The MISU Atmospheric Physics Group

HYGROSONDE  I Rocket-borne Water Vapour Measurements in the Middle Atmosphere

Objectives

In the middle atmosphere (15-100 km), water vapour is both a major input species for chemistry and a convenient tracer for transport processes. Knowledge of the water distribution is of particular interest e.g. for investigations of ozone and related species. However, accurate and spatially resolved measurements of the water vapour abundance are difficult. The only way of performing local measurements in the upper stratosphere and meososphere is to use sounding rockets.

An optical hygrometer for nighttime use on rocket payloads has been built at the Atmospheric Physics group at Stockholm University. In cooperation with Swedish Space Corporation (SSC) , the first flight of this instrument was carried out in December 1994. A second Hygrosonde flight is planned for Winter 2000/2001 in connection with the atmospheric mission of the Odin satellite.

We apply optical hygrometers of the same type also on balloons. Regular flights dedicated to stratospheric ozone chemistry are performed within the SKERRIES project in cooperation with the Swedish Institute for Space Physics.

The OH Fluorescence Technique

The Hygrosonde measurements are based on an active opical technique: OH fluorescence hygrometry. Light from a Lyman-alpha lamp is used to dissociate water vapour:

H2O + hv --> OH* + H          (121.6 nm)

OH* is produced in the excited A(2Sigma+) state. Subsequent relaxation to the ground state causes emission of a broad rotational spectrum in the near ultraviolet:

    OH* --> OH + hv'       (280-320 nm)

This radiation is detected as a direct measure of the atmospheric water abundance. Absolute calibrations of the hygrometer are obtained by laboratory measurements. At atmospheric pressures below 60 km, the OH* emission spectrum is altered by collisional processes such as quenching or rotational energy transfer. These processes are investigated by means of experimental and theoretical studies on OH fluorescence spectroscopy.

The Aerodynamic Concept

In situ measurements of water vapour from rocket payloads are extremely difficult. Outgassing and desorption can give rise to water abundances in the vicinity of the vehicle that grossly exceed the unperturbed atmospheric concentration. To avoid this contamination problem, Hygrosonde combined the active optical measurement technique with a sophisticated aerodynamic concept.

The basic idea is to perform the measurement in a volume which has not been influenced by the payload. The measurement volume is defined by the overlap of the lamp emission and the detction field of view. With the instrument positioned at the top of the rocket payload, the optics is designed to guarantee a measurement completely outside the rocket shock front. For supersonic flow and continuum flow conditions, this region can be regarded as unperturbed by the payload.

Prior to the first flight, this concept was tested by means of wind tunnel experiments in cooperation with the Aeronautical Research Institute of Sweden (FFA). Later, comprehensive model simulations and wind tunnel measurements studied Hygrosonde even under the conditions of rarefied aerodynamics which prevail in the upper mesosphere.

The First Flight

Hygrosonde was launched from Esrange, Sweden, during the night of December 4/5, 1994. The rocket flight investigated very interesting dynamic conditions as the measurements took place close to the edge of the arctic polar vortex.

In the stratosphere, an increase of the water mixing ratio was observed which is consistent with methane oxidation as middle atmospheric source of water vapour. In the mesosphere, strong vertical variations are found over height intervals of 2-4 km, consistent on the ascent and descent part of the flight.

In the vicinity of the polar vortex, these structures are attributed to the passage of the rocket through layers with air of different origin, i.e. dry air from inside the vortex and moist air from outside. Effects of such planetary-scale waves on the distribution of long-lived trace gases have been discussed by Bacmeister et al. [1995]. In the presence of 'eddy transport', the observed stratification causes a strongly enhanced vertical exchange of chemically important species.

Here are some pictures from the Hygrosonde campaign 1994 . . .

Perspectives

Hygrosonde presents a compact instrument for use on small rocket payloads. It can provide nighttime water vapour measurements throughout the stratosphere and mesosphere. After the first flight in 1994, the instrument has been improved further. The sensitivity has been enhanced e.g. by the development of a bright light source with 6 capillaries in cooperation with the State Optical Institute in St. Petersburg, Russia.

As the distribution of water vapour is a critical parameter for investigations of both chemistry and dynamics, Hygrosonde provides a valuable tool in connection with comprehensive studies of the middle atmosphere. Another important application is the validation of satellite-borne and ground-based remote sensing techniques.

A Hygrosonde launch is planned as a complement and a validation of the sub-mm water vapour measurements by the Odin satellite.

For further information, contact Mikhail Khaplanov (misha@misu.su.se) or Jörg Gumbel (gumbel@misu.su.se).

Some related references are

• Khaplanov et al., Hygrosonde - a direct measurement of water vapour in the stratosphere and mesosphere, Geophys. Res. Lett., 23, 1645, 1996.
• Gumbel et al., Fluorescence and collisional energy transfer in OH (A), Report AP-34, Department of Meteorology, Stockholm University, 1997.
• Kley and Stone, Measurement of water vapour in the stratosphere by photodissociation with Ly alpha (1216 A) light, Rev. Sci. Instrum., 49, 691, 1978.
• Bacmeister et al., Descent of long-lived trace gases in the winter polar vortex, J. Geophys. Res., 100, 11669, 1995.