A Tropospheric Water Vapour Climatology and Trends Derived from Vibrational Raman Lidar Measurements over Switzerland
Abstract
Water vapour is the most effective atmospheric greenhouse gas in terms of warming the atmosphere. Water vapour can magnify the temperature increase that CO2 would cause alone by 2-3 times. As such, it is critical to monitor changes in water vapour abundance to better understand its role in atmospheric change. I have used 10 years of lidar and radiosonde measurements from the MeteoSwiss research station in Payerne, Switzerland to calibrate the lidar, improve its water vapour retrievals, and finally calculate a lidar water vapour climatology and trend analysis.
Calculating trends with a lidar requires rigorous calibration. Therefore, my first thesis project was to improve the lidar calibrations by using the best radiosonde product available, measurements from the GCOS Reference Upper Air Network (GRUAN) and a novel trajectory method. My trajectory method improved the lidar calibration and more consistently agreed with the radiosonde measurement compared to the traditional method. Using GRUAN radiosondes enabled the calculation, for the first time, of a complete uncertainty budget of the calibration constant.
The second project was a method for removing a priori information from lidar optimal estimation retrievals. Removing a priori information from optimal estimation retrievals may be desirable in cases where there is low confidence in the knowledge of the a priori state. The a priori removal method was tested with the lidar water vapor mixing ratio retrieval. The new method increased the range of the daytime water vapour altitudes by 1 to 2\,km and the nighttime water vapour profiles by an average of 500\,m. The a priori removal method can be applied to a time series such that the prior will have no influence on a trend or climatological analysis.
In my third project, 10 years of lidar water vapour measurements were re-processed to calculate a tropospheric water vapour climatology and trends for Switzerland. The climatology showed that water vapour concentrations above Payerne are typically highest in June through September and lowest from December through March, as expected. The lidar detected water vapour concentrations increasing by 15%/decade. This increase is consistent with a 2 degree C per decade temperature increase measured by the radiosondes. The large increase in water vapour over time could have far-reaching consequences for the hydrologic cycle and weather over western Europe.