GCOS Essential Climate Variables (ECV)
Atmospheric Upper Air (over Land, Sea and Ice)
Wind Speed and Direction

Upper-Air Observation - (Also called sounding, upper-air sounding.) A measurement of atmospheric conditions aloft, above the effective range of a surface weather observation. This is a general term, but is usually applied to those observations that are used in the analysis of upper-air charts (as opposed to measurements of upper-atmospheric quantities primarily for research). Among the elements evaluated are pressure, temperature, relative humidity (e.g., by radiosonde aircraft observations), and wind speed and direction (e.g., by rawinsonde, aircraft, or wind profiling radars). Also, some mountain stations are high enough and exposed enough so that their observations may be included in the upper-air network at their elevation. See also meteorological rocket, radiosonde balloon. (AMS Glossary of Meteorology)

Introduction: Measurements of atmospheric winds are of primary importance to weather forecasting, and as a variable in the study of global climate change. Upper air wind speed and direction is a basic element of the climate system that influences many other variables. Horizontal wind may be inferred by motion vectors or by humidity and ozone tracers in geostationary imagery. Substantial information can be derived by these methods but quality control is difficult and vertical resolution is poor. Planned instruments for geostationary satellites promise improved information, but the limited vertical resolution and the problems of accurate height assignment of winds will remain areas to be improved.

Satellite Observations: For global NWP models, wind profile information – mostly over land – is available mainly from radiosondes. Satellite Doppler wind lidar technology is being developed to provide line-of-sight wind profiles of acceptable coverage and vertical resolution, but thick cloud is a limitation. Geostationary imagers offer wind profile information by cloud tracking, or through tracking of highly-resolved features in the water vapour channels in cloud-free areas. Coverage may be supplemented in future by tracking ozone features in satellite imagery. Regional NWP models also rely heavily on radiosondes (over land) and aircraft (over ocean and over the poles) for atmospheric wind profile measurements, but they would benefit from improved satellite data. At present, geostationary multi-channel visible and infrared imagers, such as INSAT, SEVIRI and VISSR, are used to measure cloud and water vapour motion vectors from which tropospheric wind estimates may be derived. Atmospheric motion vectors generated from the global ring of geostationary imagers provide improved data in terms of coverage, spatial and temporal resolution, and accuracy of both wind vectors and height assignment. Though valuable, because they offer wind information in areas of the world where otherwise there would be none, atmospheric wind vectors are single level observations which are only available where there are suitable image features to be tracked. Geostationary satellite measurements have been recently supplemented by the addition of water vapour wind motions from polar orbiters (MODIS). Plans need to be made to continue the polar orbiting wind measurements. In the longer term, laser instruments such as Doppler lidars offer the promise of directly measuring clear air winds and winds within optically thin aerosol and cloud layers. Although such active instruments will provide a global coverage of vertically resolved, highly accurate measurements, the coverage offered by polar missions, such as that planned for the research-oriented ALADIN, is limited to measurements twice a day along the satellite line of sight. Hyperspectral observations are needed to improve the vertical resolution of atmospheric motion vectors derived from geostationary satellite observations – especially in clear areas. The first opportunity for these observations may be the IRS payload on EUMETSAT’s MTG-S-1 mission. CEOS identified two actions in response to the GCOS requirements: to commit to reprocessing the geostationary satellite data for use in reanalysis projects before the end of the decade; to identify options for continuing improvements to wind determinations demonstrated by MODIS and to be demonstrated with ALADIN on the ADM-Aeolus mission.(Satellite Missions) (from the CEOS web site)

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