Windswept_ The Story of Wind and Weather - Marq de Villiers [70]
More recently NOAA, the parent body of the weather service, together with NASA, launched a network of weather satellites with varying technologies and capabilities. One of the most promising of these, at least for wind measurement, is a technique called Synthetic Aperture Radar—SAR for short. SAR measures wind by calibrating every pixel value in the radar image to what is called "absolute radar backscatter." That is, it measures precisely the size and frequency of local image interference, and matches that to wind speeds and directions. A trial in the Gulf of Maine in 2000 yielded a finely grained map down to a twenty-five-yard resolution. The same year a satellite called Radar Sat was launched carrying the new device, rather unimaginatively dubbed Scan SAR, which enabled scientists at the Jet Propulsion Laboratory to extract data for coastal winds over many hundreds of miles with a resolution of only hundreds of yards. Its use in hurricane-watching would be evident: It would enable the hurricane center to see at a glance small regions of very high winds, as a supplement to aircraft and other measurement.26 Three new wide-swath ScanSARs were launched in 2002, the European En-visat, the Canadian RadarSat-2, and the Japanese ALOS.
Nevertheless, SAR is far from perfect. It has difficulty seeing through heavy cloud cover and in high winds, just the conditions in which accurate data become most necessary.
Another promising, though still in 2005 speculative, technique is the analysis of what are called "ocean microseisms." These are simpler than they sound: When seismometers capable of measuring ground vibrations were deployed decades ago, in the early part of the twentieth century, it became apparent that the ocean itself was giving off a continuous seismic hum, the product of the earth's response to wave-on-wave interactions. More recently it was realized that storms can be located and tracked using this seismic data. Because almost seventy years of archived information exists, "this approach allows, for example, the strengths of El Nino conditions to be assessed for times when [other] ocean data were unavailable."27
By 2004, space-borne "scatterometers" were slowly building up a real-time description of global wind patterns in a system calibrated by matching results with the evidence of wind buoys. A scatterom-eter is a device that sends microwave pulses to the earth, and uses the backscatter to measure the surface roughness. On land, the device has been used mostly to map things like vegetation cover in the Sahara-Sahel region, and to track shifts in polar ice. At sea, the backscattering is caused by ripples and waves, and can be measured down to a few inches. As the Jet Propulsion Laboratory (JPL) puts it, "the idea of remote sensing of ocean surface winds was based on the belief that these surface ripples are in equilibrium with the local wind stress"— which for nonspecialists means that the direction and height of the backscatter can tell you the direction and strength of the winds.
Scatterometers do not always work smoothly, either—ambiguities are encountered in interpreting wind direction, which require several casts at different angles to resolve, and rain can still fuzzy the images—but they are nevertheless the only instruments currently deployed able to give real measurements of ocean surface wind speed and direction under both clear and cloudy conditions, day and night. As W Timothy Liu of the JPL wrote in Backscatter, "They give us not only a near-synoptic global view, but details not possible using numerical weather prediction models. Such coverage and resolution are crucial to understanding and predicting the changes of weather and climate."28
NASA first deployed the technology in a satellite called QuikScat, launched