Water vapour is a chemically, physically, and radiatively active trace gas, and its distribution in the stratosphere determines significant climatic implications. The currently discussed increase in stratospheric water vapour is believed to contribute to the recent downward trend in stratospheric temperatures, and the changing H₂O concentration modifies the stratospheric radiative balance. Furthermore, stratospheric humidity has an influence on ozone depletion by affecting both the formation temperature of polar stratospheric clouds (PSCs) and the polar vortex temperatures themselves.

Measurements

To monitor the water vapour content in the Arctic polar vortex, balloon-borne measurements are performed at the AWIPEV base in Ny-Ålesund during the winter months. The Russian Lyman-  hygrometer "FLASH-B" measures the water vapor mixing ratio with an uncertainty of  8% and a vertical resolution of about 150 m in stratospheric altitudes. To avoid contamination effects caused by tropospheric remnants in the plume of the balloon, only the descent data are taken into account. Together with similar activities by cooperation partners in Finland, these Arctic H₂O measurements are part of the EU project SCOUT-O3.

Example 1: Temporary Dehydration

Overall, the distribution of stratospheric water vapour is determined by the interaction of radiation, chemistry, and dynamics. Considering the sources, water vapour enters the stratosphere through vertical transport in the (tropical) tropopause region and is photochemically produced in the upper stratosphere through the oxidation of methane. The only sink of water vapour in the upper atmosphere is through photolysis by Lyman-α  with its efficiency increasing with altitude in the mesosphere.

Yet, in polar winter conditions a minor loss process may occur in the lower stratosphere. With very cold stratospheric temperatures, ice particles of polar stratospheric cloud (PSC) type II may grow to sizes that are effected by gravitation, and fall out like snow in the troposphere. In this case, water vapour is transported downwards to a warmer stratospheric layer where the ice particles evaporate. As this process of "dehydration" and "rehydration" is linked to very cold stratospheric temperatures and the occurrence of polar stratospheric ice clouds, it is regularly observed in the Antarctic and to a much lesser extent in the Arctic.

Example 2: Mixing Effects at the Polar Vortex Edge

The overall hemispheric water vapour distribution arises from the superimposed general circulation that accumulates trace gases by descent within the polar vortex. While in the midlatitudes rather constant values of about 4 to 5 ppmv H2O are found in the lower stratosphere, the downward transport within the polar vortex yields higher water vapour mixing ratios from the source regions. Consequently, a horizontal gradient across the vortex edge arises in the lower stratosphere with higher mixing ratios inside than outside of the vortex as far as dehydration by sedimenting PSC particles can be excluded.