Global maps of average wind stress vector (in 10 dyne/cm) for northern hemisphere winter and summer. The wind stress values are computed from ERS-1 scatterometer winds over the whole ERS-1 mission1991 - 1996. Colors indicate the wind stress magnitude and the arrows the wind directions. The spatial resolution is one degree. Typical wind features, for instance trade winds, monsoon and mid-latitude storms, and their seasonal variability, can be clearly observed.
Accurate knowledge of surface winds over the oceans is vital for the understanding and forecasting of oceanic circulation and earth climate changes. Traditionally, surface winds have been monitored using voluntary ships of opportunity and buoys, leading to large areas of the oceans where very few data are available, for instance regions away from main trading routes.
Satellite-borne radars, such as scatterometers, are of major interest, first because of their large temporal and spatial coverage and second because of their all-weather measurement capability. Such instruments are flown on the European Remote-Sensing Satellites of the European Space Agency.
Routine processing is now used to produce surface wind fields and sea ice age and extent, on a global coverage and on weekly and monthly basis. Both wind and sea ice data are available, with data extraction software and display capabilities, on CD-ROMS.
The European Remote-Sensing Satellites ERS-1 and ERS-2, launched by the European Space Agency on July 17, 1991 and on April 21, 1995 respectively, each carry a C-band scatterometer, primarily dedicated to sea surface wind field measurements. The Active Microwave Instrument of ERS in wind mode (or scatterometer) is a three-antenna radar at 5.3 GHz. The three beams sample the sea surface at azimuth angles of 45, 90 and 135 with respect to the satellite flight direction. The radar illuminates a 500 km wide swath at incidence angles ranging from 18 to 57. Radar measurements are binned over 50 km wide overlapping cells, the centres of the cells being separated by 25 km. The sea surface backscatter coefficients ( sigma0), measured by the three beams, are computed in each 25 km x 25 km grid cell.
Over open water, the principal backscattering mechanism for scatterometer off-nadir measurements is the Bragg mechanism : 0 is related to the surface wave power density spectrum within the wavelength range L= /2 sin ( phi) where L is the surface wave length, is the radar wavelength and is the incidence angle. At C-band the radar wavelength is 5.7 cm and thus the surface wavelength seen by the radar ranges from 3 cm to 10 cm. This corresponds to the range of short gravity waves. As wind speed increases the spectral power density in that wavelength range increases and so does the corresponding 0 value. The 0 dependence on wind speed is modulated by the wind direction with respect to the beam azimuth angle. Observations at three azimuth angles relative to the wind direction permit to determine this modulation curve and therefore the true wind direction. Processing the measurement triplets with a wind algorithm produces the wind vector.
C-band was selected to minimize the effects of rain and atmospheric water vapor content. The level of 0 noise is about 5 % which allows computation of the wind vector with root mean square errors of about 1.4 ms-1 in speed and 19 in direction, over the 3 ms-1 - 24 ms-1 wind speed range. Processing, validation and distribution of wind products are carried out at the French processing and archiving facility, Centre ERS d'Archivage et de Traitement (CERSAT).
The main applications of this type of data need regularly space and time gridded wind fields. Because of the limited size of the measurement swath and of the quasi-polar orbit, the data measurements are discrete in both time and space.
Scatterometer data are thus asynoptic. Methods have been developed to provide regularly spaced fields over ocean basins. An objective analysis technique is used for interpolation. Resultant wind fields are computed with a spatial resolution of 1 by 1 . The following parameters are mapped: wind vector, wind stress, wind divergence and stress curl, on a global coverage and on weekly and monthly basis. All these are available on CD-ROM, presently over the 5 year period from 1991 to 1996. Detailed information can be obtained from CERSAT at e-mail firstname.lastname@example.org.
Since August 1996, a new sensor, the NASA SCATterometer (NSCAT) has been flying on the Japanese satellite ADvanced Earth Observation System (ADEOS). It is a Ku-band scatterometer and it will provide similar information but with better spatial and temporal resolution because of its double swath and its 25 km resolution. In addition ESA and EUMETSAT are jointly planning a series of operational double-swath C-band scatterometers for flight beyond the year 2003.