A 15 Year Climatology and Trend Study of Tropospheric Relative Humidity and Temperature over Switzerland based on Raman Lidar Measurements

Abstract. Water vapor is the most important greenhouse gas, yet large uncertainties remain regarding its evolution in a warming climate, particularly for relative humidity (RH). In this study we use 15 years (2010–2024) of nighttime Raman lidar measurements from the Raman Lidar for Meteorological Observations (RALMO) in Payerne, Switzerland, to investigate free-tropospheric RH changes under clear-sky conditions. We reprocessed and homogenized the data set and derived a climatology and seasonal trends between 3 and 10 km. The temperature climatology captures the expected annual cycle with an amplitude decreasing from about 10 K near the surface to 2 K near the tropopause. The RH climatology exhibits an annual cycle in the lower and upper troposphere and a semi-annual cycle in the mid-troposphere, reflecting the influence of large-scale circulation and moisture transport. Consistent tropospheric warming of approximately 1–3 K per decade is observed, with the strongest and statistically significant trends occurring in winter and summer. In contrast, RH trends are generally small and not statistically significant, suggesting that RH has remained approximately constant over Switzerland despite atmospheric warming. Our results imply that moisture supply is sufficient in the Alpine region to compensate for the atmosphere’s increasing capacity to hold water. This work is one of few long-term studies of RH in the free troposphere where such analyses remain limited by the scarcity of suitable datasets.