Observations from space
Observations from space are becoming an ever increasing possibility as technology and skills develop. Satellites used for meteorological purposes play a significant role in modern observations both from a global perspective as well as tracking weather systems that directly affect the British Isles.
Satellites used by the Met Office
Meteorological satellites fall into two broad categories: geostationary and polar orbiting.
Geostationary satellites, located over the equator at a height of 35,800 km, orbit the earth once every 24 hours. Spinning at the same rate as the earth they stay above the same spot all the time and provide an unbroken series of images of the atmosphere below. The Met Office contributes to the European operated geostationary satellite, Meteosat, ideally positioned over the Greenwich meridian to observe weather systems that affect the UK and the rest of Europe.
Polar orbiting satellites
By contrast, polar orbiting satellites at a typical height of 850 km are much closer to the earth allowing their instruments to make measurements at far greater resolution. Orbiting the earth about every 100 minutes, they scan wide swathes of the atmosphere as they sweep from pole to pole. Over the course of the day they view most parts of the earth at least twice. Their orbits are sun-synchronous, which means that they see that same part of the earth at the same local time each day. The Met Office contributes to the European polar orbiter, Metop, and makes use of data from similar satellites operated by US agency NOAA and others.
Since the first meteorological satellite TIROS was launched in 1960, satellite images have been used to give a picture of the weather as viewed from space and they have become a familiar sight on TV weather forecasts. Movie loops of images taken from geostationary satellites give a graphic impression of the movement of clouds. Viewed in the visible part of the spectrum, as the human eye would see from space, the shadows cast by the sunlight provide a three dimensional feel to the clouds, but only where there is daylight. To view the atmosphere at night, infrared imagery is needed. In this image the highest and coldest cloud tops appear white, while lower warmer clouds and the earth's surface are in darker shades.
High precision instruments on satellites measure the radiation emitted by the atmosphere and the earth's surface. This is radiation naturally emitted by all bodies, typically in the infrared and microwave parts of the spectrum that is related to the temperature of the body. An everyday use of this technology is the passive infrared sensor used to detect intruders in a house or business premises. On a satellite these instruments, called radiometers, make accurate measurements of temperature and water vapour. Where the instrument is sensitive to a selected frequency in the infrared, the measurement is related to the average temperature of a cloud-free layer of the atmosphere. In the microwave part of the spectrum, the measurement is related to the temperature and humidity of the layer. Microwave measurements, unlike their counterparts in the infrared, provide valuable information in the presence of clouds. By having a number of radiometers on the satellite, each sensitive to a different frequency, it is possible to derive the vertical profiles of temperature and humidity through much of the atmosphere. These vertical profiles are called soundings. Having global coverage, they are of immense value to numerical models that forecast the weather.
A scatterometer is a radar system mounted on a polar orbiting satellite that directs radar pulses towards the earth's surface and measures the strength of the backscattered return beam. Over the oceans backscattering is caused by small wind generated waves. These are not the large waves that make ships heave up and down, rather, they are the much smaller ripples on the sea surface of wave length 5-20 cm that tend to lie at right angles to the wind direction. By measuring the backscatter at two or more angles of incidence, it is possible to derive the wind speed and direction close to the sea surface.
Global satellite navigation systems
The signals from global navigation satellites that help us navigate our way in a car have meteorological applications as well. Working on the same basic principles of commonly available GPS Satnav systems, the time of arrival at a ground station of a satellite signal passing through completely dry air can be calculated with immense accuracy. Water vapour in the atmosphere slows its arrival and the measured delay can be used to estimate the total water vapour content along the signal's path from the satellite. The data are particularly valuable for identifying areas of deep moist air that trigger thunderstorms on hot summer days.
As well as being received by a ground station on earth, the signal from a global navigation satellite may also be received by a second low orbit satellite providing a different source of meteorological data. As the satellite is occluded by the passage of the earth's atmosphere, the signal's path is bent by atmospheric density gradients. The variation in the bending with the height above the surface provides valuable information about the temperature and humidity through the depth of the atmosphere. The European Metop satellite includes an instrument that makes this type of measurement.
Some satellites in low polar orbits carry an altimeter instrument that very accurately measures the distance between the satellite and the earth's surface immediately below its path. These measurements have several valuable applications, particularly over the oceans. The height of the ocean surface is dependent on many factors such as the physical properties of the ocean, oceanic tides, atmospheric pressure and local gravitational anomalies of the earth's crust. Areas of the ocean where the surface height is uniformly higher than average may be attributed to warm columns of water which expand as their temperatures rises. The distinctive pattern of warm water in the equatorial east Pacific known as el Nino and its cold counterpart, la Nina, may be easily tracked from altimeter data. Currents on at the ocean surface are driven by gradients in the height of the ocean surface in the same way that winds in the atmosphere are driven by gradients in surface pressure. The surface current may therefore be inferred from altimeter measurements in the same way that the wind may be inferred from measurements of pressure. The data are particularly useful in the Ocean models at the Met Office used for forecasting the temperature and movement of water at all levels in the ocean.
Other meteorological and oceanographic applications of altimeter data include measurement of wave height and wind speed at the ocean surface, the thickness and extent of sea ice, and the rate of rise in global sea level over the long term.
Doppler wind lidars
A lidar emits pulses of light from a laser and measures the properties of targets from the nature of returned signal. In the case of a Doppler wind lidar mounted on of low orbit meteorological satellite, the targets are atmospheric molecules, cloud droplets and aerosols. The measured Doppler shift is directly related to the wind speed along the line of sight of the lidar beam. If two or more lidar beams are used, pointing in different directions, both wind speed and direction can be obtained. Launch of the first demonstration satellite by the European Space Agency ESA is due in 2017 and will provide global coverage of wind at various levels of the atmosphere.