GCOS Atmospheric Upper-Air ECV - Water Vapor

Table of Content:

 Latest update 10/28/2013

Main Climate application
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 (UT) and Lower Stratosphere (LS), 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 evidence that the ascending branch of the Brewer Dobson Circulation is changing due to climate change, which controls the balance of water vapour in the UT/LS.

Upper air humidity and related quantities such as precipitable water in layers must be measured accurately to validate models of hydrologic processes, to calibrate satellite and other remote sensing water vapour retrieval methods, to determine the radiative forcing due to water vapour and the nature of the water vapour feedback as greenhouse gases increase, and to increase knowledge of atmospheric chemistry processes in the ozone layer.

Contributing baseline GCOS observations
The Global Upper-Air Network (GUAN) of about 170 stations provides baseline upper-air humidity measurements (Cf. Upper-air temperature).

Other contributing observations
The global upper-air network includes several hundred land-based radiosonde stations in addition to those in GUAN, ships, dropsondes, and aircraft observations. Almost all soundings and many aircraft observations report both temperature and humidity measurements. Large areas have few or no radiosonde or other in-situ observations.

Broad-scale information on tropospheric water vapour is routinely provided by operational passive microwave, infrared and UV/VIS satellite instruments. Infrared instruments have more recently been enhanced by high spectral resolution infrared sounders. UV/VIS instruments provide additional information for total column water vapour.

Data from GPS receivers are used operationally to observe continuous total column water vapour (via atmospheric refractivity), and the data should be freely exchanged for climate purposes.

The capability to observe continuous total column water vapour data from ground-based GPS receivers is well established and these data are exchanged and used operationally in Numerical Weather Prediction (NWP) centres. The network of GPS receivers should be extended across all land areas to provide global coverage.

Global high vertical resolution measurements of H2O in the UT/LS by limb observations are also essential. The required limb sounding also yields invaluable information on ozone and other chemical composition variables.

Current capability
Uncertainties in the measurement of humidity are much larger than in the measurement of temperature. Differences in the average reported humidity between instrument types can exceed 10% near the surface, with newer instrument types usually showing drier readings than older instrument types. The largest disparities are in very moist and very dry conditions.

Data assimilation can be used to improve the consistency of water vapour, cloud and precipitation estimates.

GCOS Reference Upper-Air Network
Outstanding issues concerning the quality of radiosonde measurements for climate monitoring and change-detection purposes have led to a proposal for a GCOS Reference Upper Air Network (GRUAN) of eventually 30-40 sites routinely deploying high-quality radiosondes and providing other ground-based measurements such as from ozone sondes, GPS delay and lidars. In addition to making a vital direct contribution to climate monitoring across all climate zones, this network will be extensively used to calibrate and validate various satellite observations and is expected also to provide new information on humidity in the upper troposphere and lower stratosphere region needed to understand better the role of water vapour in the radiation budget. More information can be found on the GRUAN website: www.gruan.org (external link)

Data Access