Ozone and water vapour in the tropopause region was a joint project between the Physics Department at the University of Wales, Aberystwyth and the Department of Chemistry at the University of Cambridge.

Background and objectives

Air at the base of the stratosphere, within 1-2 km of the tropopause, was considered intermediate in character between stratosphere and troposphere. The ozone concentration increases steeply with height from ~80 ppbv at the tropopause to several hundred ppbv 2 km above it. The water vapour concentration was more variable, but generally decreases from ~100 ppmv at the tropopause to the standard 5-6 ppmv in the same height region. Other tracers of tropospheric origin behave likewise, which means that the lowest 2 km of the stratosphere is of quite a different chemical character to the remainder of the stratosphere.

Water vapour was a key molecule in the UTLS region, and one that has traditionally been poorly measured above the tropopause. Even though the newest generation of radiosonde at that time (e.g. the Vaisala RS90) performed much better than its predecessors in the upper troposphere it still did not measure adequately in the stratosphere. The MOZAIC humidity sensor was of this type, and was also unable to extend into the stratosphere; indeed, it cannot measure reliably below 100 ppmv in the upper troposphere. Satellite instruments extend water vapour profiles into the UTLS region but their limited resolution in a region of strong vertical gradients limits their value. Measurements of UTLS humidity have therefore relied on in-situ research instrumentation, either balloon-borne or aircraft-borne. Aircraft and large balloons are expensive and cannot provide a proper climatology for water vapour, and the long sequence of NOAA frost-point hygrometer profiles do so only in one location (Boulder). This project seeked to develop a small, relatively cheap package to measure water vapour and ozone in the region around the tropopause.

Specific objectives of this project

The hygrometers to be used in this project are:

Commercially-available frost point hygrometer (Snow White) developed by Meteorlabor, Switzerland. This device offered the possibility of water vapour measurements above the tropopause. It can be flown together with an ozonesonde and a radiosonde on a standard ozonesonde package, and was sufficiently cheap to be regarded as a throwaway device.
A surface-acoustic-wave sensor developed by Cambridge University, which was capable of greater sensitivity and accuracy than conventional frost-point hygrometers. This method is described briefly below. Flying this device with the Snow White instrument tested the capabilities of each instrument as well as providing a unique geophysical data set. Cambridge are have developed this instrument to measure methane as well as water vapour and flights with the methane instrument will be conducted towards the end of the project.
Methodology

The measurement phase of the project consisted of four month-long campaigns, for June, September and December 2001 and March 2002. An average of three flights a week were conducted during these periods - a total of 48 flights. These flights were targeted as far as possible, at different air masses in the lower stratosphere over Aberystwyth, with a particular emphasis on north-westerly jet streams. Forecast charts have been used to identify suitable conditions (350 K potential vorticity forecasts from ECMWF were available from NILU, Norway for 1 and 2 days ahead).