20 Pierre-Yves Le Traon
operational monitoring of the ocean with in-situ data is still, however, to be
developed. The Argo project (Roemmich et al., 1999) will be a major step
towards this objective.
• Satellite data, on the other hand, can provide long-term, repeated (synoptic) and
global measurements ofkey parameters (e.g. sea level, SST, winds). These are
surface parameters but they may be representative of the deep ocean (e.g. sea
level from altimeters). An additional advantage is that measurements can be
acquired and processed in ne ar-real time.
There is an obvious complementarity between satellite and in-situ data. Satellite
and in-situ measurements have different contents (e.g. vertical structure from insitu data, surface information from satellites) and very different space/time sampling. In-situ data are also required for calibration and long-term validation of satellite data.
2.1.2 Space oceanography techniques
Overview
Many space oceanography techniques have been developed in the last two
decades. I will base my discussion on a broad detinition of space oceanography
encompassing direct ohservation of the ocean, ice and ocean forcing terms (wind,
heat flux, precipitation, evaporation). Technologies range from passive radiometers measuring the natural electromagnetic signal which radiates from the sea to
active radiometers (radars) which transmit a signal and measure the moditied echo
signal. Passive radiometers allow us to look at different oceanic signals depending
on the wavelength. Wavelength bands available are actually limited by absorption
of the electromagnetic signal by molecular aerosols and water vapor.
A brief overview of space techniques and of the retrieved oceanographic parameters is given here. For a more detailed description the reader is referred, for example, to Robinson (1985), Weller and Taylor (1993) and Ikeda and Dobson (1995).
In the visible band, between 0.4 /lm and 0.8 /lm, we tind measurements of ocean
color (see below) and surface solar radiation flux [one component of the net heat
flux at the ocean surface which can be well retrieved from space data (Planton,
1994)]. In the infrared band (0.8 /lm to 100 /lm), we tind, of course, the Sea Surface Temperature (SST) and the infrared flux, although, due to cloud cover, the latter is difficult to determine precisely from satellite measurements. In the visib1e
and infrared bands, the main limitation is cloud cover. SST and ocean color measurements from space can thus only be obtained for clear skies. Sea ice surf ace
characteristics can also be distinguished in the visible and infrared bands
(AVHRR).
In the microwave band (0.1 cm to 100 cm), cloud cover is no longer a problem.
Main parameters which can be measured in the microwave band are the integrated
atmospheric water vapour (not direct1y relevant here), sea ice and surface wind
speed. The absence of cloud cover is particularly important for sea ice studies
(measurement of ice concentration and ice extent). Surface precipitation is a non-
operational monitoring of the ocean with in-situ data is still, however, to be
developed. The Argo project (Roemmich et al., 1999) will be a major step
towards this objective.
• Satellite data, on the other hand, can provide long-term, repeated (synoptic) and
global measurements ofkey parameters (e.g. sea level, SST, winds). These are
surface parameters but they may be representative of the deep ocean (e.g. sea
level from altimeters). An additional advantage is that measurements can be
acquired and processed in ne ar-real time.
There is an obvious complementarity between satellite and in-situ data. Satellite
and in-situ measurements have different contents (e.g. vertical structure from insitu data, surface information from satellites) and very different space/time sampling. In-situ data are also required for calibration and long-term validation of satellite data.
2.1.2 Space oceanography techniques
Overview
Many space oceanography techniques have been developed in the last two
decades. I will base my discussion on a broad detinition of space oceanography
encompassing direct ohservation of the ocean, ice and ocean forcing terms (wind,
heat flux, precipitation, evaporation). Technologies range from passive radiometers measuring the natural electromagnetic signal which radiates from the sea to
active radiometers (radars) which transmit a signal and measure the moditied echo
signal. Passive radiometers allow us to look at different oceanic signals depending
on the wavelength. Wavelength bands available are actually limited by absorption
of the electromagnetic signal by molecular aerosols and water vapor.
A brief overview of space techniques and of the retrieved oceanographic parameters is given here. For a more detailed description the reader is referred, for example, to Robinson (1985), Weller and Taylor (1993) and Ikeda and Dobson (1995).
In the visible band, between 0.4 /lm and 0.8 /lm, we tind measurements of ocean
color (see below) and surface solar radiation flux [one component of the net heat
flux at the ocean surface which can be well retrieved from space data (Planton,
1994)]. In the infrared band (0.8 /lm to 100 /lm), we tind, of course, the Sea Surface Temperature (SST) and the infrared flux, although, due to cloud cover, the latter is difficult to determine precisely from satellite measurements. In the visib1e
and infrared bands, the main limitation is cloud cover. SST and ocean color measurements from space can thus only be obtained for clear skies. Sea ice surf ace
characteristics can also be distinguished in the visible and infrared bands
(AVHRR).
In the microwave band (0.1 cm to 100 cm), cloud cover is no longer a problem.
Main parameters which can be measured in the microwave band are the integrated
atmospheric water vapour (not direct1y relevant here), sea ice and surface wind
speed. The absence of cloud cover is particularly important for sea ice studies
(measurement of ice concentration and ice extent). Surface precipitation is a non-