Summary of the WALES mission
Insights into the distribution of water vapour, the dominant greenhouse gas, are crucial to understand the key elements in the global energy and water cycle, interaction with clouds, radiation and dynamics and transports in the Earth’s atmosphere. Major improvements are required in the global observation of water vapour, both for climate and numerical weather prediction applications.
Validation of key components of models and improved parameterisations in models (water vapour distribution, heating profiles, effects of convection, precipitation, chemistry) are essential for better climate as well as operational weather forecasts. Credibility of climate and weather forecasts as those initiated by the Intergovernmental Panel on Climate Change (IPCC) depend on improvement in measurability and better parameterisation.
The primary objective of WALES is to overcome the shortcomings of radiosondes and passive satellite sensors. The former do not cover the globe uniformly and do not provide reliable water vapour observations in the upper troposphere and lower stratosphere, nor at low humidity and low temperature levels (e.g. polar regions). The latter do not provide observations with sufficient vertical resolution and high accuracy and low bias.
The mission is proposed to employ an active lidar technique as a new and innovative component of the meteorological Global Observing System (GOS) and of the Global Climate Observing System (GCOS):
to directly sample the four-dimensional (spatial and temporal) variability of atmospheric water vapour over the entire globe.
to improve the ability to quantitatively measure water vapour with high vertical resolution, thus demonstrating the large benefit of the proposed active technique, and
- to establish an independent set of global water vapour profiles that will help answer questions about the relationships between changes in atmospheric forcing, radiation budgets and water vapour-cloud interactions, and to validate other measurement techniques such as passive infrared and microwave.
|Horizontal sampling||typ. 100 km|
|Vertical coverage||0 - 16 km|
|Vertical sampling||1 - 2 km|
|Dynamic range||0.01 - 15 g/kg|
|Systematic error||< 5 %|
|RMS error||< 20 %|
|Mission duration||> 2 years|
Mission Elements and Products
Single satellite carrying a differential absorption lidar (DIAL) operating in the 935 nm range, RMS error < 10%
Receiving stations providing near-real time capabilities for reception and processing; pre-processed data to be ingested into data assimilation systems
Global water vapour profiles with high vertical resolution and high accuracy. Each humidity estimate is accompanied by an estimate of the associated random error.
Backscatter profiles, cloud tops/bases, optical thicknesses, planetary boundary layer height and surface reflectance/albedo can be derived as 'spin-off' products.
|Total mass||~ 1500 kg|
|Total power||~ 1400 W|
|Data rate||~ 250 kbit/s|
|DIAL instrument||4 wavelengths|
|Orbit||sun-synchronous dawn-dusk (6/18 h)|
|Altitude||~ 450 km|
|Lifetime||2 (+1) years|
Last update: 19 October 2001