Geoscience Reference
In-Depth Information
To estimate the surface radiative balance on a global scale,
satellite-borne sensors have been developed. Some of them measure
solar and infrared radiation directly at their flight altitude, enabling the
planet's energy balance to be obtained. However, physical or statistical
modeling of the relationship between this measurement and the
surface balance is required to establish the radiative transfer throughout
the atmosphere.
From satellites, the surface temperature of the oceans is obtained
with sensors measuring the Earth's radiation in thermal infrared (but
in a narrow band, contrary to the long wave radiation). Hypotheses for
emissivity are formulated to obtain the surface
temperature; the
atmospheric absorption is removed using two similar bands of long
waves (10 and 12
µm). However, the infrared radiation emission layer
is extremely thin, less than one mm thick, and the temperature thus
obtained differs from the temperature measured by the
in situ
sensors
(a measurement performed at several tens of centimeters, down to
more than a meter). The two quantities are compared using heat
transfer models of the oceanic surface layer.
From space, it is possible to estimate the chlorophyll concentration
in the ocean surface layer, using measurements from visible imagers in
the blue-green spectrum: “ocean-color” instruments such as Sea-
viewing Wide Field-of-view Sensors (SEAWIFS), or multiobjectives
ones, such as Moderate Resolution Imaging Spectroradiometer
(MODIS) and Medium resolution Imaging Spectrometer (MERIS).
The major problem is the aerosol content in the atmosphere, which
contaminates the surface measurement and influences the reading.
Another difficulty, especially in coastal regions, is distinguishing the
signal from the phytoplankton from the one of mineral particles or
dissolved colored matter.
3.2.5.2.
Turbulent fluxes
Direct measurements using the correlation method or turbulent
dissipation method are only possible on ships or large moored buoys,
because of the electric power, mass sensors and volume, and the high-
data rate and storage required. The quality of the measurements is
therefore affected by several factors: the physical obstacle due to the
ship superstructure (the flow wake), and the movements caused by
