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Stratospheric Processes And their Role in Climate
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| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
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Stratospheric Processes And their Role in Climate
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| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
There are still many aspects of water vapour properties, distribution, and roles in the atmosphere and the climate system that are not well understood. On October 12-15, 1999 the American Geophysical Union convened a Chapman Conference on Water Vapour in the Climate System to review the present status of knowledge of the distribution of water vapour in the atmosphere and its role in atmospheric processes. About 100 scientists gathered at the Bolger Center in Potomac, MD, for in-depth discussions and exploration of new ideas and data. The center, on extensive grounds, provided an excellent venue for people to interact in and out of the sessions. A previous conference of the same title was held in 1994 (Elliott and Gaffen, 1995). It focused on tropospheric water vapour, and emphasised the difficulties in measuring water vapour with sufficient accuracy and resolution to allow observational tests of theory. Recent discussions, including some led by SPARC, have emphasised the importance of Upper Troposphere/Lower Stratosphere (UT/LS) water vapour. This second Chapman Conference was broadened to include discussion of this topic.
This review selectively (and subjectively) notes some of the highlights of interest to the SPARC audience, with emphasis on the UT/LS. More complete information, the program and abstracts may be found at http://www.agu.org/meetings/cc99bcall.html.
For all its prevalence and importance, water vapour is very hard to measure accurately, even in the lower atmosphere. The best current capacitive radiosondes were stated to be very good sensors if not contaminated, but they have a dry bias at low levels, and are too moist at low humidities, where a temperature-dependent correction factor may reach a factor of 2 at -70°C. Measurement programs by the US. Atmospheric Radiation Measurement (ARM) program have sought to use the advanced instrumentation of ARM to quantify the water vapour measurement uncertainty of the sondes, and find ways of improving the accuracy, with a goal of order 2%. At present, using sondes, ground based microwave, GPS, Raman lidar normalisation and solar radiometer comparisons, the full range of differences is still at least 12%.
The discussion of measurement capabilities and difficulties in the UT/LS at the conference paid considerable attention to the work done within the SPARC Water Vapour Assessment (WAVAS) activity (cf. SPARC Newsletter 13). The wide variety of sensors that can operate in the UT/LS were summarised; no single technique or instrument covers all latitudes. Individual instruments reach an accuracy on the order of 5%, although intercomparisons between different instrumental techniques showed even larger discrepancies. It was concluded that no technique available today has proven to be an absolute standard, and future calibration and characterisation will be required for high-quality measurements. Inexpensive balloon-borne frost-point hygrometer were suggested as a way to overcome some of these difficulties.
Results from the MOZAIC program, in which capacitive sensors are flown on commercial aircraft to obtain measurements over long tracks in the UT/LS were presented, with emphasis on the need for frequent calibration of the sensors in this demanding regime. Another aircraft measurement program noted the need for large corrections when operating near the speed of sound, and mentioned plans to develop a tunable diode sensor to alleviate some of these problems.
Satellite data are characterised by a variety of different spectral bands and viewing geometries, but all in general have good coverage and precision, making them suitable for studies of relative variations. WAVAS is using different techniques to make intercomparisons among them, allowing the errors to be estimated. The quality of nine data sets being evaluated will be described in the WAVAS final report. The utility of the high vertical and horizontal resolution of a future satellite instrument was also illustrated. Recent progress in determining the total column of water vapour under all weather conditions from GPS signals, and from GOME data under clear skies, was also described. Airborne DIAL systems have been successful in seeing vertical distributions and small horizontal scale variations in the UT/LS.
Several presentations dealt with the role of water vapour in radiative transfer in the Earth's atmosphere. For studies of radiative balance or remote sensing, a spectral perspective was felt to be essential in disclosing processes, as well as for calculation purposes. One theme was the importance of the pure rotation spectrum of water vapour (wavelengths longer than about 20 micrometers), which dominates cooling in the UT. The SHEBA experiment in the cold, dry Arctic has provided unique information on these features, and the continuum created by broadening of the spectral lines of water by other kinds of molecules (foreign broadening), which is inextricably linked with the lines in the band. Uncertainty in this quantity results in an uncertainty in UT cooling of order 0.2K/day. (By contrast, the continuum created by water molecules (self-broadening) dominates transfer between bands (e.g. in the 10-micrometer window region), which is effective near the surface.) Cavity ringdown spectroscopy was described as a new approach to measure continuum absorption.
Water vapour has many roles in atmospheric chemistry. It is the major source of the hydroxyl radical (OH), which is the major oxidising agent and cleanser in the atmosphere. Additionally, ice provides absorbent sites for polar molecules, and a matrix for chemical reactions, while liquid water provides sites for aqueous chemistry. Finally, liquid water is a solvent, resulting in wet removal processes and acid rain. In the stratosphere water is created by the oxidation of methane and is responsible for ozone destruction in the upper stratosphere, while water vapour chemistry is responsible for processes that cool the mesopause by 9K/day. These strong links between the hydrological cycle and atmospheric chemistry mean that future climate changes are tightly linked with atmospheric chemistry.
Fifty years ago Brewer used airborne measurements of water vapour in the UT/LS to infer that the Lagrangian mean transport in the stratosphere, later termed the Brewer-Dobson circulation, consists of rising motion through the tropical tropopause, poleward drift, and sinking at high latitudes. Recent measurements, which have again illustrated the importance of water vapour as a tracer of atmospheric motions, have added detail to this picture. Reviews pointed out that measurements by HALOE, MLS, and aircraft have illustrated the upward propagation of the effects of the seasonal cycle in tropical tropopause temperatures on water vapour values in the tropical stratosphere, the so-called "tape recorder" effect. The reduction of this signal up to ~30hPa indicates mixing with extra-tropical air, while from the relatively unattenuated signal above this level one can deduce an isolated "tropical pipe" in the tropical stratosphere, and also estimate mean upwelling rates that have been used to confirm estimates from radiative calculations.
Trends in UT/LS water were a subject of much discussion. Analysis of roughly 40 years of LS Northern mid-latitude data from numerous in situ and more recently satellite sources indicated that there has been an increase of ~0.05ppmv/year since the late 1950's. This is larger than can be explained by the measured increase in methane or measured changes in tropical tropopause temperatures, and may indicate a change in the circulation.
Speakers pointed out that the vertical resolution of current climate models is not adequate to resolve the important processes controlling UT water vapour and temperature, or changes in tropopause height. Water vapour is a particularly challenging problem because it decreases by 4 orders of magnitude between the surface and tropical tropopause, while current models typically use less than 15 tropospheric layers. In response to CO2 increases, all current climate models predict a larger temperature increase in the upper tropical troposphere than at the surface, and an increase in UT specific humidity of ~40%. The stratospheric temperature decreases in response to increased radiative cooling. Much greater vertical resolution is needed to have confidence in model predictions of UT/LS water vapour.
Several speakers discussed the injection of water into the stratosphere through the tropical tropopause. One proposed mechanism suggested that convective overshooting inserted water vapour and cirrus into a tropopause layer, suggested to lie between roughly 150-50hPa. Entry into the stratosphere occurs away from the overshooting region, where radiative heating leads to upward motions greater than those of the Brewer-Dobson circulation. However, an attempt to model this process, including detailed cloud microphysics, indicated that the process is more complicated than suggested. Cloud crystal growth and sedimentation can short circuit the apparent water vapour transport, while the vertical pumping of water vapour depends very strongly on the horizontal scale of the cloud. Another participant concluded that the water vapour depends sensitively on cloud microphysics that present GCMs neglect. However, he suggested that this sensitivity was probably masked by inadequate vertical resolution. Clouds appear to be a source of moistening in the UT in the tropics. A 3D model study indicated downward motion in the stratosphere over Indonesia, inconsistent with a fountain in this region.
New measurements are also providing more insight into distributions and processes in the extratropical middleworld, the UT and stratosphere. Four years of data from the MOZAIC program over the Atlantic between Europe and North America at altitudes of 9-12km found about 30% of the cases where the air was supersaturated by as much as 150% with respect to ice. (This can have a significant impact on the formation and lifetime of aircraft contrails in this altitude region.) They also showed strong seasonal variations, with higher specific humidity in the summer than the winter, but with relative humidities about 10% lower in the summer than the winter, and about 30% lower in the subtropics than in mid-latitudes. These changes are associated with the seasonal march of the subtropical and polar jet streams, where strong latitudinal boundaries occurred. Measurements obtained by the RB57 high altitude research aircraft flying along the tropopause also saw such boundaries, with highly variable regions on the poleward edge of the subtropical jet, consistent with recent mixing of air into the extratropical and providing insights into the dynamics of STE in this region. A related study using MLS retrievals of water vapour at 215hPa found strong evidence for mixing by Rossby wave breaking in the North Atlantic region, although this effect was weak in the central and eastern Pacific region. Finally, a study using NVAP data, based on nadir viewing operational satellites, indicated a dependence of UT water vapour on SST anomalies south of the jet, where deep convection and meridional fluxes dominate. North of the jet stronger upper level cyclonic flow dries the UT when the baroclinicity of the storm tracks is enhanced.
GEWEX (The Global Energy and Water Cycle Experiment), a sister program to SPARC under the World Climate Research Program, has as one of its components a subprogram called the Global Water Vapour Program (GVaP). Taking advantage of the conference, the US GEWEX Panel and the GVaP Science Working Group held a "town meeting" to discuss the GVaP Science and Implementation Plan. SPARC has been encouraged to work closely with GVaP.
Convenors for the Conference were Dian Gaffen (NOAA), John Gille (NCAR & U. of Colorado, and Becky Ross (NOAA). The conference was sponsored by the American Meteorological Organization, EUMETSAT, NASA, NOAA, NSF, and SPARC.
Elliott, W.P, and D.J. Gaffen, 1995: Chapman conference probes water vapour in the climate system, EOS, 76,67.