## 2 Motivation and Background
### Current Capabilities for Measuring Water Vapour, Clouds and Aerosols in the UTLS and above
[PERSON] _et al._[29] suggest that retrieved water vapour profiles have low measurement error, and are subject to a random error of less than 2.0% for the stratosphere (with higher values in the troposphere and mesosphere). The Global Ozone Monitoring by Occultation of Stars (GOMOS) uses stellar occultation to retrieve vertical profiles of atmospheric constituents, with a vertical resolution of 2-4 km. The signal-to-noise ratio for water vapour is low, but the retrieval has recently been improved through refined calibration procedures and is expected to improve from the new processor version [30]. The largest impact on numerical weather prediction (NWP) in recent years has come from radio occultation measurements, such as those from the Constellation Observing System for Metereology, Ionosphere and Climate (COSMIC) and the Global Navigation Satellite Systems Receiver for Atmospheric Sounding (GRAS). By assimilating the measured bending angle of atmospheric refraction, the atmospheric temperature, pressure and water vapour content fields can be constrained in NWP models. [PERSON] _et al._[27] compare atmospheric profiles from COSMIC and radiosondes, finding dry biases in the latter depending on radiosonde type and time of day. This demonstrates the importance of the radio occultation measurements as a reference for other humidity measurements, however they are only accurate to 0.1 g/kg, and the very low amounts of water vapour in the UTLS require a more sensitive technique. The majority of spaceborne systems for cloud and aerosol measurements also employ passive instruments. NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) has been spaceborne since 1999, acquiring data in 36 spectral bands. Aerosol optical depth is derived globally, and size distribution can be derived over the oceans, at a spatial resolution of up to 250 m. MODIS wavelengths also detect clouds, and the discrimination of cloud from aerosol plumes is challenging. The Multi-angle Imaging Spectroradiometer (MISR; 1999-present) takes images at nine viewing angles simultaneously at visible and near-infrared frequencies, which allows the discrimination of surface and aerosol signatures. However it has a narrow swath and a long repeat cycle of 16 days. The Stratospheric Aerosol and Gas Experiment III (SAGE III) employed a UV/visible spectrometer to make measurements of aerosol (up to 40 km) and water vapour (up to 50 km), along with cloud detection (6-30 km), at 1 km vertical resolution. It was launched in 2002 but ceased operating in 2006. Active monitoring of clouds and aerosol became possible with the launch in 2006 of the cloud-profiling radar and LiDAR on Cloudsat and CALIPSO (Cloud-Aerosol LiDAR and Infrared Pathfinder Satellite Observations) respectively. The two spacecraft fly in close formation so that the observations from the two instruments are near-simultaneous, and together the instruments provide much greater information on the vertical structure of clouds. However, they only measure a narrow swath. The Earth Clouds, Aerosol and Radiation Explorer (EarthCARE), currently in development, will combine a cloud-profiling radar and a backscatter LiDAR on one spacecraft, together with a multi-spectral imager and broadband radiometer. Exploiting these measurements simultaneously will allow more sophisticated process studies of aerosol-cloud-radiation interactions. Finally, Process Exploration Through Measurements of Infrared and Millimeter Wave Emitted Radiation (PREMIER) is a candidate mission for ESA's seventh Earth Explorer mission. If selected, this mission would employ passive limb-sounding to determine concentrations and dynamics of many atmospheric constituents, including water vapour. The altitude range for water vapour measurements is 6-55 km, with a target accuracy of 5% and a vertical resolution of 2 km. Whichever mission of the three candidates is picked, the earliest launch would be 2016.