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GCOS Atmospheric Surface ECV * *over land, sea and ice Definition: Water Vapor - (Also called aqueous vapor, moisture.) Water substance in vapor form; one of the most important of all constituents of the atmosphere. Its amount varies widely in space and time due to the great variety of both “sources” of evaporation and “sinks” of condensation that provide active motivation to the hydrologic cycle. Approximately half of all of the atmospheric water vapor is found below 2-km altitude, and only a minute fraction of the total occurs above the tropopause. Water vapor is important not only as the raw material for cloud and rain and snow, but also as a vehicle for the transport of energy (latent heat) and as a regulator of planetary temperatures through absorption and emission of radiation, most significantly in the thermal infrared (the greenhouse effect). The amount of water vapor present in a given air sample may be measured in a number of different ways, involving such concepts as absolute humidity, mixing ratio, dewpoint, relative humidity, specific humidity, and vapor pressure. (AMS Glossary of Meteorology) Introduction: Water vapour is a key gas in the atmosphere since it is both radiatively and chemically active. It is the strongest of the greenhouse gases (GHGs) on the planet, though largely influenced indirectly rather than directly by anthropogenic activity. In the upper troposphere and lower stratosphere, it is a key indicator of convection and radiative forcing. In the stratosphere, water vapour is a source gas for OH which is chemically active in the ozone budget. There is recent evidence that the Brewer Dobson circulation is changing in the Tropics due to climate change, which alters the balance of water vapour in the Upper Troposphere (UT) and Lower Stratosphere (LS) markedly and has a strong feedback on climate change. Calibration of the data from the various satellite sensors is a very important issue, and for this the implementation of the GRUAN will provide essential data. International Data Centers and Archives for Atmospheric Upper-Air:
Coordinating Bodies:
Satellite Observations: The observations for water vapour (atmospheric humidity) are a core requirement for weather forecasting and are largely dealt with in the framework of the Coordinating Group for Meteorological Satellites (CGMS). A wide range of sensors is available to measure column water vapour – microwave imagers like SSM/I and traditional imagers like AVHRR or MERIS on LEO platforms, and GOES and SEVIRI on GEO platforms. Vertical profiles are provided by microwave sounders like SSM/T2, AMSU-B, HIRS/4 and MHS, by hyperspectral infrared sounders like IASI and AIRS, or by radio-occultation observations provided by GPSMET or GRASS on MetOp. These data are supplemented by instruments on Aqua (AIRS+, AMSR-E, AMSU-A), Aura (HiRDLS, MLS, TES), and the FY-3 series (MWHS), amongst others.
All of these are being improved as technology allows. In broad terms the challenges are to improve vertical resolution of observations and temporal sampling, to overcome cloud problems and improve the ability to process sounding data over land. For instance, NPOESS will feature the combination of the CrIS interferometer and ATMS sounder to derive accurate water vapour profiles.
The 3-dimensional field of humidity is a key variable for global and regional weather prediction (NWP) models that are used to produce short- and medium-range forecasts of the state of the troposphere and lower stratosphere. Polar satellites provide information on tropospheric humidity with global coverage, good horizontal resolution and acceptable accuracy, but with poor vertical resolution.
In the case of observations for regional NWP models, polar and (mainly) geostationary satellites provide estimates of total column water vapour accurate to within 10–20%. Enough information is collected to infer moisture concentration within several thick layers vertically, with good horizontal resolution. Vertical resolution is marginal for mesoscale prediction, and the infrared information is available only for cloud-free fields of view. Despite this coarse vertical resolution, the high temporal resolution of the geostationary satellite observations allows derivation of products like the instability index for convective initiation, which is used for nowcasting applications.
Until recently, performance in cloudy areas was poor, but the new microwave measurements from AMSU offer substantial improvements. Geostationary infrared soundings (e.g. by the GOES sounders and SEVIRI on MSG) are also helping to expand coverage in some regions by making measurements on repeat timescales of fifteen minutes to one hour, thus creating more cloud-free observations.
Over oceans, coverage is currently supplemented by information on total column water vapour from microwave imagers.
Satellite sounding data are difficult to use over land, but progress in data interpretation is expected in the near future. Recent research has shown that the GPS-based radio occultation (RO) technique also has the potential to provide, in the middle to lower troposphere, high resolution profiles of atmospheric refractivity, combining the effects of temperature and water vapour in this region of the atmosphere.
In response to the GCOS IP, CEOS undertook to ensure continuity by 2011 of GPS RO measurements with, at a minimum, the spatial and temporal coverage established by COSMIC. CEOS will also continue efforts to exploit the complementary aspects of radiometric and geometric determinations of temperature and moisture in the upper air.(Satellite Missions) (Source: CEOS) Data, Metadata, Products and Information Access
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