Water Vapour in the Upper Troposphere and Stratosphere
Water vapor is the most important greenhouse gas in the Earth’s atmosphere. Due to the high sensitivity of atmospheric radiative forcing to changes in greenhouse gases in the cold tropopause region, even small, not yet fully understood, variations in water vapor in the lower stratosphere are an important source of the decadal variability of the surface temperature. This implies the need for highly accurate measurements of low water vapor concentrations in the UTS and a detailed understanding of observed variability and the underlying processes.
While climate models successfully simulate the effects of water vapour on the climate in the troposphere, the simulation of key processes that determine the water vapour distribution in the stratosphere are still inadequate. This applies, in particular, to the transport of water vapour from the tropical troposphere to the stratosphere and the accompanying drying processes. Even state-of-the-art models are thus incapable of describing the fluctuations of stratospheric water vapour observed during the last few decades. This is a critical issue since recent analyses (e.g. Solomon et al., Science, 2010 ) emphasize that changes in atmospheric water vapour are an important trigger of decadal changes in surface climate.
Measurements carried out by IEK-7 as part of campaigns in tropical zones of Australia (SCOUT-O3), Brazil (TROCCINOX) and Africa (AMMA) have shown that changes in the water vapour content in the stratosphere are closely related to changes of water vapour transport through the tropical tropopause layer . This raised the question of the paths along which air masses move from the troposphere into the stratosphere and of the processes governing the drying of humid tropospheric air before it enters the stratosphere. Water input into the stratosphere is the overall result of very complex interactions between deep convection (up to an altitude of approx. 15 km), a slow diabatic ascent through the tropical tropopause layer (TTL) and the movement of these air masses towards the ascending branch of the wave-driven Brewer-Dobson circulation. According to one widely accepted theory, the air masses travel very large horizontal distances (circling the earth several times) while slowly traversing the TTL (time scale of weeks) and are “freeze-dried” in the coldest regions they pass through before entering the stratosphere. Trajectory calculations made by IEK-7 agree well with this theory and show that the temperature in the coldest region an air parcel passes through is the most important factor determining the amount of water vapour it contains before it enters the stratosphere. The drying process itself is closely related to the formation of cold cirrus clouds.
While results of recently published studies suggest that these clouds form close to the TTL only if the water vapour saturation is very high (“supersaturation puzzle”), measurements of IEK-7 are in line with current theories. The results of these recent studies are therefore evidently due to the fact that the measuring instruments used were not accurate enough. IEK-7's results also show that, in contrast to the conclusions of several other studies, the role of overshooting convection is of secondary importance for water vapour transport into the stratosphere.
On the whole, the Lagrangian transport model (CLaMS) developed by IEK-7 has decisive advantages over Eulerian models, since the “temperature history” of an air parcel, which is important for the drying process, can be more easily tracked. At an altitude of more than ~16 km, modelling the influence of convection in great detail is apparently less important.
Apart from the processes described here, the oxidation of methane and molecular hydrogen in the stratosphere also contributes to the water vapour budget. IEK-7 has made a decisive contribution to quantifying this effect, primarily by analysing a long time series of balloon measurements that is unique in the world. These measurements started in Jülich in 1978 and are being continued today in cooperation with Frankfurt University . Furthermore, IEK-7 also conducts research into the potential impact of a future hydrogen economy on the development of stratospheric water vapour as well as possible effects on the ozone layer and climate [4,5].
Persistent discrepancies between individual hygrometers were found during an intensive laboratory study, AquaVit-1 , and it proved possible to narrow this down during the intercomparison aircraft campaign MACPEX by NASA in 2011, where IEK-7 participated as the only non-US group by contributing the FISH instrument . For the same purpose, we reviewed two decades of FISH water vapor measurements  and found discrepancies of less than 10 % between FISH and other hygrometers. Thus, FISH is established as one of the core instruments worldwide for measuring water vapor in the UTLS. Also, other in situ hygrometers in the framework of IAGOS or the SCIAMACHY limb sounder satellite used FISH as the reference instrument for their comparisons [9,10,11].
An intercomparison of FISH water vapor with reanalysis and operational analysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF)  shows a tendency of the reanalysis data to overestimate water vapor values in the lower stratosphere (values below 1.0 ppmv) and to underestimate high water vapor mixing ratios in the upper troposphere.
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