Water vapour in the atmosphere is one of the key substances, controlling both weather and climate. Water vapour is the dominant greenhouse gas in the Earth's atmosphere. Moreover, it plays a central role in atmospheric chemistry. Despite its importance to atmospheric processes over a wide range of spatial and temporal scales, water vapour is one of the least understood and poorly described components of Earth's atmosphere.
The primary objective of the DLR project WALES is the preparation of a spaceborne mission to overcome the shortcomings of radiosondes and passive satellite sensors in mapping the global water vapour distribution. While the former do not cover the globe uniformly and do not provide reliable water vapour observations in the upper troposphere and lower stratosphere, the latter suffer from insufficient vertical resolution and accuracy. In contrast, a spaceborne multi-wavelength H2O-DIAL (differential absorption lidar) could provide global water vapour observations suitable for a reliable assessment of its temporal and spatial evolution. These analyses would lead to an improved description of climate processes in general circulation models and to benefits in numerical weather prediction.
First proposals for a spaceborne H2O-DIAL made during the 90s suffered from a much too high power-aperture product, driving the system costs to unrealistically high values. To overcome this problem a measurement scheme was developed that uses a higher number of wavelengths (four or more) each one especially adapted to a restricted altitude range of the atmosphere.
One major step taken during the last years to validate the four-wavelength concept for a spaceborne DIAL was the realization of an airborne demonstrator. This instrument not only implements the basic concept but also is based on a laser technology, which is principally suited for in-space operation by using high efficiency solid state lasers and non-linear conversion techniques.
Measurements of the airborne demonstrator on board HALO during international field campaigns provide a better understanding of water vapour transport, mixing and transformation (cloud) processes relevant to weather and climate.