MSG-2 will advance long-term monitoring of Earth’s energy balance

This week’s launch of MSG-2 will ensure that satellite images continue to be available to European weather forecasters well into the next decade. It also marks a new chapter in a long-term space experiment measuring the available energy that drives the weather as a whole, and helping to establish how much the Earth is heating up.

MSG-2’s main instrument is the Spinning Enhanced Visible and Infra-red Imager (SEVIRI) which returns detailed 12-wavelength images of the Earth and its atmosphere every 15 minutes, for use in operational meteorology.

As in the case of the original MSG-1 – launched into geostationary orbit back in August 2002 – the drum-shaped satellite also carries a smaller scientific instrument called the Geostationary Earth Radiation Budget (GERB), designed to measure the net balance between incoming radiation from the Sun, and outgoing radiation from the Earth, known as the ‘Earth Radiation Budget’ – which is the energy source for the planet’s atmospheric system and the ultimate driver of weather phenomena and climate.

Results from MSG-1’s original GERB are being used to enhance scientific understanding of climate processes, how human activities are modifying the climate balance and as inputs to improve the accuracy of complex numerical models. GERB results are also being combined with high-resolution SEVIRI imagery to see how hour-by-hour variations in clouds affect the radiation balance.  

“Already GERB data from the first instrument are being used, for example, to study how the daily variation of tropical clouds over Africa affect the climate balance, and also how the huge dust storms that sweep from the Sahara out over the Atlantic can affect the weather and the climate,” says Prof. John Harries of Imperial College, Principal Investigator for GERB.

“New aspects of these processes are being observed, with studies carried out by members of the GERB International Science Team (GIST), which includes scientists from the UK, Belgium, France, Germany, Spain, Italy and the USA. This wide representation demonstrates the strong international interest in these unique data.”

The intention is to fly a GERB on all four MSG satellites, to continue monitoring of the Earth Radiation Budget for at least a decade and a half and identify any long-term trends.

In the average 24-hour day the Sun shines out radiation equivalent to 1.37 kilowatts per square metre of the top of the Earth’s atmosphere. If this energy was retained within the atmosphere, our planet would swiftly become a Venus-like hothouse. However once the Earth has reached balance, this energy is actually returned back to space, either as short-wavelength light or else longer-wavelength heat energy. It is the subtle variability of this balance, at different locations and times, that needs to be understood.

  As light, some of it is directly reflected back by clouds or the Earth’s surface or scattered backward by air molecules in the atmosphere. Other sunlight is absorbed by trace gases, dust, or water vapour in the high-altitude stratosphere, clouds in the low-altitude troposphere or else by the land surface or ocean.

The air, land or sea that absorbs this sunlight becomes heated up by it, and this heat is eventually emitted back into space at longer, thermal wavelengths – invisible to the human eye but capable of detection by sophisticated sensors.

What GERB does is continuously measure the sunlight reflected or scattered back from the Earth plus the heat radiation radiating from it. First it measures the total radiation, next a filter is passed in front of its measuring array so it measures only the short wavelength radiation. A figure for long wavelength radiation can simply be derived by subtracting the short wavelength value from the total.

While the incoming energy of the Earth’s Radiation Budget stays broadly consistent within seasonal norms, the outgoing energy can vary considerably over a period of hours of days because what is termed ‘radiative forcing’, mainly from changes in concentrations of clouds, aerosols or water vapour.

GERB, like SEVIRI, performs a new measurement every 15 minutes, working on a continuous basis. The sensor has a spatial resolution of 50 km – a value which can be reduced to 10 km after data processing – so even small-scale spatial or temporal radiation fluxes can be identified.

  Prof. Harries adds: “The initial GERB has now been operating since early 2003. The instrument works in a very tough environment, of the vacuum of space, but also under a force of some 16g, caused by the spin of the spacecraft.

“Despite this, the instrument has been a major success, a credit to the British, Belgian and Italian engineers – led by the UK’s Rutherford Appleton Laboratory (RAL) – that designed it.

“Scientifically, highly accurate images of the infrared emission and solar reflectivity of the Earth are being made every 15 minutes, a response time never before achieved, which allows us too study many processes that were previously not resolved by spacecraft.

“The GERB team led by Imperial College continues to pin down every last detail of how GERB works, to maximise the accuracy, but already the performance has been a great success. The ground segment, operated by RAL, the Royal Meteorological institute in Brussels, and Imperial, is working highly effectively.”

The idea for such a sensor was first proposed in 1958, and a number of earth radiation budget instruments have flown before the first GERB launched on MSG-1. What makes the GERB series unique is that, for the first time, they observe the Earth disc from geostationary orbit, approximately 36 000 km up.

In such an orbit – also known as the ‘Clarke Belt’ after its discoverer Arthur C. Clarke – a satellite is able to cover more than a quarter of the Earth’s surface, and also moves at the same velocity as the rotating Earth, to remain hanging at the same spot in the sky relative to the ground.

Such a fixed position means that day-to-night variations which throw up problems for radiation budget instruments in a lower orbit can be properly characterised, and steady temporal sampling is maintained. At the same time GERB results can also be blended with data from such lower-orbit sensors to create a fuller picture of the entire Earth Radiation Budget.

“This is a true ‘first’ for Europe,” Prof. Harries explains. “While we have US colleagues working alongside us on the GIST, collaborating on the scientific exploitation of GERB data, this is the first Earth Radiation Budget sensor to be flown in the geostationary orbit, and the whole concept, design, development, testing and operation has been a unified European effort.”

  In response to an Announcement of Opportunity by ESA, lead funding for the original GERB instrument came from the UK Natural Environment Research Council (NERC). The instrument was developed by a European consortium headed by Imperial College, London, where the Principal Investigator (PI) is Prof. John Harries.

The UK Rutherford Appleton Laboratory (RAL) has held the Project Manager role, as well as being the technical authority. Major technical and financial contributions have come from Belgium (Royal Meteorological Institute of Belgium – RMIB, Advanced Mechanical and Optical Systems – AMOS) and Italy (Officine Galileo). The University of Leicester provided the detector arrays that ultimately sense the radiation from the Earth.

Subsequent GERB instruments are being funded by MSG operator EUMETSAT, the European Organisation for the Exploitation of Meteorological Satellites, by contracts to the same consortium members and structure as for the first instrument.

RAL is responsible for processing, archiving and distribution of GERB data, with RMIB performing additional processing. Imperial College in London is performing instrument calibration as well as scientific leadership of the consortium. The role of the GIST has already been described.

Meteosat Second Generation (MSG) is a joint project between ESA and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and follows up the success of the first generation Meteosat weather satellite series with a larger design boasting higher performance. The first in a planned series of MSG satellites was launched in 2002, entering into service with EUMETSAT in early 2004 and now renamed Meteosat-8.