Geoscience Reference
In-Depth Information
vertical
transport
u,v,T,q
horizontal
advection
τ
H
E
land-surface
model
Figure 9.13
Role of land-surface model in lowest grid box: momentum is trans-
ported to the surface, heat and moisture is exchanged with the surface (here positive
luxes during daytime). Exchange with neighbouring cells through horizontal advec-
tion and vertical (turbulent) transport. The dot indicates the grid point within the box
where velocity, temperature and humidity are stored.
of the terrain, whereas smaller mountains, hills and valleys will not be visible. This
subgrid orography has two effects. First, it modiies the effective roughess length (for
momentum): the atmosphere not only feels the roughness of the surface itself (grass,
trees etc.) but also the roughness due to variations in elevation. In this way the momen-
tum exchange between the surface and the atmosphere is affected (Wan and Porté-Agel,
2011
). Second, orography may generate orographic drag due to gravity waves excited
by the variations in terrain height. This orographic drag affects momentum transport
throughout the atmosphere (Jiménez and Dudhia (
2012
) provides an example of how this
drag is dealt with in NWP models).
Question 9.9:
Consider
Figure 9.13
and suppose that there is no horizontal advection
from or into the grid box for each of the quantities.
a) What happens with the values of
u, v, T
and
q
in the grid box if the shear stress, heat
lux and moisture lux at the surface have the sign as given by the arrows in the igure
(assume the luxes at the top of the grid box have the same direction, but are smaller
in magnitude than the surface lux).
b) If the dimensions of the gridbox are ∆
x
, ∆
y
and ∆
z
in the two horizontal and vertical
directions, respectively and the heat lux at the top and bottom of the gridbox are
indicated by
H
(∆
z
) and
H
(0), give an expression for the change in time of the tem-
perature in the gridbox.
9.2.2 General Structure of a LSM
Any LSM needs to provide the turbulent luxes of momentum, heat and water vapour
(and possibly other trace gases) to the atmospheric part of the model. Furthermore,
the upwelling longwave radiation (dependent on surface temperature) and relected
shortwave radiation are fed into the model.
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