Satellite Oceanography for Ocean Forecasting
21
radiative parameter which can be inferred from the satellite radiative measurements (microwave radiation is related to atmospheric liquid water content, not
direct1y to rain rate, but a good correlation between the two parameters existsbesides, precipitation is poorly known and satellites provide very useful information). Note that visible and infrared techniques can also provide useful information
on precipitation. Latent heat flux or evaporation rate can also be inferred from SST,
wind speed and integrated water vapour (although data on surf ace level humidity
would be needed). Finally, one should note that SST can also be retrieved from
microwave radiation but with a lower resolution and accuracy compared to infrared radiation. There is also an interesting capability for measuring Sea Surface
Salinity from microwave measurements.
Main active microwave radiometers or radars for satellite oceanography are:
altimeters, scatterometers and Synthetic Aperture Radars. Satellite altimetry provides measurements of the sea surface topography, significant wave height and
wind speed modulus. It also provides measurements of ice sheet topography (e.g.
mass balance of Antarctica and Greenland ice sheet). We shall describe altimeter
measurement principles in the next section. A scatterometer measures the electromagnetic signal backscatter coefficient in three directions. The backscatter coefficient depends on wind speed and direction. By analyzing backscatter in different
directions, we can estimate the wind speed direction. There is a 180 0 ambiguity
because differences in backscatter 180 0 apart are very small. This ambiguity is
generally removed through consistency check from previous measurements and/or
using atmospheric ocean models. Scatterometers thus provide wind speed and wind
direction measurements. They are also useful for measuring sea ice, although not
as useful as microwave radiometers. Synthetic Aperture Radar (SAR) is used
mainly to measure the wave spectrum. These measurements may also be used to
map the winds and ocean currents at small scales but the signal is complex and difficult to invert (Johannessen, 1995). SAR images also provide useful measurements ofsea ice parameters (Johannessen et al., 1995).
Finally, one has to consider satellite data collection and/or location systems for
in-situ measurements. They are vital for operational oceanography since they allow
the near real time data collection and, if needed, the location of drifting instruments
for ve10city estimations (e.g. surface drifters, ALACE, profiling floats). In particular, it inc1udes the well known ARGOS system (www.c1s.fr/htmVargos).
A1timetry, SST and ocean color
A ranking of satellite oceanography techniques for ocean forecasting objectives
would probably give altimetry, SST, scatterometers and probably ocean colour.
The reason why altimetry is so useful will be detailed in the next section. We will
not discuss further scatterometry as we focus here on ocean parameters. The avai1ability of winds with high space and time resolution is, however, a critic al requirement for ocean forecasting (see Milliff et al, 1999 for a review).
It is c1ear that SST measurements are crucial for understanding and predicting
the ocean/atmosphere system. SST is an important factor that influences (and is
21
radiative parameter which can be inferred from the satellite radiative measurements (microwave radiation is related to atmospheric liquid water content, not
direct1y to rain rate, but a good correlation between the two parameters existsbesides, precipitation is poorly known and satellites provide very useful information). Note that visible and infrared techniques can also provide useful information
on precipitation. Latent heat flux or evaporation rate can also be inferred from SST,
wind speed and integrated water vapour (although data on surf ace level humidity
would be needed). Finally, one should note that SST can also be retrieved from
microwave radiation but with a lower resolution and accuracy compared to infrared radiation. There is also an interesting capability for measuring Sea Surface
Salinity from microwave measurements.
Main active microwave radiometers or radars for satellite oceanography are:
altimeters, scatterometers and Synthetic Aperture Radars. Satellite altimetry provides measurements of the sea surface topography, significant wave height and
wind speed modulus. It also provides measurements of ice sheet topography (e.g.
mass balance of Antarctica and Greenland ice sheet). We shall describe altimeter
measurement principles in the next section. A scatterometer measures the electromagnetic signal backscatter coefficient in three directions. The backscatter coefficient depends on wind speed and direction. By analyzing backscatter in different
directions, we can estimate the wind speed direction. There is a 180 0 ambiguity
because differences in backscatter 180 0 apart are very small. This ambiguity is
generally removed through consistency check from previous measurements and/or
using atmospheric ocean models. Scatterometers thus provide wind speed and wind
direction measurements. They are also useful for measuring sea ice, although not
as useful as microwave radiometers. Synthetic Aperture Radar (SAR) is used
mainly to measure the wave spectrum. These measurements may also be used to
map the winds and ocean currents at small scales but the signal is complex and difficult to invert (Johannessen, 1995). SAR images also provide useful measurements ofsea ice parameters (Johannessen et al., 1995).
Finally, one has to consider satellite data collection and/or location systems for
in-situ measurements. They are vital for operational oceanography since they allow
the near real time data collection and, if needed, the location of drifting instruments
for ve10city estimations (e.g. surface drifters, ALACE, profiling floats). In particular, it inc1udes the well known ARGOS system (www.c1s.fr/htmVargos).
A1timetry, SST and ocean color
A ranking of satellite oceanography techniques for ocean forecasting objectives
would probably give altimetry, SST, scatterometers and probably ocean colour.
The reason why altimetry is so useful will be detailed in the next section. We will
not discuss further scatterometry as we focus here on ocean parameters. The avai1ability of winds with high space and time resolution is, however, a critic al requirement for ocean forecasting (see Milliff et al, 1999 for a review).
It is c1ear that SST measurements are crucial for understanding and predicting
the ocean/atmosphere system. SST is an important factor that influences (and is