1.1 Introduction

Measurements of water vapour in the upper troposphere and in the stratosphere require tremendous technical effort due to the large gradients around the tropopause and the low stratospheric mixing ratios of a few ppmv in contrast to the moist tropospheric air masses. Further, in the stratosphere, the spatial and temporal variability of the H2O abundance is relatively small, i.e. changes of a few tenths of 1 ppmv need to be detected with a similar accuracy of the measurement.

No single existing instrument is capable of H2O measurements at all altitudes, with adequate global and temporal coverage. A combination of different instruments and techniques is necessary in order to meet the objectives of this Assessment. Owing to the importance of water vapour in the atmosphere, a large number of techniques for its measurement from different platforms have been developed and are used both for mechanistic case studies and the determination of its climatological distribution. Figure 1.1 shows a diagram of the altitude range and platforms for the most important techniques.

Figure 1.1 Typical vertical profile of water vapour in the upper troposphere and the stratosphere, the altitude range where the techniques described in this Assessment can be applied, and the carriers available for integration of the different instruments.

The purpose of this Chapter is to describe individual well characterised hygrometers for the upper troposphere and the stratosphere that produced data used in the following Chapters of this Assessment. Emphasis is put on the description of instrument quality parameters such as accuracy and precision as determined from laboratory and field studies, the description of the calibration procedures, and technical differences between the instruments. For a detailed description of the applied techniques of each instrument the reader is referred to the literature. It is not intended and not possible to give a complete review of all hygrometers used up to now, and no instruments that are still under development or which are not described in the peer reviewed literature are included. However, this summary of the most advanced instruments may provide an update and complement to previous reviews and summaries such as those of Wexler and Ruskin [1965] and Deepak et al. [1980].

Absolute calibration of hygrometers is crucial but very difficult, in particular at low mixing ratios of a few ppmv or frost points of -60° to -80°C as occur in the UT/LS. In this Chapter it is described to which reference the absolute calibration of each instrument is traceable. Primary standards for humidity measurements are gravimetric hygrometers and precision humidity generators, the latter based on saturating air with respect to water or ice at a given temperature. Reviews of these methods, which have been developed in the middle of this century and continuously improved, are given by Wexler and Wildhack [1965] and more recently by Sonntag [1994] and Wiederhold [1997]. Often, chilled mirror hygrometers are used as calibration standards since they are based on a fundamental hygrometric technique. Absorption cross sections of H2O and interfering trace gases in the vacuum ultraviolet (VUV) and infrared (IR) spectral region have been calculated or determined based on such techniques and are used for calibration of a large number of instruments. For some instruments, a combination of different calibration methods is applied. Further possible error sources such as contamination and sampling problems which affect the in situ hygrometers and retrieval algorithms for the remote sensing instruments are discussed.

Another focus of this Chapter is to list available data sets obtained with the described instruments. An overview of the data sets from aircraft, balloons and the ground since 1970 is given in Figures 1.2a and b for the tropopause region and for the stratosphere, respectively. Data obtained before 1980 have to be regarded as ‘historic’ since they are available from published reports only; however, some of these data sets have been re-evaluated and digitised recently. A corresponding Table for satellite data sets is given in Section 1.4.1 (Table 1.22). The following Chapters are based on the data sets listed in these Tables. Most of them are archived at the SPARC Data Center (http://www.sparc.sunysb.edu) and can be made available by SPARC and/or by the respective principal investigator upon request.

 

Figure 1.2a

Figure 1.2a Timetable of water vapour measurements in the upper troposphere and in the lower stratosphere (<420 K) using different airborne and ground-based hygrometers. Measurements at these altitudes from profiling observations listed in Figure 1.2b are not included.

 

Figure 1.2b

Figure 1.2b Timetable of water vapour measurements in the stratosphere above 420 K using different airborne, balloon-borne and ground-based hygrometers.