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

Frequency Dependence of the Correlation between Ozone and Temperature Oscillations in the Middle Atmosphere

This study investigates the frequency dependence of the correlation or anticorrelation of ozone and temperature in the middle atmosphere. The anticorrelation of ozone and temperature plays a role for a possible super recovery of upper stratospheric ozone in the presence of man-made cooling of the middle atmosphere due to increasing carbon dioxide emissions. The correlation between lower stratospheric ozone and temperature indicates the dependence of lower stratospheric temperature trends on the ozone evolution in addition to greenhouse gas emissions. Ozone and temperature measurements of the microwave limb sounder (MLS) on the satellite Aura from 2004 to 2021 are utilized for Bern (46.95◦ N, 7.44◦ E) at middle latitudes and for the equator region. The time series are bandpass filtered for periods from 2 days to 5 years. The correlation coefficient depends on the period of the oscillation in temperature and ozone. The strongest correlation and anticorrelation are found for the annual oscillation. The anticorrelation between ozone and temperature in the upper stratosphere is about −0.7 at a period of two days and −0.99 at a period of one year. Thus, the temperature dependence of the ozone reaction rates also leads to an anticorrelation of ozone and temperature at short periods so that ozone can be considered as a tracer of planetary waves. At the equator, a dominant semiannual oscillation and an 11 year solar cycle are found for nighttime ozone in the upper mesosphere. The semiannual oscillation (SAO) in ozone and temperature shows a strong correlation indicating a dynamical control of the ozone SAO in the upper mesosphere. The SAO in the equatorial nighttime values of ozone and temperature is possibly due to a semiannual modulation of vertical advection by the diurnal tide

Response of Total Column Ozone at High Latitudes to Sudden Stratospheric Warmings

The total column ozone (TCO) at northern high latitudes is increased over a course of 1–2 months after a major sudden stratospheric warming as a consequence of enhanced ozone eddy transport and diffusive ozone fluxes. We analyzed ground-based measurements of TCO from Oslo, Andøya and Ny Ålesund from 2000 to 2020. During this time interval, 15 major sudden stratospheric warmings (SSWs) occurred. The observed TCO variations are in a good agreement with those of ECMWF Reanalysis v5 (ERA5), showing that TCO from ERA5 is reliable, even during dynamically active periods. ERA5 has the advantage that it has no data gaps during the polar night. We found that TCO was increased by up to 190 DU after the SSW of February 2010, over one month. The composite analysis of the 15 SSWs provided the result that TCO is increased on average by about 50 DU over one month after the central date of the SSW

Ozone and water vapor variability in the polar middle atmosphere observed with ground-based microwave radiometers

Leveraging continuous ozone and water vapor measurements with the two ground-based radiometers GROMOS-C and MIAWARA-C at Ny-Ålesund, Svalbard (79∘ N, 12∘ E) that started in September 2015 and combining MERRA-2 and Aura-MLS datasets, we analyze the interannual behavior and differences in ozone and water vapor and compile climatologies of both trace gases describing the annual variation of ozone and water vapor at polar latitudes. A climatological comparison of the measurements from our ground-based radiometers with reanalysis and satellite data was performed. Overall differences between GROMOS-C and Aura-MLS ozone volume mixing ratio (VMR) climatology are mainly within ±7 % throughout the middle and upper stratosphere and exceed 10 % in the lower mesosphere (1–0.1 hPa) in March and October. For the water vapor climatology, the average 5 % agreement is between MIAWARA-C and Aura-MLS water vapor VMR values throughout the stratosphere and mesosphere (100–0.01 hPa). The comparison to MERRA-2 yields an agreement that reveals discrepancies larger than 50 % above 0.2 hPa depending on the implemented radiative transfer schemes and other model physics. Furthermore, we perform a conjugate latitude comparison by defining a virtual station in the Southern Hemisphere at the geographic coordinate (79∘ S, 12∘ E) to investigate interhemispheric differences in the atmospheric compositions. Both trace gases show much more pronounced interannual and seasonal variability in the Northern Hemisphere than in the Southern Hemisphere. We estimate the effective water vapor transport vertical velocities corresponding to upwelling and downwelling periods driven by the residual circulation. In the Northern Hemisphere, the water vapor ascent rate (5 May to 20 June in 2015, 2016, 2017, 2018, and 2021 and 15 April to 31 May in 2019 and 2020) is 3.4 ± 1.9 mm s−1 from MIAWARA-C and 4.6 ± 1.8 mm s−1 from Aura-MLS, and the descent rate (15 September to 31 October in 2015–2021) is 5.0 ± 1.1 mm s−1 from MIAWARA-C and 5.4 ± 1.5 mm s−1 from Aura-MLS at the altitude range of about 50–70 km. The water vapor ascent (15 October to 30 November in 2015–2021) and descent rates (15 March to 30 April in 2015–2021) in the Southern Hemisphere are 5.2 ± 0.8 and 2.6 ± 1.4 mm s−1 from Aura-MLS, respectively. The water vapor transport vertical velocities analysis further reveals a higher variability in the Northern Hemisphere and is suitable to monitor and characterize the evolution of the northern and southern polar dynamics linked to the polar vortex as a function of time and altitude

Prediction and Comparison of low-Reynolds Airfoil Performance

This semester project is aiming to compare different airfoils in order to select the most promising one as blade shape for scaled the straight-bladed giromills used in wind tunnel experiments. For this matter, a large amount of steady-state CFD-simulations have been performed trying to characterize aerodynamic airfoil's performance at low Reynolds number. It is a continuation of a previous semester project which already selected the numerical model to use for the simulation and developed an automation process for batching several simulations which facilitated the creation of the dataset presented in this report. In total, thirteen profiles have been investigated through various angles of attack and for chord Reynolds numbers of 1x10^4, 2x10^4 and 4x10^4. The airfoils chosen consist of the symmetrical NACA profiles of different thicknesses (NACA 0005 to NACA 0021), three modified NACA with sharp leading edge (NACA 0005-05, NACA 0009-05 and NACA0012-05), two 5% cambered NACA profiles (NACA 5505 and NACA 5510) and three specific airfoils for low Reynolds number (E387, S1223 and BW3). Based on the tangential force coefficients of the airfoils, NACA5505 and BW3 showed the overall best performances at this range of Reynolds which is in accordance with the existing studies on the subject as they are both thin airfoils with around 5% maximum camber at mid chord

Harmonized retrieval of middle atmospheric ozone from two microwave radiometers in Switzerland

We present new harmonized ozone time series from two ground-based microwave radiometers in Switzerland: GROMOS and SOMORA. Both instruments have measured hourly ozone profiles in the middle atmosphere (20–75 km) for more than 2 decades. As inconsistencies in long-term trends derived from these two instruments were detected, a harmonization project was initiated in 2019. The goal was to fully harmonize the data processing of GROMOS and SOMORA to better understand and possibly reduce the discrepancies between the two data records. The harmonization has been completed for the data from 2009 until 2022 and has been successful at reducing the differences observed between the two time series. It also explains the remaining differences between the two instruments and flags their respective anomalous measurement periods in order to adapt their consideration for future trend computations. We describe the harmonization and the resulting time series in detail. We also highlight the improvements in the ozone retrievals with respect to the previous data processing. In the stratosphere and lower mesosphere, the seasonal ozone relative differences between the two instruments are now within 10 % and show good correlation (R > 0.7) (except during summertime). We also perform a comparison of these new data series against measurements from the Microwave Limb Sounder (MLS) and Solar Backscatter Ultraviolet Radiometer (SBUV) satellite instruments 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 that point

An Indoor Microwave Radiometer for Measurement of Tropospheric Water

This article presents the first detailed description of the innovative measurement setup of an indoor tropospheric microwave radiometer [TROpospheric WAter RAdiometer (TROWARA)] that avoids water films on radome. We discuss the performance of a commercial outdoor microwave radiometer [Humidity And Temperature PROfiler radiometer (HATPRO)] for measuring tropospheric water parameters in Bern, Switzerland. The HATPRO is less than 20 m from the TROWARA and has different instrument characteristics. Brightness temperatures measured by HATPRO are analyzed by comparing them with coincident measurements from TROWARA and Radiative Transfer Simulations based on the [European Centre for Medium-Range Weather Forecasts (ECMWF)] operational analysis data (denoted as RTSE). To find the source of brightness temperature bias, a gradient boosting decision tree is used to analyze the sensitivity of eight feature factors to bias. Data processing routines of the two radiometers use different algorithms to retrieve integrated water vapor (IWV) and integrated cloud liquid water (ILW), whereas the same physical algorithms based on the radiative transfer equation are applied to obtain the opacity and rain rate. Using 62 days of data with varied weather conditions, it was found that TROWARA brightness temperatures are in good agreement with RTSE. HATPRO brightness temperatures are significantly overestimated by about 5 K at 22 GHz, compared to TROWARA and RTSE. HATPRO brightness temperatures at 31 GHz agree well with TROWARA and RTSE (within about ±1 K). The overestimated brightness temperatures in the K-band and the HATPRO retrieval algorithm lead to an overestimation of IWV and ILW by HATPRO. The opacities at 31 GHz match very well for TROWARA and HATPRO during no rain with a verified R2of 0.96. However, liquid water floating or remaining water films on the radome of the outdoor HATPRO radiometer induce an overestimation of the rain rate. The physical reason for the overestimated 22-GHz brightness temperatures of the HATPRO is mainly the result of the combined effect of instrument calibration, the surrounding environment of the instrument, and the Sun elevation angle. This can be a problem with the Generation 2 HATPRO radiometer and this problem was resolved in the Generation 5 HATPRO radiometer

Real-time pollen monitoring using digital holography

We present the first validation of the SwisensPoleno, currently the only operational automatic pollen mon-itoring system based on digital holography. The device pro-vides in-flight images of all coarse aerosols, and here wedevelop a two-step classification algorithm that uses theseimages to identify a range of pollen taxa. Deterministiccriteria based on the shape of the particle are applied toinitially distinguish between intact pollen grains and othercoarse particulate matter. This first level of discriminationidentifies pollen with an accuracy of 96 %. Thereafter, in-dividual pollen taxa are recognized using supervised learn-ing techniques. The algorithm is trained using data obtainedby inserting known pollen types into the device, and out ofeight pollen taxa six can be identified with an accuracy ofabove 90 %. In addition to the ability to correctly identifyaerosols, an automatic pollen monitoring system needs to beable to correctly determine particle concentrations. To fur-ther verify the device, controlled chamber experiments us-ing polystyrene latex beads were performed. This providedreference aerosols with traceable particle size and numberconcentrations in order to ensure particle size and samplingvolume were correctly characterized