Looks like rain …

By Mick Hamer FORECASTING the risk of rain and floods is about to be made more accurate by a microwave sensor that has just been launched into orbit. At present, weather satellites show only where clouds are, not how much water they contain. The new sensor will probe the humidity within cloud formations, giving forecasters a much better indication of when a downpour is likely to occur. Current weather satellites use infrared and visible light to provide forecasters with data on weather systems. But these instruments cannot see through cloud cover. The new microwave instrument, known as the advanced microwave sounding unit (AMSU-B), will provide forecasters with data on humidity in and beneath clouds. The instrument was built by the Anglo-French company Matra Marconi for Britain’s Meteorological Office, and was carried into orbit on a satellite belonging to the US National Oceanic and Atmospheric Administration (NOAA). The computer models used by weather forecasters have improved steadily over the past decades, but they are only as good as the information put into them. “We need a good description of what the atmosphere is like now,” says John Eyre, head of satellite applications at the Met Office. Most of Europe’s weather blows in from the Atlantic. Apart from satellites, the only sources of information about rain and humidity over the ocean are weather ships and land-based radar, which has a useful range of no more than 100 kilometres or so. In the early 1970s, there were eight weather ships in the Atlantic, but now there is only one. “Satellites give us global coverage,” says Eyre, “but the disadvantage is that you are looking at things from several hundred miles away.” The lack of information about humidity under clouds has been an important gap, says Eyre. Over a three-day forecast, some errors becomes less important. But other errors grow rapidly over time. “Most of the errors that grow rapidly are in frontal areas and they are predominately cloudy,” says Eyre. The NOAA satellite will carry three microwave instruments. AMSU-B will look for the distinctive microwave frequencies absorbed by water, while AMSU-A1 and AMSU-A2 will measure temperature. Philip Evans, head of the Met Office’s AMSU-B programme, says that the device will be able to measure humidity at different heights in the atmosphere, regardless of cloud cover. He says the instrument has been tested extensively in vacuum tanks to check its resistance to vibration and solar radiation, and to calibrate the humidity readings. The final stage of testing will be to calibrate the readings from the satellite against measurements taken by aircraft. In about a month, the first readings from the AMSU-B will be given to Met Office forecasters, who advise TV and radio forecasters. And in about six months’ time, after the final calibration,
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