Integrated Models in Hydrology and Meteorology - Transport in the Atmosphere-Vegetation-Soil Continuum - page 327

Those luxes need to be modelled (decribed) in terms of variables that are avail-
able in the atmospheric model in combination with parameters that characterize the
surface. Such a description is called a parameterization.
For a LSM the relevant forcing variables , providing the boundary upper conditions
for the land-surface model, are:

• The radiative forcing (incoming shortwave and longwave radiation)

• The liquid water forcing (precipitation)

• The atmospheric state in the lower gridbox (wind speed, temperature and humidity)

To determine the luxes, the LSM also needs to keep track of some internal variables
(among others):

• The soil conditions (temperature and water content)

• The state of the surface (e.g., snow cover, amount of intercepted water)

The list of model parameters very much depends on the level of complexity of the
LSM, but will at least contain albedo, surface emissivity, roughness length, as well
as information on the amount and type of vegetation (including properties related to
stomatal behaviour). These parameters are usually based on land-use classiications
(based on remote sensing data) where for each land-use class a set of parameters
is given. In some cases these parameters are time-dependent, for example, seasonal
variations in albedo or vegetation fraction. In more complex models (see Section
9.2.3 ) some of these parameters become variables within the LSM (and thus are mod-
elled themselves).
Provided that the lowest grid point of the model is located at a height within the
surface layer (or constant lux layer), surface layer similarity can be used to compute
the luxes of momentum, heat and water vapour using:

1. Wind speed, temperature and humidity known at that the lowest atmospheric grid point
2. The roughness lengths for momentum and heat
3. The surface values for wind speed (equal to zero), temperature and humidity. Usually the
surface speciic humidity is replaced by q sat ( T s ) in combination with a canopy resistance
(see Chapter 7 and Section 9.2.4 ).

The similarity relationships presented in Chapter 3 can be used to combine the infor-
mation under items (1) to (3) to compute the luxes. To arrive at the values needed
under item (3), the energy balance needs to be computed. This generally involves the
solution of a number of coupled problems:

• Net radiation is coupled to the surface by an albedo, surface emissivity (both relatively

constant) and a highly variable surface temperature.
Soil heat lux depends on the surface temperature as well (as well as on the soil tempera-

•

tures).
The partitioning of the available energy (

• Q* - G ) over sensible and latent heat lux deter-

mines the surface temperature (e.g., high evaporation gives a low surface temperature).