Geoscience Reference
In-Depth Information
4
Surface Energy
Fluxes
Introduction
Terrestrial surfaces influence and are influenced by the overlying atmosphere
through the exchange of energy, water, and other atmospheric constituents. In this
chapter we consider the exchange of energy that occurs mainly in the form of sur-
face fluxes of radiant energy, latent heat (when water vapor evaporates from or
condenses onto the land), sensible heat (that warms or cools the air in contact with
the surface), and heat that diffuses into or out of the ground.
The surface flux of any entity is the amount of that entity flowing through and
normal to the surface in unit time, per unit surface area. In the case of energy flux
exchange with terrestrial surfaces, this is the rate of flow of energy per unit area
of land surface. In
Système International
(SI) units, surface energy fluxes are
expressed in units of Joules per second per square meter, but one Joule per second
is one Watt, consequently the units of surface energy fluxes are Watts per square
meter (W m
−2
). For terrestrial surfaces, the maximum rates of surface energy
transfer are constrained by the incoming energy from the Sun. At the top of the
atmosphere, the time average energy flux arriving from the Sun when directly
overhead is
1366 W m
−2
: this value is called the
Solar Constant
. Typically 25-75%
of the Sun's energy is absorbed as it travels through the atmosphere depending
mainly on cloud cover, so the incoming energy arriving at the surface as solar
radiation could be
∼
1000 W m
−2
in clear sky conditions at midday near the
equator. However, it is typically much less than this at other times of day and at
higher latitudes, and there is no incoming solar radiation during the night.
The energy incoming as solar radiation is shared between several surface fluxes,
consequently the order of magnitude of such energy fluxes is typically a few 10s
to a few 100s of W m
−2
.
∼
