Doctor of Philosophy
Planetary Science and Exploration
Sica, Robert J.
Federal office of Meteorology and Climatology,Payerne, Switzerland; University of Western Ontario
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.
Summary for Lay Audience
Water vapour is a critical constituent of Earth’s atmosphere and the most effective greenhouse gas. Its strength as a greenhouse gas leads to a strong water vapour feedback mechanism in both the troposphere and the stratosphere. The water vapour feedback increases the rise in temperature in the atmosphere by 2 - 3 times that of CO2 alone. Estimating the strength of the water vapour feedback requires accurate trend analyses of both temperature and water vapour, which in turn require well-characterized measurements. This thesis is composed of three projects, two of which work to improve the quality of water vapour measurements, and the last uses the final processed measurements to calculate height-resolved trends for water vapour and temperature in the troposphere for Switzerland.
The measurements used in this thesis come from the Raman Lidar for Meteorological Observation (RALMO) and radiosondes. Lidar measurements are unique in that they have relatively high spatial and temporal resolution. The first project is a new method for calibrating RALMO’s water vapour measurements using GCOS Reference Upper Air Network (GRUAN) radiosondes. The new method uses radiosonde trajectories to reduce the bias introduced by the lack of co-location of the radiosonde with the lidar.
The second project in the thesis is a method to remove prior information from our water vapour retrievals. In cases where the uncertainties in the prior information are not well known, it may not be desirable to keep the prior inside the retrieval. The method is tested on water vapour and temperature retrievals, and increases the maximum valid height of the daytime water vapour measurements. The method will be used to retrieve water vapour profiles from the Upper Troposphere and Lower Stratosphere.
The last project in the thesis is a water vapour climatology and trends analysis using RALMO measurements in conjunction with GRUAN and other radiosondes. The results showed that water vapour is increasing in the atmosphere above Switzerland at an average of 15% per decade and that the surface temperature is increasing at 2 degrees C per decade. This project is the first presentation of water vapour trends as a function of altitude.
Hicks-Jalali, Shannon, "A Tropospheric Water Vapour Climatology and Trends Derived from Vibrational Raman Lidar Measurements over Switzerland" (2019). Electronic Thesis and Dissertation Repository. 6375.
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