Geoscience Reference
In-Depth Information
transpiration lux is extracted from those soil layers where roots are present, in pro-
portion to the root density (which depends on the type of vegetation in the tile). If
in a given soil layer the soil moisture content is below permanent wilting point, no
water is extracted from that particular layer. The upper boundary condition of the
soil column is precipitation diminished by interception and surface runoff (occurring
if rainfall exceeds the iniltration capacity). At the lower boundary free drainage is
allowed.
The energy balance is heavily modiied if snow is present on the surface. First, the
albedo of the surface increases. Second, the coupling between the surface temper-
ature and soil temperature is decreased (see
Section 2.3.8
). The exact conductivity
depends on the snow density, which changes in time (ECMWF,
2009
). Finally, direct
sublimation of snow may occur, but at temperatures above the freezing point of water
there will also be snow melt. The timing of this melt has a large impact on the simu-
lated surface energy balance (see Balsamo et al.,
2011
).
Question 9.16:
The thicknesses of the soil layers in TESSEL are based on the period
of the temporal variations they should be able to represent. TESSEL uses a loamy soil.
Assume that the soil properties of a loamy soil are halfway in between those of a sandy
soil and a clay soil (see
Table 2.2
).
a) Determine the damping depth of a dry loamy soil for forcings with a period of 1 day,
1 week, 1 month and 1 year.
b) Determine the damping depth of a saturated loamy soil for forcings with a period of
1 day, 1 week, 1 month and 1 year.
c) Compare those damping depths with the thicknesses of the soil layers as used in
TESSEL.
9.2.7 The Role of Observations
Observations play a crucial role in the development, testing and use of LSMs. In
the development phase observations are used to determine the underlying physical
relationships needed to describe transport processes. Examples are the lux-gradient
relationships (
Chapter 3
), parameterizations for the unsaturated soil-hydraulic con-
ductivity (
Chapter 4
) and the empirical relationships that describe the regulation of
the canopy resistance (
Chapters 6
and
9
).
Then, LSMs need a large number of parameters that describe properties of the
surface: for example, albedo, soil properties, vegetation fraction, minimal stomatal
resistance, and so forth. Some models include a model or parameterization for some
of these parameters (e.g., dynamic vegetation, albedo that depends on vegetation frac-
tion). Otherwise, these parameters are assumed to be immutable (or at most slowly
varying on a seasonal time scale). In that case they are usually derived from remote-
sensing observations as those are the only means to obtain this information on a global
scale at suficient spatial resolution (e.g., Hall et al.,
1995
; Masson et al.,
2003
).
Subsequently, LSMs are tested in two ways: off-line and on-line. In off-line test-
ing the model is decoupled from the full atmospheric model: the variables that are
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