Earth’s wind patterns are generated by the transfer of heat in the atmosphere as affected by the planet’s rotation through the Coriolis effect. Source: ESA

Throughout history, weather has had an important impact on human activity. This remains true into the present day, with meteorological conditions affecting everything from agriculture to construction. To work around and mitigate the effects of inclement weather, forecasts provide early warning for the timing and location of bad conditions.

Weather forecasts are more accurate than ever thanks to advanced weather instruments and complex meteorological models. However, there is much room for improvement.

A new satellite operated by the European Space Agency (ESA) promises to enhance forecast accuracy by providing meteorologists with information that has been lacking in the past: direct measurements of global wind profiles. Aeolus, named after the "keeper of winds" from Greek mythology, will measure the wind from space using a sophisticated Doppler lidar instrument, supplying key data for meteorologists to improve forecasts of both short-term weather and long-term climate changes.

Numerical Weather Prediction

Modern numerical weather prediction models generate forecasts by ingesting rich sets of data on the current state of the weather from sensors around the world and in space. These sensors, part of the Global Observing System (GOS), provide real-time data on atmospheric conditions like temperature, pressure, moisture and cloud cover. But a large gap in the data exists for directly measured wind speeds at 30 km in altitude, especially in the Southern Hemisphere, where wind speed observations are mostly indirect.

Aeolus will serve as a prototype for future satellites that can provide this wind profile data, allowing scientists to gain an improved understanding of atmospheric dynamics, including the interplay of wind, pressure, temperature and humidity. Ultimately, meteorologists will incorporate this knowledge into improved numerical weather predictions that increase forecast accuracy.

“The lack of global wind observations limits our understanding and the prediction of weather and climate,” said Lars Peter Riishøjgaard of the World Meteorological Organization in an ESA press release. “Aeolus will provide a pioneering demonstration of the most promising technology to fill this gap.”

Aladin emits laser pulses and senses backscattered light from the atmosphere to measure wind speeds and generate global wind profiles. Source: ESA

Direct Detection Doppler Wind Lidar

Aeolus’s payload consists of a single instrument, the atmospheric laser Doppler instrument (Aladin), comprising a laser, receiver and telescope.

Aladin generates wind profiles based on two principles: light scattering and the Doppler effect. The instrument operates by emitting pulses of ultraviolet light from its laser. Backscattered light from the atmosphere is collected by the telescope and detected by the receiver. The distance between the satellite and the atmospheric particles that scatter the laser light (and thus, their altitude) is determined by tracking how long it takes to receive the backscattered signal. Meanwhile, the motion of the particles in the wind produces a Doppler shift in the wavelength of the scattered light, a change that is related to wind speed by a mathematical function. Combining the two data points – wind altitude and wind speed – provides a complete wind profile picture.

Aeolus’s Aladin sophisticated Doppler wind lidar instrument is the first of its kind in space. Source: ESA

Aladin’s solid-state, diode-pumped, frequency-tripled, Nd:YAG laser emits ultraviolet light at a wavelength of 355 nm, a spectral region that produces the strongest backscatter signal from molecules in the atmosphere.

The instrument’s receiver incorporates both Mie and Rayleigh spectrometers for accurate detection of both wind-driven particles as well as air itself. Aerosol and cloud particles cause Mie scattering with unique spectral and wavelength properties detectable by the Mie spectrometer. Air molecules produce Rayleigh scattering of the laser pulses, which is sensed by the Rayleigh spectrometer.

Aladin’s 1.5 m telescope features a silicon carbide structure and mirror to minimize weight while providing high optical quality and stability.

Aeolus will capture around 100 wind profiles per hour with an accuracy of 1 m/s in the planetary boundary layer (up to an altitude of 2 km) and 2 m/s in the free troposphere (up to an altitude of 16 km). The satellite will also find average wind velocity over 100 km tracks.

Aeolus was launched into space by a Vega rocket on August 22 from Kourou, French Guiana. After a commissioning period of a few months, during which controllers will verify instrument function and calibration, the 1,360 kg spacecraft will spend at least three years in a sun-synchronous, polar orbit 320 km above Earth mapping the planet’s winds.

(Read: Lidar technology is also being harnessed to provide early warning detection of turbulence onboard aircraft.)