Integrated Models in Hydrology and Meteorology - Transport in the Atmosphere-Vegetation-Soil Continuum - page 325

• Temperature changes because of horizontal advection of heat from neighbouring cells.

• Moisture and other scalars change because of horizontal advection from neighbouring

cells as well.

But much more happens than horizontal exchange alone: many processes occur at
scales that are much smaller than can be resolved on the grid of an atmospheric model
(which has typical horizontal distances between cells of 10-100 km and vertical dis-
tances of 10-500 m). The processes that cannot be resolved encompass:

• Turbulent exchange of momentum, heat and moisture (in particular vertical exchange)

• Cloud formation and precipitation

• Radiation processes

Because these processes cannot be explicitly resolved, they have to be parameterized.
This means that they have to be expressed in terms of variables that are known in the
model. For example, radiation transfer through clouds needs to be calculated from
information on the liquid water content within a grid box only, whereas in reality the
radiation transfer would depend not only on moisture content but on the structure
of the clouds as well. In most atmospheric models all of these processes are applied
within each vertical column separately. So, as far as the parameterized processes are
concerned, the atmosphere consists of a large collection of parallel, independent col-
umns.
Whereas most grid boxes in an atmospheric model only have other grid boxes as
their neighbour, the grid boxes at the lower end of the atmospheric column have Earth
(either land surface, water or ice) as its lower boundary.
At this lower boundary momentum, heat and water vapour are exchanged between
the land surface and the atmosphere (see Figure 9.13 ). This exchange will affect the
values of horizontal wind speed, temperature and humidity in this grid box (e.g.,
a positive evapotranspiration will increase the speciic humidity in the box). Thus,
the role of an LSM in an atmospheric model is to provide the correct exchange of
momentum, heat and water vapour between the surface and the atmosphere.
Two major complications arise in the speciication of those luxes, in particular
over land:

• On the scale of a single vegetation unit (i.e., maize ield or a forest) the luxes originate

from a number of different sources (e.g., soil and leaf). Those luxes are regulated both
by the external atmospheric conditions and by the state of the land surface (in particular
vegetation cover and soil moisture content). The various levels of complexity in which
this can be dealt with in LSMs is discussed in Section 9.2.3 .
On the scale of a single grid box the properties of the land-surface are not homogeneous: •

within a square of say 50 × 50 km one will ind, cities, grassland, forests and open water).
Methods to deal with this large-scale heterogeneity are discussed in Section 9.2.5 .
Apart from the fact that the • properties of the land-surface vary within a grid box, the ele-

vation may vary as well: at the grid-scale the model can only follow the mean elevation