Geoscience Reference
In-Depth Information
To obtain the water balance (
E-P
), both the evaporation flux
E
and
precipitation rate
P
must be measured. A new obstacle emerges:
measuring precipitations over the oceans. On a buoy, sea spray makes
the rain gauges unusable; on a ship, the disturbance in the flow
renders this measurement very uncertain, unless a rain radar can be
used. In any case, the spatio-temporal variability of precipitations is
such that local measurements are not reliable for use on a regional
scale. Estimation by satellite is once again the only reasonable
alternative. However, a sensor (radar or radiometer) only receives
radiation that has been backscattered or emitted by droplets being in
the atmosphere at the overpass time. Indeed, the precipitations are
mostly used as hourly or daily rates. The development of statistical
methods, established with data that are most often based on
simulations (rain rate and satellite measurements) or reconstructed
using other instruments, implies a reliable relationship and a thorough
validation, which are unfortunately impossible to realize over ocean.
The eventual uncertainty is broadly from 50 to 100% on the
precipitation rate.
The calculation of the CO
2
flux, the only gaseous component for
which researchers have attempted to get a global estimation of the flux
at the surface, relies on the measurement of the wind and the CO
2
partial pressure in the surface water. It is determined on ships by
carrying out a spectrometric measurement of the carbon dioxide in the
air, previously equilibrated with the seawater. On ships, other methods
are being developed using automatic sampling, also enabling the
quantity of inorganic carbon dissolved in the water to be determined
(for example, as done by the observation service CARbone AUStral
(CARAUS)). Approaches using indirect automatic methods for
determining
pCO
2
have also been developed in the 1990s for
installation on buoys (CARbon Interface OCean Atmosphere
(CARIOCA) system, by colorimetry).
The flux estimation is made by combining the
in situ
data with
those of the wind and surface temperatures, as estimated from space,
and, more recently, of the surface salinity to better estimate the
exchange coefficient. The uncertainty on this estimation remains,
however, in the order of 50-100%.
