The present campaign is focused on water vapour, one of the most challenging meteorological atmospheric variable to measure. The World Meteorological Organization (WMO) and the Comité International des Poids et Mesures (CIPM) have signed the Mutual Recognition Arrangement (MRA), recognizing the need of improved measurements and taking steps to improve the matters. In response, the MeteoMet project has been launched in 2011 by 18 european meteorological institutes. Currently, water vapour (WV) is the second key meteorological variable considered after temperature.
Water vapor plays an important role in the radiative balance on Earth since it is the principal source of infrared opacity. Its contribution to the greenhouse effect is about 60 % to 75% (Kiehl and Trenberth, 1997; Schmidt et al., 2010; Lacis et al., 2013). Simulations based on radiative-convective models and observations have demonstrated that the surface warming caused by an increase in greenhouse gases like CO2 could lead to a moistening of the troposphere (Dessler, 2013; Dessler et al., 2008; Dessler and Wong, 2009; Minschwaner and Dessler, 2004; Soden et al., 2005). This coupling could double the warming induced by CO2 only (Banerjee et al., 2019; Dessler et al., 2013). The increase in tropospheric temperatures has been found to increase stratospheric water vapor (SWV), implying the existence of a stratospheric water vapor feedback coefficient of about +0.3 W/(m2 K) (Dessler et al., 2013). Therefore, SWV has a great influence on the global climate radiative and chemical equilibrium. Various studies, based on radiative-chemical models, have shown a correlation between the variations in SWV and the changes in stratospheric ozone and the changes in stratospheric and mean global temperatures (Dvortsov and Solomon, 2001; Riese et al., 2012; Solomon et al., 2010). Stratospheric water vapor is a significant contribution to the radiative equilibrium of the stratosphere, and therefore to the global radiative equilibrium. Observational studies have shown that a moistening of the stratosphere could lead to a warming of the mean surface temperature (Forster and Shine, 1999; Wang et al., 2017), with disparities at different latitudes.
In the stratosphere, the humidity is driven by many complex processes and interactions for which important lack of information remains. However, being able to understand and predict the evolution of the stratospheric humidity is essential to determine the future climate. Several studies based on observational methods and models have estimated an increase of stratospheric water vapor from 0.6 to 1%/year (Scherer et al., 2008; Rosenlof et al., 2001a; Hurst et al., 2011b; Oltmans et al., 2000). The observed trend may be closely linked to climate change due to modifications in the transport of air parcels through the tropical tropopause (Rosenlof et al., 2001b; Rosenlof and Reid, 2008). Basically all models uniformly predict an increase of stratospheric water vapor in the future [e.g., (Gettelman et al., 2010)] and most observations have shown positive trend although some other analyses have not found evidence of such a long-term trend(e.g. HALOE satellite data between 1992 and 2002 (Randel et al., 2004)). Today, the origin of such an increase and the processes responsible for the modulation of the stratospheric water vapor content (and of the repartition of other greenhouse gases)are still under investigation and are a matter of active research.
Factors which impede further progresses in this field are 1- the scarcity of observations at the tropics which are the main gate of tropospheric species to the stratosphere and 2- the instrumental discrepancies found between atmospheric hygrometers (about 50% at the sub10 ppm level) well above the stated instrumental uncertainties (5 % -10%).
In the same vein of previous AquaVIT campaigns (1&2), AquaVIT-4 aims to address instrumental biases as a complement of balloon-borne comparisons.