Integrated Models in Hydrology and Meteorology - Transport in the Atmosphere-Vegetation-Soil Continuum - page 342

Note that all tiles share the same soil temperature (the subscript '1' refers to the irst soil
layer). The surface temperature is not the temperature of the irst soil layer but the temper-
ature of a skin layer that has no heat capacity. Because this skin layer cannot store heat, it
needs to be in immediate equilibrium with the energy lux. This energy balance Eq. ( 9.34 )
is supplemented with the following expressions for the sensible and latent heat lux:

TT
r

−

a s , i

Hc =− ρ

i p

a , i

(9.35)

− ()
+

qq T
rr

a sat s , i

LE =− ρ L

v i v

a , i extra , i

where the nature of the extra resistance r extra, i depends on the tile (either a canopy
resistance, or a soil evaporation resistance). For each of the tiles the set of equations
given by Eqs. ( 9.34 ) and ( 9.35 ), in combination with the similarity relationship from
Chapter 3 , is solved, giving values for the sensible and latent heat lux for each tile.
The weighted sum of those luxes from separate tiles is passed to the atmospheric
model as the total lux representative of the entire grid box. Details on the implemen-
tation of the coupling between a tiled land surface model and the atmospheric column
can be found in Best et al. (2004).

Coupling to the Soil
In principle each tile in a tiled LSM could have its own soil properties. However, there is
no reason why soil properties should vary with land use. Therefore, TESSEL uses a sin-
gle soil type within a grid box, for all tiles. Between grid cells, the soil hydraulic proper-
ties vary: the properties related to the dominant soil type within the grid box are assigned
to all tiles (as of the introduction of HTESSEL; Balsamo et al., 2009 ). However, all grid
boxes have identical soil thermal properties (heat capacity and thermal conductivity).
The evolution of the soil temperature is determined from the solution of the heat
diffusion equation (Eq. ( 2.31 )). The soil in TESSEL is divided into four layers with
a thickness that increases with depth (7 cm, 21 cm 72 cm and 189, i.e., a total soil
column of 289 cm). The thicknesses have been chosen such that, for forcings with
a frequency between 1 day and 1 year (see Chapter 2 ), the phase and amplitude of
the soil temperature at each depth is close to those that would be obtained if a large
number of layers would be used. The upper soil layer represents the diurnal cycle, the
second layer represents variations at the timescale of 1 day to 1 week, the third layer
variations between 1 week and 1 month and the deepest layer represents variation
with a period longer than 1 month (Viterbo and Beljaars ( 1995 ). The upper boundary
condition of the soil column is the surface soil heat lux (see Eq. ( 9.34 )), whereas at
the lower boundary the lux is taken zero.
Vertical soil moisture transport in the soil is governed by Richards' equation
( Chapter 4 ), which is solved for the same layers as used for the soil temperature. The
sink term in Richards' equation is the uptake of soil moisture by the roots. The total