A Raman Lidar Tropospheric Water Vapour Climatology and Height-Resolved Trend Analysis over Payerne Switzerland

Abstract. Water vapour is the strongest greenhouse gas in our atmosphere and its strength and its dependence on temperature leads to a strong feedback mechanism in both the troposphere and the stratosphere. Raman water vapour lidars can be used to make high vertical resolution measurements, on the order of 10s of meters, making height-resolved trend analyses possible. Raman water vapour lidars have not typically been used for trends analyses, primarily due to the lack of long enough time series. However, the RAman Lidar for Meteorological Observations (RALMO), located in Payerne, Switzerland, is capable of making operational water vapour measurements and has one of the longest ground-based and well-characterized data sets available. We have calculated a 11.5-year water vapour climatology using RALMO measurements in the troposphere. Our study uses nighttime measurements during mostly clear conditions, which creates a natural selection bias. We have also calculated the geophysical variability of water vapour. The climatology shows that the highest water vapour specific humidity concentrations are in the summer months, and the lowest in the winter months. The percent variability of water vapour in the free troposphere is larger than of the boundary layer. We have also determined water vapour trends from 2009–2019. We first calculate precipitable water vapour trends for comparison with the majority of water vapour trend studies. We detect a precipitable water vapour trend of 2.1 mm/decade using RALMO measurements, which is significant at the 90 % level. The trend is consistent with a 1.38 °C/decade surface temperature trend detected by coincident radiosonde measurements under the assumption that relative humidity remains constant;however, it is larger than previous water vapour trend values. For the first time, we show height-resolved increases in water vapour through the troposphere. We detect positive tropospheric water vapour trends ranging from 5% change in specific humidity per decade to 15% specific humidity per decade depending on the altitude. The water vapour trends at 5 layers are statistically significant at or above the 90% level.