Trends in Tropospheric and Lower Stratospheric Water Vapor Above Switzerland Derived from a 10 Year Raman Lidar Data Set

Water vapour is a critical constituent of Earth’s atmosphere and the most important natural greenhouse gas. It is responsible for a strong feedback doubling the temperature increase due to the increase in carbon dioxide. Upper tropospheric and lower stratospheric (UTLS) water vapour remains particularly difficult to measure, due to its very low concentration and low air temperatures. Large discrepancies between observations still remain regarding whether water vapour is in fact increasing or decreasing in the UTLS. Additionally, the trends at different altitudes in the troposphere is largely unknown with the majority of tropospheric water vapour trend studies focusing on total column and surface water vapour. Detecting water vapour trends throughout the entire troposphere and in the UTLS can help understand surface temperature trends and improve water vapour feedback schemes in climate and forecasting models.

Carefully calibrated and quality-controlled Raman water vapour lidars can be used to make frequent measurements of tropospheric and lower stratospheric water vapour at high vertical resolution of (< 1 km). The RAman Lidar for Meteorological Observation (RALMO), located in Payerne, Switzerland, was designed for operational water vapour measurements and has one of the longest quality-controlled data sets available. The data set ranges from 2008-2019 with an average of 50% uptime over the entire period. We processed the entire data set using an optimal estimation retrieval technique providing a complete uncertainty budget on a profile per profile basis. The lidar was calibrated combining external calibration against co-located radiosoundings from the GCOS Reference Upper Air Network (GRUAN) and internal calibration based on the lidar's solar background measurements. Tropospheric profiles are retrived from raw data integrated over an entire night, while UTLS profiles are retrieved from monthly integrated data. We will show that the monthly UTLS profiles are an accurate estimate of the monthly mean water vapor profiles, and hence suited for trend analyses.

For the first time, we show non-uniform increases in water vapour through the troposphere with trends ranging from +7% specific humidity per decade to +20% specific humidity per decade depending on the altitude. Almost all of the water vapour trends in the troposphere are statistically significant at the 95% Level. Combining these results with temperature measurements from the co-located operational radiosonde record leads to the conclusion that relative humidity is increasing from 2008 to 2019 throughout the troposphere.