## 2 Motivation and Background
### Current Capabilities for Measuring Water Vapour, Clouds and Aerosols in the UTLS and above  
The longest stratospheric measurement record comes from the radiosonde network. Sondes provide high resolution vertical profiles but are sparsely distributed across the globe, and both sonde type and reporting practices differ at different sites. [PERSON] _et al._[26] compared the performance of two types of operational radiosonde with a more accurate dew-point hygrometer and found that the radiosondes were insensitive to humidity changes in the upper troposphere, leading to potential errors in the climate record derived from these measurements. [PERSON] _et al._[27] also detect a dry bias which increases with altitude, and note the difficulty of collocating sonde profiles with other data sources, in part because of the drift of the balloon as it ascends. Satellites are now also used to measure atmospheric humidity, providing much better spatial and temporal coverage than sondes. Infrared sounders such as SEVIRI on Meteosat Second Generation and the High-resolution Infrared Radiation Sounder (HIRS) provide information on upper-tropospheric humidity, but these nadir sounders have broad weighting functions that limit their vertical resolution. Deriving profile information from a top-of-atmosphere brightness temperature requires good _a priori_ information on the atmospheric temperature structure, and the largest differences between different observations, models and reanalyses are seen in the upper troposphere and stratosphere [3]. Limb sounders have narrower weighting functions, with a pronounced peak at the altitude corresponding to the tangent point. This improves vertical resolution whilst still offering good coverage, at the expense of complex radiative transfer and difficult retrievals in the lower atmospheric layers due to increased scattering. Several satellites currently make passive limb measurements of water vapour, usually as part of missions measuring a wide range of atmospheric species. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Enivsat, launched in 2002, is a mid-infrared Fourier transform spectrometer which can measure water vapour in the mesosphere, lower thermosphere, stratosphere and upper troposphere, at a vertical resolution of 3 km. The Odin satellite, launched in 2001 has a combined payload of the Sub-Millimeter Radiometer (SMR), and the Optical Spectrograph and Infrared Imaging Spectrum (OSIRIS), and also can be used to derive water vapour at 3 km vertical resolution, although 50% of its operational time is spent making astronomical rather than Earth observations. Higher vertical resolution is achieved by the Microwave Limb Sounder (MLS), launched in 2004 on Aura. It also observes thermal emission in the limb, and has a vertical resolution of 1.5 km at 200 hPa, reducing to 5 km in the mesosphere. The Tropospheric Emission Spectrometer (TES), also on Aura, has both nadir- and limb-viewing modes for water vapour. TES has a high spectral resolution but a relatively small swath (5.3 \(\times\) 8.5 km) and nadir footprint (0.53 \(\times\) 5.3 km). In limb mode, TES observes a region with a ground footprint of 26 km \(\times\) 41.8 km, with a height resolution of 2.3 km and vertical coverage from 0 to 33 km. Various occultation techniques are also used to measure water vapour. Water vapour profiles have recently been retrieved from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) solar occultation data for an altitude range of 15-45 km with a vertical resolution of 2.6 km. However, the spectral resolution of SCIAMACHY is not high enough to resolve water vapour lines, which leads to additional correction steps in the retrieval algorithm [28]. The Atmospheric Chemistry Experiment, onboard the Canadian satellite SCISAT-1, carries a Fourier transform spectrometer for solar occultation profiles of water vapour. Vertical resolution is 3 to 4 km and the instrument has been making measurements since 2003.