title: Harmonized microwave radiometer observations of middle-atmospheric ozone over Switzerland
Creators: name: Sauvageat, Eric
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 530 Physics
subject: 570 Life sciences; biology
subject: 620 Engineering

description: This thesis is concerned with ozone measurements in the middle atmosphere over Switzerland. Its main focus is the time series measured by two ground-based microwave radiometers located in Switzerland: The GROund-based Millimeter-wave Ozone Spectrometer (GROMOS) in Bern (46.95◦ N, 7.44◦ E, 560 m) and the Stratospheric Ozone MOnitoring RAdiometer (SOMORA) in Payerne (46.82◦ N, 6.94◦ E, 491 m). These two instruments have measured hourly ozone profiles in the middle atmosphere (20−75 km) for over two decades. As anomalous time periods and inconsistencies in the long-term trends derived from these two instruments were detected, a harmonization project was initiated in 2019. The goal was to fully harmonize the calibration and retrieval routines of GROMOS and SOMORA to better understand and reduce the discrepancies between the two data records. This dissertation presents in detail this harmonization work, the resulting time series and the recent research work done with the harmonized series.
Chapter 1 introduces middle-atmospheric ozone, the quantity of interest of this thesis. In particular, the research background is explained, and the aims and expected impacts of this dissertation are listed.
Chapter 2 lays the basis of passive microwave ground-based radiometry, the ozone measurement technique used throughout this thesis.
The harmonization project between GROMOS and SOMORA is described in Chapter 3. It has been completed for the data from the two instruments from 2009 until 2022 and has been successful at reducing the discrepancies previously observed between the two time series. In the stratosphere and lower mesosphere, the seasonal ozone relative differences between the two instruments are now within 10% and show good correlations (R > 0.7), except during summertime. The new time series were validated against satellite measurements from the Microwave Limb Sounder (MLS) and from the Solar Backscatter Ultraviolet Radiometer (SBUV) over Switzerland. Seasonal mean differences with MLS and SBUV are within 10% in the stratosphere and lower mesosphere up to 60 km and increase rapidly above.
The careful harmonization of the processing algorithms explains some of the remaining differences between the two instruments and enables to flag their respective anomalous measurement periods to adapt their consideration in future trend studies. These results are shown in a first peer-reviewed publication reproduced in Chapter 4.
The harmonized calibration and retrieval algorithms have also been applied to the GROMOS data from 1994 to 2009. With a simple homogenization procedure, the time series of GROMOS now extends from 1994 to 2023 and are ready to compute new strato–mesospheric ozone trends. The harmonization of SOMORA data before 2009 is ongoing.
During my thesis, I also investigated a spectral bias affecting the Acqiris AC240 digital spectrometer, widely used in the field of microwave remote sensing and notably as back-end in GROMOS and SOMORA. A negative bias of ∼ 10% was found on the ozone profile retrieved from the AC240 compared to other, more recent digital spectrometers. The bias origin remains unclear, but it can be accounted for by a simple correction scheme. These investigations and results are reproduced in the form of a second peer-reviewed publication in Chapter 5.
At last, I investigated the ozone diurnal cycle in the middle atmosphere above Switzerland. Specifically, I updated the previous observations of the ozone diurnal cycle derived from GROMOS measurements, which had some discrepancies against model data. The strato–mesospheric ozone diurnal cycle is now in better agreement with SOMORA and with different model datasets. Also, I show the first observations of short-term variability of the ozone diurnal cycle. Chapter 6 presents the investigation of the ozone diurnal cycle and its variability over Switzerland in the form of the third and last peer-reviewed publication of this dissertation.
Finally, Chapter 7 presents the conclusions of this thesis and offers an outlook on ongoing and future work done on ozone microwave remote sensing in Switzerland.