PIs: Susanne Crewell, Annette Rinke, Georg Heygster

The central aim of this project is to determine the strength and variability of water vapour feedback mechanisms contributing to the Arctic Amplification by effects on radiation, clouds and temperature, and how they changed over the recent decades and may change in the future.

To achieve the overarching goal, first, new information about the spatial and temporal variability of water vapour in the Arctic will be provided on large scales (Arctic–wide) and over more than a decade by compiling new data sets of satellite remote sensing, new reanalyses (Arctic System Reanalysis; ASR) and regional climate model (RCM) simulations. Specifically, the advantage of microwave remote sensing to sense water vapour in clear and cloudy situations will be exploited and an operational daily circum–Arctic precipitable water vapour (PWV, i.e., vertically integrated water vapour) dataset will be constructed. This merges two different algorithms from microwave satellite instruments to mitigate the problem of variable surface emissivity of ice and water. Furthermore, brightness temperatures in pixel resolution will provide information on the vertical water vapour distribution and be compared to their counterpart calculated from RCM simulations. The build–up of a unique multi–decade database is an important goal that will become possible as microwave imagers and sounders on polar orbiting, operational meteorological satellites will continue to provide comprehensive observations with especially good coverage at high latitudes.

The intensive observational efforts of the TR 172 in spring/summer 2017 and 2019 will be exploited to investigate the effects of small–scale water vapour variability not captured by satellite products. Here the ground–based, ship– and airborne data will be used to analyse resolution effects and in particular the role of humidity inversions and whether the RCM is able to reproduce these in sufficient detail. Furthermore, these periods will serve as a reference to investigate the potential of other satellite products to be potentially integrated into the large scale data base.

Based on the new observational large–scale data, the regional patterns of monthly water vapour (both the column integrated PWV, and different vertical layering) and their interannual variability, changes and decadal trends will be examined. Furthermore, the variability on the daily timescale associated with cyclone activity will be assessed. Circum–Arctic RCM simulations will be evaluated to what extent they can reproduce the findings. By comparing both atmosphere and atmosphere–ice–ocean RCM simulations over 1979–present, the impacts of atmospheric stability, regional atmospheric circulation and coupled feedback mechanism on the PWV patterns and their variability will be studied. While the focus will be on the HIRHAM and HIRHAM–NAOSIM models, the multi–model ensemble of Arctic CORDEX RCM simulations will also be analysed.

Finally, PWV–related feedback processes which affect the Arctic Amplification primarily by modifying the downward longwave radiation (LWD) will be quantified, and it will be analysed how clouds may impact the PWV–LWD feedback. For this, the spatial regional patterns of the sensitivity of LWD to changes in PWV and of the relationship between LWD and surface air temperature will be calculated for all seasons, based on the RCM and reanalyses. The LWD response to changes in the vertical humidity distribution and the importance of humidity inversions will be also examined. It will be analysed how these feedback processes have changed regionally over the recent decades and how state–of–the–art RCMs can reproduce the observed strength and patterns of such relationships. Finally, future changes in water vapour and related feedback processes will be estimated with an ensemble of RCM projections under the RCP 8.5 scenario.

In order to test the hypothesis, we will address the following central questions:

• How does the water vapour feedback influence the Arctic Amplification?

• What are the relative effects on radiation, clouds and temperature?

• How does the water vapour and related feedback processes change temporally and regionally over the last decades and will change in the future?

• Regional climate simulations, derivation of synthetic satellite observations, and feedback assessment (D03, E01, E04)
• Evaluation of high-resolution WV products at super sites (B03, B06, E02)