## 2 Motivation and Background
### Observational Requirements for a New Mission Concept  
The main purpose of the mission concept presented in this paper is to provide new observational data on upper air quantities for climate studies, focussing on water vapour. The Database of Observational Requirements formulated by the World Meteorological Organisation (WMO) [31] gives different threshold and target values on vertical resolution needed for water vapour measurements in the different atmospheric layers, depending on the application. For Nowcasting and Very Short Range Forecasting the threshold vertical resolution in the higher troposphere is 3 km and the target resolution is 1 km. However for Global Climate Observing System (GCOS) applications, threshold and target vertical resolution are considerably lower at 2.0 and 0.1 km respectively. Such high resolution in the vertical enables the detection and proper sampling of small scale tropopause folds, which may have extensions as low as 1 km to 4 km [32], as well as resolving the strong gradients in water vapour around (and defining) the tropopause. The data would also be of comparable resolution to high resolution GCMs in this atmospheric region [8].  
Horizontal resolution requirements can be lower since stratospheric water vapour features are considerably slower-varying on a horizontal spatial scale than on the vertical scale. Horizontal resolution requirements have been discussed in the WALES proposal [4] to be about 150-200 km for the mid-troposphere and the UTLS regions respectively to deliver best input to atmospheric models. Finer scale features such as tropopause folds exist in the UTLS region, with extensions of about 100 km [32]. A system aiming at horizontal detection of such folds requires a target resolution rather higher, of the order of 10-50 km.  
To be able to resolve the atmosphere from the UTLS region upwards, the target lowest measurement height is 8 km. To detect polar mesospheric clouds (noctilucent clouds) we specify an upper threshold for the altitude of 100 km.  
Water vapour concentrations in the atmosphere are highly variable. To cover the atmosphere from the UTLS down to the surface it has been suggested that a dynamic range of 0.01-15 g/kg is necessary [4]. To cover also water vapour in the stratosphere, considerably higher sensitivity is required, since concentrations are generally lower at around a few ppm (_i.e._, 0.001 g/kg) or less.  
To properly characterise the variability of upper tropospheric and stratospheric water vapour, it is desirable to sample both sub-seasonally and sub-diurally, over a number of years. Global coverage is also required: good coverage over the poles would allow the monitoring of polar stratospheric and mesospheric clouds, while the tropics require coverage for understanding the role of deep convection in stratospheric variability, and tropopause folds occur at midlatitudes. A mission lifetime of at least 4 years is recommended as providing the minimum number of repeat observations for attempting a climatology of UTLS water vapour.  
Such water vapour observations alone would be beneficial to our scientific understanding of the UTLS and above, but it would be even more useful to monitor clouds and aerosols from the same observational platform. Passive imaging radiometers such as MODIS provide this capability; a resolution of 200 m \(\times\) 200 m and swath of 400 km are suggested as providing suitable complementary information to the water vapour observations, and continuity with existing similar instruments.