## 2 Motivation and Background  
### The Importance of Upper Air Water Vapour, Clouds and Aerosols  
Water vapour is the most important greenhouse gas, contributing about 50% towards the Earth's greenhouse effect, followed by clouds which contribute around 25% [5]. The radiative balance of the atmosphere is particularly sensitive to changes in the UTLS, as this is where most of the Earth's thermal radiation escapes into space, and where cirrus clouds trap this outgoing radiation. For example, decreasing water vapour in the stratosphere has been identified as slowing the rate of increase of global surface temperature between 2000 and 2009 by about 25% [6], however it is not clear if the observed decadal variability in stratospheric water vapour is a driver or rather a response to climate change. Water vapour in the stratosphere originates from transport through the tropopause region [7] and from the oxidation of methane, especially in the higher stratosphere. Transport is mainly induced by deep convection in the tropics passing the cold trap tropopause ([6] and references therein). Absolute amounts are low, less than 10 ppmv [8], but nevertheless are of crucial importance, both radiatively and for atmospheric chemistry, for example through an indirect destructive effect on the ozone layer as shown in [9]. Furthermore water vapour can be used as a tracer to study atmospheric dynamics such as stratosphere-troposphere exchange processes or the injection of trace gases into the stratosphere [10]. Important processes of this kind are for example the tropopause folds at midlatitudes or tropopause inversion layers [11], which are associated with strong peaks in the atmospheric stability above the tropopause. Their formation mechanisms are not well understood but the radiative effects of water vapour in combination with ozone may have a substantial impact [12]. The UTLS is also of interest because it is the region where aircraft fly, emitting carbon dioxide, water vapour, nitrous oxide and aerosols, forming contrails and having an impact on cirrus clouds and the radiative budget. Aerosol-cloud interactions are key here, and also poorly understood: the fourth report of the Intergovernmental Panel on Climate Change attributes the largest uncertainties and lowest scientific understanding of radiative forcing to aerosols and their direct and indirect albedo effects [13]. Aerosols in the UTLS affect the Earth's radiation budget by reflecting and absorbing incoming radiation. The scattering of shortwave radiation leads to a cooling of the climate system, and the absorption of longwave radiation to an increased heating rate [14]. Aerosols also can affect the climate in an indirect way by modifying cloud properties, for example droplet size, quantity of cloud drops, cloud albedo, liquid water content and cloud lifetime. As a third effect, aerosols may have an influence on atmospheric chemistry through their indirect modification of the concentration of several gases, especially greenhouse gases [15]. The stratospheric concentrations of aerosol particles are in general quite small compared to tropospheric aerosol, apart from the high concentrations found after volcanic eruptions. The aerosol optical thickness in the stratosphere is typically an order of magnitude smaller than in the troposphere [16]. Cirrus clouds are ice clouds found in the upper troposphere, and cover about 30% of the globe on average over a year [17]. Cirrus clouds affect the Earth's radiation balance by reducing the amount of shortwave radiation reaching the surface, and reducing the longwave radiation emitted back to space. The net effect on the surface radiation budget is dependent on the microphysical properties of the clouds such as ice water content, number density and size of ice crystals as well as thickness. Theoretical calculations suggest that optically thin cirrus (e.g., a contrail) generally has a warming effect whereas optically thick cirrus has a cooling effect.