Harmonized microwave radiometer observations of middle-atmospheric ozone over Switzerland

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

Detecting trends of stratospheric ozone and tropospheric water vapour at mid-latitudes using measurements from multiple techniques

This thesis investigates and quantifies changes in stratospheric ozone and tropospheric water vapour at mid-latitudes since the mid-1990s. Recent studies have shown that estimates of such changes from various ground-based measurement techniques are not always consistent. A possible reason for these differences may be inhomogeneities in the data. Data inhomogeneities arise from modifications in the instrument setup, measurement failures, problems or adjustments in the calibration and retrieval procedures, or from temporal sampling biases. To explain differences in observed changes, data inhomogeneities have first to be identified by intercomparing various datasets. In a second step, the inhomogeneities can be considered in the trend estimation to obtain optimal estimates of the true changes. This thesis aims to obtain more consistent trend estimates of stratospheric ozone profiles and tropospheric water vapour at mid-latitudes. For this purpose, we compared ozone and integrated water vapour (IWV) time series from various measurement techniques. The observations were intercompared to identify anomalies, biases, and inhomogeneities in the data. Trends in recent decades were then estimated by considering these irregularities in the trend estimation. To this end, two advanced trend analysis methods were tested and applied on the data. The trend models use the full error covariance matrix of the observations, which can be adapted to account for data correlations and inhomogeneities. We used stratospheric ozone observations measured by ground-based microwave radiometers, lidars, and ozonesondes, as well as satellite and reanalysis model data.We found good agreement between various ozone datasets. However, we also identified some anomalies and inhomogeneities in the ozone data and showed that they affect the trend estimates. Stratospheric ozone trend profiles are presented for northern (central Europe) and southern mid-latitudes (New Zealand). In both hemispheres, we observe a recovery in ozone concentrations in the upper stratosphere after the turn-around of ozone-depleting substances (ODSs) in 1997. We found trends that generally lie between 1% and 3% per decade, providing a confirmation of ozone recovery in the upper stratosphere as expected from reduced ODS emissions. In the lower stratosphere, we found inconsistent trends, suggesting that further research on lower-stratospheric ozone changes is required. Observations of IWV were used from a microwave radiometer, from a Fourier-transform infrared spectrometer, from a network of ground stations of the Global Navigation Satellite System (GNSS), and from reanalysis model data. We present trends derived from these IWV measurements in Switzerland. They show that IWV increased by 2% to 5% per decade since 1995, which is generally consistent with rising temperature. Also, we show that the advanced trend model used is well suited to reduce trend biases caused by GNSS-antenna updates. In conclusion, this thesis presents optimized trends of stratospheric ozone and IWV for recent decades at mid-latitudes. Further, it helps to better understand inconsistencies between trend estimates from multiple techniques by investigating the effect of data irregularities on the trends. Consequently, the results of this thesis contribute to a better understanding of ozone and water vapour changes in a changing climate