PhD student filters water vapour information from satellite data

PhD student Rüdiger Lang has developed a method to obtain information about water vapour from satellite data not specifically measuring this. The research is part of a project from the FOM Institute for Atomic and Molecular Physics (AMOLF), the Space Research Organisation Netherlands (SRON) and the Free University of Amsterdam.

Water vapour has a greenhouse effect three times stronger than that of carbon dioxide. Therefore a good picture as to the presence, distribution and influence of water vapour in the atmosphere is important. Observations of the atmosphere and the earth`s surface from a number of satellites orbiting the earth contain hidden information about water vapour.

In laboratory experiments, Rüdiger Lang determined the exact `fingerprints` which water molecules leave behind on the signal that satellites measure for the atmosphere as a whole. An instrument in such a satellite looks through the earth’s atmosphere and measures the characteristics of the sunlight which is reflected by the earth`s surface (land, sea, ice, clouds).

The reflected light contains a large number of absorption lines. These are narrow parts of the spectrum where the light is weakened by specific molecules or atoms in the atmosphere. The form of the lines depends on the concentrations of molecules or atoms and their distribution throughout the atmosphere. As water molecules influence their environment, they leave behind a `fingerprint` on the absorption lines. The researchers found that the most accurate predictions were obtained when the very weak absorption lines were used.

An important element in the research was the development of efficient detection methods. Both existing and future satellites gather enormous quantities of measurement data. Unless this can be processed efficiently, it takes far too long to obtain up-to-date data. The methods developed will also be usable for other greenhouse gases.

The greenhouse effect is a socially important issue, yet without the greenhouse effect the earth would be uninhabitable. The quantity of water in the atmosphere alone ensures that the average temperature on the earth is 14oC higher than would be the case if there were no water in the atmosphere. As the average temperature on earth is 14oC then without this water large parts of our planet would be in an ice age. Information about the quantity of water is particularly important if the greenhouse effect is to be fully understood. Water occurs in the atmosphere in three different forms: as ice crystals, water drops and gas (water vapour). Therefore, the influence of water vapour in the atmosphere is complex and the effect is further complicated by the fact that the quantity of water in the atmosphere can vary considerably from place to place.