S P A R C Stratospheric Processes And their Role in Climate A project of the World Climate Research Programme Home Initiatives Organisation Publications Meetings Acronyms and Abbreviations Useful Links

SPARC INITIATIVES : Water Vapour in the Upper Troposphere and Stratosphere

Chair: D. Kley (Germany), e-mail: d.kley@fz-juelich.de
Co-Chair: J.M. Russell III (USA) e-mail: james.russell@hamptonu.edu

Water vapour in the lower stratosphere and upper troposphere is critically important for overall atmospheric radiative transfer, but, at present, knowledge of the mean distribution of water vapour, variability on seasonal and other timescales, and the controlling processes is limited.

This SPARC initiative aimed at improved monitoring of water vapour in the stratosphere and a development of a water vapour climatology for the lower stratosphere and upper troposphere. A review of the state of knowledge on the distribution of water vapour in the stratosphere and upper stratosphere before the Assessment started is available in the SPARC Implementation Plan.

In August 1998 during the Workshop held in Boulder, USA, it has decided to make an Assessment of our knowledge. The work is now finished and published as a SPARC Report N°2, available from the SPARC Office, and on the SPARC Web site. The results can also be found in several publications:
S. Oltmans et al., GRL, 2000 ; K. Rosenlof et al., Science, 2001 (see Publications below).

SPARC Assessment of Upper Tropospheric and Stratospheric Water Vapour : WAVAS
(Assessment co-chairs : Dieter Kley and Jim Russel) (to receive a copy, please contact the SPARC Office)

The SPARC Water Vapour Assessment, WAVAS, address the issue of the concentration, distribution and variability (including trends) of water vapour in the upper-troposphere (UT) and lower-stratosphere (LS). The main question, clearly related to the role of water vapour in the greenhouse effect, is to determine whether the UT water vapour concentration is under thermodynamic or under dynamic control, and to understand what controls the LS water vapour concentration.

The assessment consists of three main chapters:

1 - Instrumentation and data sets (Profiling instruments, Group-based aircraft sensors, Satellite sensors)
2 - Data quality (Evaluation and Comparison)
3 - Distribution and variability.

The main findings are summarized below:

• A significant increase in the number and quality of stratospheric water vapour measurements has occurred over the past 25 years, particularly with the advent of satellite observations. Stated accuracy of most in situ and remote instruments as well as direct or indirect comparisons of coincident field measurements cluster within a ±10% range.
• The concentration of stratospheric water vapour in the "overworld" (Q ? ~380 K) is determined by dry air upwelling through the tropical tropopause, methane oxidation in the stratosphere, and transport by the poleward-and-downward (Brewer-Dobson) mean circulation. At the tropical tropopause, air transported into the stratosphere is dried by a complex combination of processes that act on a variety of spatial and temporal scales. Water vapour in the upper troposphere is controlled by local and regional circulation patterns and seasonal changes of upper tropospheric temperature.
• There has been a 2 ppmv increase of stratospheric water vapour since the middle 1950s. This is substantial given typical current stratospheric values of 4-6 ppmv. Photochemical oxidation of methane in the stratosphere produces approximately two molecules of water vapour per molecule of methane. The increase in the concentration of tropospheric methane since the 1950s (0.55 ppmv) is responsible for at most one half of the increase in stratospheric water vapour over this time period. It is not clear what is responsible for the remainder of the observed increase in stratospheric water vapour.
• Upper tropospheric relative humidity (UTH) has been monitored for about 20 years by instruments on operational satellites. In the upper troposphere, no major inconsistencies were found between existing satellite-based measurements that would preclude their use in describing the long-term behaviour of upper tropospheric humidity. The data are also of sufficient quality for climatological and process studies.
• Assessing long-term changes in the UTH is difficult because of high variability during El Niño - Southern Oscillation events, other natural modes of variability in the large-scale circulation, and the competing effects of changes in water vapour concentration and temperature. Although both positive and negative statistically significant long-term changes can be found in different latitudinal bands, no striking global trend emerges from preliminary analyses.
• The operational radiosonde network does not produce water vapour data that can be used for either analyses of long-term change, process studies in the upper troposphere, or for validation of UTH measurements. However, emerging data sets from improved quality, quasi-operational aircraft and ground-based instrumentation show promise and should be used more extensively for process studies, climate analyses and validation of satellite data.

Workshops

• Upper-Troposphere/Lower-Stratosphere Water Vapour, NCAR, Boulder, Colorado, USA, 26-28 August 1998.
• WAVAS Chapter 1 Workshop, Summer/fall 1999, Paris, France

Publications

Tropospheric Relative Humidity
D. Kley et al., SPARC Newsletter 11, July 1998.

Plan for the SPARC Water Vapour Assessment,
D. Kley and J.M. Russell, SPARC Newsletter 13, July 1999.

The increase in stratospheric water vapor from balloon-borne, frostpoint hygrometer measurements at Washington, D.C. and Boulder, Colorado
S. Oltmans, H. Vömel, D. Hofmann, K. Rosenlof and D. Kley, G.R.L., 27 (21) 3453-6, 2000.

SPARC Report N°2 : SPARC Assessment of Upper Tropospheric and Stratospheric Water Vapour,
WCRP N° 113, WMO/TD-N° 1043 (December 2000) - Summary . (to receive a copy, please contact the SPARC Office)

Stratospheric water vapor increases over the past half-century
Rosenlof K., S. Oltmans, D. Kley, J.M. Russell III, et al., G.R.L., 28 (7), 1195-1198, 2001