The quality of Numerical Weather Prediction (NWP) models relies partly on the availability of surface wind and wave observations.
Multiple views of the same storm using three different satellites.
Until recently, observations at sea were mainly provided by voluntary observing ships. Away from ship lanes, in many areas, there is a lack of observations. Moreover, the coarse resolution of NWP models limits the interpretation and the description of the atmospheric surface features. Strong gradients in the surface wind and wave fields exist at scales of a few kilometers; they are dramatically smoothed in NWP models. Unprecedented views of surface wind and wave fields in storms are now provided by microwave sensors on-board polar orbiting satellites. Microwave measurements penetrate thick convective storm clouds, observing wind and wave fields with good accuracy. Microwave data are now available to meteorological centres where they are assimilated in NWP models and also used for nowcasting. Microwave radars of the European Remote-Sensing Satellites (ERS), radiometers of the Defense Meteorological Satellite Program (DMSP) and the dual-frequency altimeter TOPEX-POSEIDON have shown their ability to improve the description and location of storms, especially in the case of tropical cyclones for which very few traditional observations exist.
The European Remote-Sensing Satellites (ERS-1 and ERS-2), launched by the European Space Agency respectively on July 17, 1991 and April 21, 1995, carry the first satellite-borne C-band (5.3 GHz) Active Microwave Instrument (AMI) capable of measuring, in scatterometer mode, surface wind speeds and directions over the oceans. Algorithms to process the three-beam scatterometer data have been developed at Institut Franais de Recherche pour l'Exploitation de la Mer (IFREMER).
Scatterometer systems are becoming more and more operational and better adapted to storm observation. A Ku-band (14.3 GHz) scatterometer, NSCAT, was launched by NASA on August 17, 1996. In addition ESA and EUMETSAT are jointly planning a series of operational C-band scatterometers for flight beyond the year 2001. These scatterometers have two-sided systems covering at least 1000 km in width with a resolution of 25 km. The Special Sensor Microwave Imager (SSM/I) radiometers provide brightness temperatures at three different frequencies (19.35, 37.0 and 85.5 GHz) from which are estimated: wind speed when not raining, integrated atmospheric water vapor content, liquid water content, and a rain index. The SSM/I instruments have a wide swath (1400 km) and are in a high inclination sun-synchronous orbit, covering almost the whole planet within a day. The resolution varies from 15 km to 50 km depending on the frequency.
Altimeters on-board the ERS satellites and TOPEX-POSEIDON also allow high resolution wind speed and wave height measurements. Surface wind and wave parameters at various scales help to describe the wind and wave patterns in storms as well as to study of air-sea interaction mechanisms in such energetic events. However, altimeters are limited to a narrow footprint of about 7 km at the nadir of the satellite and do not measure wind direction. Overleaf, a tropical cyclone has been selected to illustrate the capabilities of the measurements coming from these different microwave instruments. The profile of wind speeds and wave heights across the storm shown on the previous page are shown on the diagram.
Significant Wave Height & Wind Speed:
- a: metres
- b: metres/s
Wind speeds (blue line) and wave heights (red line) derived from theTOPEX/POSEIDON satellite for the same date as the SSM/I image on thepreceding page. Large spikes near 18 latitude correspond tocontamination of measurements by rain.