proposed to use a blending height as a reference height. Horizontal gradients in PBL variables vanish at the blending height
due to horizontal mixing. In other words, the effect of spatial heterogeneity on the
PBL vanishes at the blending height. Brutsaert et al. (1992) chose the PBL as a
reference.
The advantage of choosing a higher reference height is two- fold. Coupling with
surface conditions is weaker, so air temperature at the top of the PBL changes over
larger spatial scales in comparison with near surface air temperature.
Carlson et al. (1981) developed a detailed land surface - atmospheric boundary
layer model to study sensible and latent heat fluxes, soil moisture availability
(thermal inertia) in a urban - rural environment. The function of the model was to
partition net radiation into sensible and latent heat fluxes and to determine the
daily amplitude of surface temperature. Effective land surface variables were defined to match the observed surface temperatures with simulated ones through heat
balance modeling. Four different sub-systems were considered: a mixing layer, a
50 m surface turbulent layer, a thin transition layer which contains many surface
obstacles and aim ground layer. The model was applied with observations provided by the imaging radiometer on-board the Heat Capacity Mapping Mission
(HCMM).
The necessity of using observations of the atmospheric boundary layer to study
heat fluxes at the land surface was further demonstrated by Diak and Whipple
(1993). Diak et al. (1994) linked top-of the-atmosphere radiances, simulated to
reproduce the observations of the High resolution Interferometer Sounder (HIS),
to heat fluxes and skin temperature at the surface using a planetary boundary layer
(PBL) model. The interest of this approach is that HIS spectral radiances relate to
both the PBL (at some wavelengths) and the surface (at other wavelenghts). A
further refinement of this approach was presented by Anderson et al. (1997) who
coupled a dual- source model for sensible heat transfer at the surface with a parameterization of the development of the PBL in response to surface heating. A
sequence of observations of radiometric surface temperature T rad gives the rate of
change of T rad from which heat fluxes at the surface are estimated by model inversion. The coupling with PBL growth eliminates the need for observations of near
surface air temperature. Soil and vegetation temperatures within a mixed target are
estimated using a parameterization of the angular dependence ofTrad.