Geoscience Reference
In-Depth Information
wind increases from the surface according to a logarithmic law, by
linking
l
m
to a characteristic height called the roughness height (
z
0
):
u
z
*
[3.5]
u
=
log
κ
z
0
By establishing empirical laws to determine z0, it becomes
possible to link the fluxes to the average wind.
Using similar dimensional relations, the characteristic heights for
temperature and humidity
z
T
and
z
q
, as well as
T
*
and
q
*
have been
defined such that the covariances are replaced by the product
u
*.
T
*
and
u
*.
q
*
in the equations for heat fluxes. Monin-Obukhov's theory
develops these concepts, permitting the calculation of flux in any
stability condition. The methods developed since the 1980s for
calculating flux, called global aerodynamics or “bulk”, are based upon
Monin-Obukhov's work. Fluxes are expressed as simple functions of
the mean differences in wind, temperature and humidity between the
surface and measurement altitudes. This approach requires the
introduction of an exchange coefficient (
C
D
, C
H
, C
E
) for each flux,
which is a function of the roughness height (or the typical magnitude
u
*
,
T
*
and
q
*
):
2
[3.6]
τρ
=
.
CUUs
.(
−
)
[3.7]
HCCU
=
ρ
.
.
.(
−
s
.(
T s
−
)
PH
[3.8]
LE
.
=
ρ
. .
LC
.(
U
−
Us
).(
q
−
qs
)
E
This approach is the most commonly used, since it allows the
estimation of fluxes using “ordinary”
measurements, such as those
made on moored buoys. Fluxes can be iteratively calculated. Mean
quantities are measured over several tens of minutes, at a height of
two meters to several tens of meters, then algorithms developed by
various authors can be used to calculate the exchange coefficients
(see, for example, the analysis of [BLA
87]). This is also the method
used to calculate the fluxes in numerical weather models or for climate
simulations. But, this simplicity has led to widespread use, sometimes
