Geoscience Reference
In-Depth Information
- latent heat flux (evaporation flux
E
times the latent heat of
evaporation
L
):
[3.4]
LE
.
=
ρ
. .
Lw q
'
'
These equations convey the fact that the atmosphere, through its
turbulent movements, transports air vertically, along with its
properties. The larger and more positive the difference in temperature
between the surface and lower atmosphere, the more free convection
is observed, with a significant heat flux. Similarly, a difference in
humidity favors evaporation.
Horizontal wind plays two roles in the turbulent fluxes: it acts on
the friction directly, thereby on the transmission of energy to the
ocean and atmosphere. But a strong wind also favors heat fluxes by
increasing the turbulence. Considering the major role of horizontal
wind, approximate formulations have been developed which are easier
to manipulate than the covariances: the flux parameterizations.
The direct determination of the fluxes requires the variables
(temperature, humidity, horizontal and vertical wind) in the
atmospheric surface layer to be measured with a very accurate
measuring tool, and a high acquisition rate (up to at least 10-20 Hz).
Indeed, this measurement is difficult to implement over the sea
surface, since such instruments cannot be installed on isolated buoys.
Moreover, these types of instrument have only existed since the
1980s.
Since the middle of the 20th Century, this technical difficulty has
led researchers to look for formulations that can relate fluxes to mean
variables, which are easier to measure.
Prandtl (1925) defined the “friction velocity” as equal to the square
root of the momentum flux, divided by the density. Considering that
turbulent eddies progressively lose their coherence, he defined a
“mixing length”,
l
m
, such that the friction speed is equal to
l
m
times the
vertical gradient of horizontal speed. Using this, it can be shown that
