Unravelling the mysteries of clouds

Clouds and aerosols are still considered the great unknowns in understanding our climate. With the EarthCARE Earth observation satellite, Airbus has made the finishing touches to the crucial ‘key’ that will unlock the mysteries of clouds, helping make more accurate atmospheric models and climate forecasts.

The 2.3-tonne satellite is now being transported from the Airbus site in Friedrichshafen, Germany, to the European Space Agency's ESTEC technology centre in Noordwijk, the Netherlands. There it will be put through its paces until the middle of next year. Among other things, it will be subjected to the stresses and strains that occur during launch and the environmental conditions that await it in orbit. In other words, its space readiness will be extensively tested.

The science

Clouds play an important role in our climate because they regulate the amount of solar energy that reaches the Earth’s surface and the amount of energy that is reflected back into space. The more energy absorbed by the Earth, the warmer it gets. If less energy is absorbed, it becomes cooler.

However, the exact role that clouds and aerosols play in the climate system is still a big unknown in climate forecasts and in understanding the Earth's water cycle.

Clouds need aerosols to form. These micrometre-sized particles - smaller than a human hair - float in the air. Many are natural, like sea salt carried away by the oceans or dust particles from the Sahara. There are also human-produced aerosols. For example, combustion produces soot and sulphur dioxide, which comes mainly from power stations. Nitrogen oxides from car exhausts are another cause. The more aerosols in the air, the more clouds reflect because they are made up of more but smaller water droplets. When there are fewer aerosols, there are also fewer water droplets, which become larger and can then fall more easily as raindrops.

Larger clouds generally reflect more light and have a greater cooling effect. When clouds contain more water droplets, they become whiter and reflect more sunlight, which cools the Earth's surface.

Cloud height also matters. Higher clouds tend to be colder, so they give off less heat and keep it trapped in our atmosphere instead of radiating it out into space. Low clouds are warmer, emitting more heat. So this means that higher clouds tend to warm the Earth's surface and atmosphere. In addition, aerosols influence the life cycle of clouds and thus indirectly contribute to warming of the Earth, i.e. its radiation and energy budget. Measurement data on aerosols should now help to better understand these processes.

The satellite and its mission

EarthCARE will provide global profiles of clouds and aerosols together with measurements of solar radiation reflected from the planet and thermal radiation emitted by the Earth. To this end, the satellite carries two large instruments: a lidar called ATLID to measure the vertical profiles of aerosols and thin clouds, and a cloud profiling radar (CPR) to measure the vertical profiles of thick clouds and precipitation. The CPR is provided by the Japanese Space Agency JAXA.

ATLID, built by Airbus in Toulouse, France, operates in the ultraviolet spectrum at a wavelength of 355 nm and uses the Doppler effect to provide vertical profiles from around 100 metres above the ground to a maximum height of 20 kilometres, or from 500 metres to a maximum height of 20 to 40 kilometres. The measuring principle makes use of the fact that an emitted light signal is scattered differently by molecules or aerosol particles as it passes through the atmosphere. ATLID is the second European lidar in orbit - Airbus is already globally renowned as a specialist in space-based lidar systems through the Aeolus satellite (in space since August 2018).

Two other instruments, a cloud imager (MSI) and a broadband radiometer (BBR), which measures the reflected solar radiation as well as the emitted thermal radiation of the clouds, complete the sensor equipment on the satellite. By using all four instruments simultaneously, 3D cloud and aerosol scenes can be directly correlated with reflected solar radiation and emitted thermal radiation.

EarthCARE ready for transport - Copyright Airbus

EarthCARE will orbit the Earth at an altitude of about 400 kilometres. The orbit is as low as possible to optimise the use of lidar and radar, but not too low, otherwise aerodynamic drag would affect fuel consumption and mission lifetime.

Because global coverage is required, EarthCARE flies in an almost polar orbit. It crosses the equator in the early afternoon, which ensures optimal illumination and minimal solar radiation for the passive instruments. At 1,600 watts, the system's power requirements are considerable and are mainly determined by the two active instruments, ATLID and CPR.

Visually, the satellite is dominated by the large CPR antenna, which has a diameter of 2.5 metres. The long rear solar array gives the satellite an overall length of 18 metres. The solar array consists of five sections with an area of 21 square metres. The tail-like arrangement helps minimise drag given the satellite's low orbital altitude.