Geoscience Reference
In-Depth Information
Terrestrial surfaces often have heterogeneous land cover and the nature of this
cover can substantially alter the amount of solar radiation captured at the surface
and the way in which this is then shared as surface energy fluxes. In hydrometeorology
and hydroclimatology, heterogeneous terrestrial surfaces are often imagined as
being made up of a patchwork of
ideal surfaces
, with each ideal patch assumed to be
opaque to radiation and acceptably homogeneous in terms of those surface
characteristics that influence surface energy fluxes. It is further assumed that above
a certain level in the atmospheric boundary layer (say 50-100 m) the atmosphere is
well-mixed so atmospheric variables can be considered independent of the
underlying patch. This height is called the
blending height
. The land surface
characteristics that influence surface energy exchange and are assumed to be
homogeneous across an ideal patch include the vegetation-dependent reflection
coefficient for solar radiation, thermal emissivity, aerodynamic roughness, and
the ability to capture and store precipitation on plant canopies or in the soil accessible
by plant roots. Often in atmospheric models ideal patches are also assumed to be
horizontal, and even if not horizontal, it is at least assumed that the flow of energy
into or out of the atmosphere is vertical.
Latent and sensible heat fluxes
In SI units the evaporation flux,
E
, has the units of kilograms of evaporated water
per second per square meter of land surface. Conveniently, because the density of
water is close to 1 kg l
−1
, 1 kg s
−1
m
−2
of evaporated water is equivalent to 1 mm s
−1
depth of water evaporated. However, surface evaporation rates of the order 1 mm s
−1
do not occur in the natural world: evaporation rates are more typically on the
order of a few mm day
−1
because the latent heat needed to support evaporation is
constrained by the Earth's surface energy balance.
It is very common to quantify evaporation rates from terrestrial surfaces not in
terms of mass flow but in terms of the flow of energy leaving the evaporating surface
as latent heat of vaporization in the water vapor. To express the rate of evaporation
as the
latent heat flux,
λ
E
, in W m
−2
, the evaporation flux
E
, in kg s
−1
m
−2
, is multiplied
by
, the latent heat of vaporization of water in J kg
−1
. Hydrologists more used to
working in terms of the mass balance of water rather than the surface energy
balance find it useful to remember that an evaporation rate of 3.5 mm d
−1
is
equivalent to a daily average latent heat flux of 100 W m
−2
, and an evaporation rate
of 1 mm d
−1
is equivalent to a daily average latent heat flux of 28.6 W m
−2
(i.e., about
30 W m
−2
).
The flow of energy as latent heat away from or toward the land surface is very
important, but it is not the only way the energy can be exchanged with the
atmosphere. A second important way is by the surface directly warming or cool-
ing the air in contact with the surface, with heat then either diffusing outward to
the air above or inward from the air above, respectively. The associated flow of
energy is called the
sensible heat flux
because it is associated with changes in air
λ
