Abstract
Abstract. Vertically integrated water vapour (IWV) is expected to increase globally in a warming climate. To determine whether IWV increases as expected on a regional scale, we present IWV trends in Switzerland from ground-based remote sensing techniques and reanalysis models, considering data for the time period 1995 to 2018. We estimate IWV trends from a ground-based microwave radiometer in Bern, from a Fourier transform infrared (FTIR) spectrometer at Jungfraujoch, from reanalysis data (ERA5 and MERRA-2) and from Swiss ground-based Global Navigation Satellite System (GNSS) stations. Using a straightforward trend method, we account for jumps in the GNSS data, which are highly sensitive to instrumental changes. We found that IWV generally increased by 2 % per decade to 5 % per decade, with deviating trends at some GNSS stations. Trends were significantly positive at 17 % of all GNSS stations, which often lie at higher altitudes (between 850 and 1650 m above sea level). Our results further show that IWV in Bern scales to air temperature as expected (except in winter), but the IWV–temperature relation based on reanalysis data in the whole of Switzerland is not clear everywhere. In addition to our positive IWV trends, we found that the radiometer in Bern agrees within 5 % with GNSS and reanalyses. At the Jungfraujoch high-altitude station, we found a mean difference of 0.26 mm (15 %) between the FTIR and coincident GNSS data, improving to 4 % after an antenna update in 2016. In general, we showed that ground-based GNSS data are highly valuable for climate monitoring, given that the data have been homogeneously reprocessed and that instrumental changes are accounted for. We found a response of IWV to rising temperature in Switzerland, which is relevant for projected changes in local cloud and precipitation processes.
Highlights
Atmospheric water vapour is a key component in the climate system
Our study presents a complete trend analysis of integrated water vapour (IWV) in Switzerland based on data from the Swiss Global Navigation Satellite System (GNSS) station network, a microwave radiometer located in Bern, an Fourier transform infrared (FTIR) spectrometer located at Jungfraujoch and from reanalysis models
Our study presents trends of integrated water vapour (IWV) in Switzerland from a ground-based microwave radiometer, an FTIR spectrometer, GNSS stations and reanalysis data
Summary
Atmospheric water vapour is a key component in the climate system. It is the most abundant greenhouse gas and is responsible for a strong positive feedback that enhances temperature increase induced by other greenhouse gases (e.g. IPCC, 2013; Stocker et al, 2001). Water vapour is involved in important tropospheric processes such as cloud formation and precipitation; it influences size, composition and optical properties of aerosols; and it is responsible for atmospheric energy and heat transport via evaporation and condensation (Kämpfer, 2013). Measuring changes in atmospheric water vapour is important because they reflect externally forced temperature changes in the climate system and can be an indicator for changes in involved processes such as cloud formation and precipitation.
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