Geoscience Reference
In-Depth Information
rolling, pitching and displacement. Moreover, few temperature and
humidity sensors allow such measurement at sea, without being
damaged by the effects of spray in a marine environment…
Aerodynamic mass methods are, therefore, mainly used for local
measurements. On a ship, the problem of the flow disturbance
remains. In practice, the sensors should be placed on a mast as far as
possible from the ship superstructures at the bow, and measurements
can only be used for winds blowing toward the ship (at ±30°C ).
Present strategies point toward measurements on buoys or even on
drones sailing on the water surface.
From space, the methods enabling turbulent fluxes to be estimated
are based only on the use of simplified aerodynamic formulae (a
neutral case, due to the lack of information on the atmospheric
stratification). Scatterometers (radars pointing obliquely from one side
or either side of the satellite's tracks), altimeters (along the track on
the vertical) and microwave imaging radiometers (at frequencies of
10-18 GHz) enable the surface wind to be estimated; the
scatterometer is the only instrument that also establishes wind
direction. The general principle of this measurement is that the small-
scale roughness (centimeter scale) caused by the wind friction on the
sea modifies the backscattering (radars) and the emission (radiometers)
of electromagnetic waves. The relationship between this roughness and
the wind (at 10
m) is calculated taking a wind profile for neutral
stratification. In practice, statistical relationships are established
between local measurements and satellites, or with simulated data
sets.
Concerning heat fluxes, the problem is even more complex: there
are currently no means for estimating the atmospheric temperature
near the ocean surface using satellites, and the methods for obtaining
the low-level humidity are devalued by significant errors. The sensible
heat flux, generally small over the sea (most often <50 Wm
-2
), cannot
be estimated from space. As for the latent heat flux (which often
reaches 150-200 Wm
-2
), the uncertainty is equal or more than 30%
when satellite data for the wind, surface temperature (to obtain the
surface humidity) and atmospheric humidity are combined…
