14 2 WATERVAPOUR IN THEATMOSPHERE
overlap (NRL6and NOAA7/ESRL8GMD9frost point hygrometer,cf. sec-tion 3.1.4), so the observations must be handled cautiously with respect to changes on decadal time scales. SCHERER et al. (2008) present trend estimates from a reanalysis of the 1980–2000 data, which are 10−40%
lower than previously reported, the correction being largest at highest alti-tudes. Around 2000/2001, several observations indicate a pronounced drop in stratospheric water vapour, which is consistent with an unusually cold anomaly in TTL temperature (RANDELet al.,2006).
The reasons for the observed trend in stratospheric water vapour are all but well established. Increased methane levels may account for a third of the 1% yr−1 trend. A variety of mechanisms are considered to completely account for the observations, e.g. increased SO2levels from anthropogenic emissions, which result in formation of smaller ice crystals that are more readily lifted into the stratosphere (NOTHOLTet al.,2005). It is likely that different mechanisms are at play at different altitudes (IPCC,2007).
If the observed trend were due to a rise in[H2O]e, temperatures around the tropical tropopause should have increased during 1960–2000. Observations however indicate a slight cooling, adding further obscurity to the matter. On the other hand, the trend data, having been won from mid-latitude measure-ments, so far do not allow inferring quantitative constraints on a possible change in [H2O]e neither. The combined uncertainties in the observations and in the wind and temperature data are too large (SCHERERet al.,2008).
2.4 EFFECTS ONCLIMATE 15 reactive OH radical which is directly involved in catalytic ozone
destruc-tion, prevalently at 30−40 km. Catalytic schemes essentially speed up the reactions that destroy odd oxygen in CHAPMAN’s original scheme, like
X+O3 −→ XO+O2 (2.2a)
XO+O −→ X+O2 (2.2b)
Net O+O3 −→ 2 O2
A more detailed description of the reactions involved is given for example by WAYNE(2000). Additionally, increased levels of stratospheric water vapour indirectly contribute to ozone destruction by easier formation of polar strato-spheric clouds (HINTSAet al.,1999). This is both through better availability of water and through lower temperatures, caused by water vapour radiative cooling.
Climate effects also stretch across atmospheric layers, and include the oceans:ROSENLOFand REID(2008) report on lower stratospheric temper-atures above the western Pacific being significantly anti-correlated with sea surface temperature (SST) of the underlying ocean. The connection is al-most simultaneous and is present on the scale of individual monthly anoma-lies. They suggest that this connection could be moderated by intensified deep convection in the troposphere, as introduced by higher SST. Modifi-cations in cloud cover, and consequently in outgoing longwave radiation, would be another possible explanation. The data correspond well with the significant drop in stratospheric water vapour found around 2000/2001.
There is indication from modelled scenarios that the global meridional circulation will accelerate in response to global warming. Changes in strato-spheric dynamics in turn affect the propagation of atmostrato-spheric waves. The stratosphere thus exerts a feedback to ground weather and climate, in par-ticular at higher latitudes. For example, cold anomalies in northern hemi-spheric winter exhibit some correlation to the phase of the quasi-biennial oscillation (QBO). The exact mechanisms of such correlations remain to be established, but will form an important part of future climate modelling (BALDWINet al.,2007).
3 Water Vapour Observation Techniques
A large number of techniques exist to measure water vapour from various platforms,in situand by remote sensing. This is due to both the importance of water vapour in atmospheric processes and the experimental challenges that are involved in its observation. These intricacies have motivated the pur-suit of a comprehensive assessment of upper tropospheric and stratospheric water vapour observations at the turn of the century (KLEY et al.,2000).
Significant uncertainties remain, and have recently sparked an initiative to sum up new results over the past decade (SCHILLERet al.,2008).
The major focus are measurements in the upper troposphere/lower strato-sphere (UT/LS), because in this region, water vapour exerts large dynamic and radiative effects, and its transport into the stratosphere is determined.
Both are critical parameters in future climate modelling. The UT/LS also is a region of very sparse measurements, which is due to the challenging thermodynamic conditions there. In addition, even the most sophisticated scientific instrumentation may easily suffer from dry biases because of the difficulty to access areas of deep convection. Further demand for detailed and accurate measurements in the UT/LS has been created by the surpris-ing observation of pronounced supersaturations with respect to ice (PETER
et al.,2006). At the same time, the discrepancies between various sensors are so large that the interpretation of microphysical processes may change by choice of the observational data on which it is based (e.g.VÖMELet al., 2007a). For this reason, an extensive laboratory intercomparison effort has been carried out at the unique AIDA1facility of FZ Karlsruhe2, Germany, in 20073. The laboratory allows emulating all temperature, pressure and water vapour levels that are found up to the lower stratosphere. Campaign results
1 Aerosol Interactions and Dynamics in the Atmosphere 2 Forschungszentrum Karlsruhe
3 cf.http://imk-aida.fzk.de/campaigns/RH01/Water-Intercomparison-www.htm
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have not yet appeared in peer-reviewed literature, but WEINSTOCK et al.
(2008) indicate that the sensor discrepancies could not be reproduced in the laboratory set-up, so it must be further refined to match in-flight conditions.
The outlined issues limit our capacity to discern the parameters that con-trol water vapour entering to the stratosphere, which is required to model fu-ture trends in the stratospheric water vapour budget. Observations of strato-spheric water vapour are found to reasonably agree, the majority of sensors clustering within 10% (∼0.2−0.7 ppmv) of each other (KLEYet al.,2000).
Measurements are instead challenged by the smallness in stratospheric vari-ability, which in turn necessitates changes of the order of a few tenths of 1 ppmv to be resolved. Accurate long-term monitoring forms a key require-ment for our ability to detect and attribute trends in stratospheric water vapour abundance.
In light of these sobering remarks, this chapter aims to give an orientation about the various techniques for water vapour observation, and their individ-ual benefits and shortcomings. The discussion covers a representative por-tion of sensors for the individual methods, while mostly maintaining focus on stratospheric applications. Furthermore, with respect to remote sensing methods, it is mostly restricted to sensors in current operation for tropical stratospheric measurements.