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

Tropical waves and rainfall over Africa: Variability, mechanisms and potential for forecasting

Excessive rains or prolonged drought can have severe impacts on the economy, agriculture, water resources, spread of diseases and ecosystems in many African countries. As current global numerical weather prediction systems fail to deliver accurate rainfall forecasts over tropical Africa, novel forecasting strategies are needed. Tropical waves are known to modulate precipitation over this region on timescales of a few days to several weeks. The aim of this dissertation is to quantify the influence of all major waves on rainfall variability over Africa, to investigate the involved mechanisms and, to test their potential for forecasting rainfall, with a focus on northern tropical Africa during the extended monsoon season. Despite the importance of rainfall variability for vulnerable societies in tropical Africa, the relative influence of tropical waves for this region is largely unknown. This thesis closes this gap and presents the first systematic comparison of the impact of six wave types on precipitation over northern tropical Africa during the transition and full monsoon seasons, using two satellite products and a dense rain gauge network. Composites of rainfall anomalies based on different datasets show comparable modulation intensities in the West Sahel and at the Guinea Coast, varying from less than 2 to above 7 mm per day depending on the wave type. Tropical disturbances (TDs, including African Easterly Waves, AEWs) and Kelvin waves dominate the 3-hourly to daily timescale and explain 10-30% of precipitation variability locally. On longer timescales (7–20 days), only the Madden-Julian Oscillation (MJO) and Equatorial Rossby (ER) waves remain as modulating factors and explain up to one third of rainfall variability. Eastward inertio-gravity (EIG) waves and mixed Rossby-gravity (MRG) waves are comparatively unimportant. An analysis of wave superposition shows that low-frequency waves (MJO, ER) in their wet phase amplify the activity of high-frequency waves (TD, MRG) and suppress them in the dry phase. Furthermore, this dissertation gives the first systematic comparison of the dynamics and thermodynamics associated with tropical waves affecting rainfall variability over northern tropical Africa: Reanalysis and radiosonde data were analyzed for the period 1981–2013 based on space-time filtering of outgoing longwave radiation. The identified circulation patterns are largely consistent with equatorial shallow water theory. The slow modes, MJO and ER, mainly impact precipitable water, whereas the faster TDs, Kelvin waves, and MRG waves primarily modulate moisture convergence. Monsoonal inflow intensifies during wet phases of the MJO, ER, and MRG waves, associated with a northward shift of the intertropical discontinuity for MJO and ER waves. This study reveals that MRG waves over Africa have a distinct dynamical structure that differs significantly from AEWs. During passages of vertically tilted imbalanced wave modes, such as MJO, TDs, Kelvin, and partly MRG waves, increased vertical wind shear and improved conditions for up- and downdrafts facilitate the organization of mesoscale convective systems. The balanced ER waves are not tilted and rainfall is triggered by large-scale moistening and stratiform lifting. The MJO and ER waves interact with intraseasonal variations of the Indian monsoon and extratropical Rossby wave trains. The latter causes a trough over the Atlas Mountains associated with a tropical plume and rainfall over the Sahara. The presented results unveil which dynamical processes need to be modeled realistically to represent the coupling between tropical waves and rainfall in northern tropical Africa. The potential of tropical waves as predictors for African rainfall was tested. The spatio-temporal correlation patterns of tropical waves highlight their potential for synoptic rainfall forecasting. The observed spatio-temporal properties agree with values predicted by shallow-water theory, with the exception of MRG and EIG waves, which have a strong phase dispersion at low wavenumbers. Unfiltered precipitation fields show correlations patterns that are physically explainable by tropical waves and other atmospheric phenomena such as the position of the tropical rainbelt. These correlations serve as predictors in a logistic regression model. It was shown that this model successfully predicts rainfall occurrence over Africa with a lead time of one day. The statistical model is calibrated and outperforms the climatological forecast and current numerical weather prediction models by about 20%. The fact that tropical waves explain large portions of synoptic to intraseasonal rainfall variability in almost the entire tropics emphasize the potential of the proposed statistical model. This PhD thesis has laid the foundation to exploit this potential and to significantly improve short-term weather forecasts in Africa and throughout the tropics

Space-borne observations of meteoric metal layers in the upper atmosphere.

The upper mesosphere/lower thermosphere (MLT) is an important transition region. However, it remains poorly understood relative to other parts of the Earth’s atmosphere, largely due to a lack of observations. Metal species, produced by meteoric ablation act as useful tracers of upper atmospheric dynamics and chemistry. Of these meteoric metals, K has long been an enigma. Limited lidar data at extra-tropical latitudes shows that the K layer displays a semi-annual seasonal variation rather than the annual pattern seen in other metals such as Na and Fe. This is a rather surprising feature as both Na and K are Group 1 alkali metals and, thus, should exhibit similar behaviour. The aim of this thesis was to produce the first near-global K retrieval which could be used to evaluate this unusual behaviour, as well as providing a new dataset with which to test our understanding of the MLT region. The K retrieval uses dayglow measurements of K at ~770 nm from the Optical Spectrograph and InfraRed Imager System (OSIRIS) instrument on-board the Odin satellite. This retrieval is shown to be capable of retrieving K number density profiles with a 2 km vertical resolution and a typical peak layer error of ±15%. It is shown to compare well with the limited available lidar data. A first near-global look at the global K layer is presented, which shows that the unusual semi-annual seasonal behaviour is global in extent. The OSIRIS data is used to validate the National Center of Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM) modelled K layer; showing good overall agreement and providing support for a new K chemistry scheme which is included in the model. Both OSIRIS and WACCM datasets are used to examine the response of the Na and K metal layers to the 11-year solar cycle. Unlike Na, K shows an anti-correlation with the 11-year solar cycle. The associated temperatures appear to be the predominant source of this anti-correlation. Finally, the response of the WACCM modelled K, Na and Fe layers is examined with respect to longer-term (50-year) changes within the MLT region. K is the only metal to demonstrate a pronounced response to the recent cooling temperature trend

Characterization of Gravity Waves in the Lee of the Southern Andes utilizing an Autonomous Rayleigh Lidar System

Die weltweit größten Gebirgswellen werden an den südlichen Anden angeregt, wo sie anschließend vertikal und horizontal ins Lee propagieren und dort in der mittleren Atmosphäre ihren Impuls auf den Grundstrom übertragen. Viele Fragen im Bezug auf Anregung, genaue Ausbreitung, Wechselwirkung und Dissipation dieser Wellen sind immer noch unbeantwortet. Aus diesem Grund wurde im Auftrag des DLR in Río Grande (53, 7◦ S, 67, 7◦ W), Argentinien, ein Rayleigh Lidarsystem installiert, das vertikale Temperaturprofile aufnimmt, um Schwerewellensignaturen zu detektieren. Die Analyse des Lidar-Datensatzes, der automatisiert zwischen November 2017 und Oktober 2020 erhoben wurde, ist der Kern dieser Doktorarbeit. Neu ist hierbei nicht nur die Messung an diesem geographischen Ort, sondern auch die hohe Kadenz der Messungen. Die Messabdeckung von durchschnittlich zwei Messungen innerhalb drei Nächte ermöglicht es einen Temperaturhintergrund zu definieren, der zeitliche Skalen von 9 Tagen bis hin zu einem Jahr und vertikale Skalen ab 15 km abdeckt. Zusätzlich werden tägliche Gezeiten aus den Nachtmessungen des Lidars mit einer neuen Methodik extrahiert, die zur Validierung auch auf Reanalyse Daten des ECMWF angewandt wird. Der Vergleich zeigt gute Übereinstimmungen, wobei die Amplituden der täglichen Gezeit in den Lidardaten in der Mesosphäre größer sind und auch wesentlich stärker variieren als in den Reanalyse Daten. Zudem führt Gezeiten-Aliasing wahrscheinlich zu unerwartet kleinen/großen Amplituden in den jährlichen/halbjährlichen Schwingungen. Die untersuchten Wellenenergien sind die größten, die je in der Stratosphäre gemessen wurden und erreichen ein Sättigungslimit bei 60 km Höhe. Das Erreichen eines Sättigungslimits in derart niedrigen Höhen wurde so bisher nicht beobachtet und lässt darauf schließen, dass Wellen bereits mit sehr großen Amplituden erzeugt werden und auch während der vertikalen Propagation gute Wachstumsbedingungen vorfinden. In Zusammenhang mit der Sättigung steht auch eine beobachtete Abnahme der Schwerewellenintermittenz in der Mesosphäre. Die Entwicklung eines neuen spektralen Werkzeugs hilft bei der Bestimmung von Wellenlängen. Hierbei wird deutlich, dass etwa 50 % der Wellen vertikale Skalen von über 16,5 km aufweisen. Dies ist ein wichtiges Ergebnis, wenn man bedenkt, dass bisherige Lidar-Studien sich meist auf vertikale Wellenlängen <15 km fokussiert haben. In Einzelfällen wird ein Energiezuwachs in der Stratosphäre beobachtet, der das erwartete exponentielle Wachstum übersteigt. Dies könnte ein Hinweis darauf sein, dass die Wellen horizontal durch das Beobachtungsvolumen des Lidars hindurch propagieren. Um die Propagation der Wellen zusammen mit ihrem Anregungsmechanismus zu untersuchen, wurde eine Raytracing-Studie durchgeführt. Es wird zum einen deutlich, dass gemessene Wellenenergien in der mittleren Atmosphäre in erster Linie von den Eigenschaften der Hintergrundatmosphäre abhängen und erst in zweiter Linie von der Stärke der Anregung. Zweitens hat sich herausgestellt, dass die Anregung die Ausbreitungsrichtung der Gebirgswellen definiert. Dreht der Wind mit der Höhe, kommt es verstärkt zu lateraler Ausbreitung teilweise über mehrere 100 km leewärts. Ein horizontaler Windgradient vermag dies durch Drehung des Wellenvektors nicht zu kompensieren. Dies ist ein wichtiges Ergebnis und sollte in zukünftigen Parametrisierungs-Schemata von Klimamodellen berücksichtigt werden

Modern Climatology - Full Text

Climatology, the study of climate, is no longer regarded as a single discipline that treats climate as something that fluctuates only within the unchanging boundaries described by historical statistics. The field has recognized that climate is something that changes continually under the influence of physical and biological forces and so, cannot be understood in isolation but rather, is one that includes diverse scientific disciplines that play their role in understanding a highly complex coupled “whole system” that is the Earth’s climate. The modern era of climatology is echoed in this book. On the one hand it offers a broad synoptic perspective but also considers the regional standpoint as it is this that affects what people need from climatology, albeit water resource managers or engineers etc. Aspects on the topic of climate change – what is often considered a contradiction in terms – is also addressed. It is all too evident these days that what recent work in climatology has revealed carries profound implications for economic and social policy; it is with these in mind that the final chapters consider acumens as to the application of what has been learned to date. This book is divided into four sections that cover sub-disciplines in climatology. The first section contains four chapters that pertain to synoptic climatology, i.e., the study of weather disturbances including hurricanes, monsoon depressions, synoptic waves, and severe thunderstorms; these weather systems directly impact humanity. The second section on regional climatology has four chapters that describe the climate features within physiographically defined areas. The third section is on climate change which involves both past (paleoclimate) and future climate: The first two chapters cover certain facets of paleoclimate while the third is centered towards the signals (observed or otherwise) of climate change. The fourth and final section broaches the sub-discipline that is often referred to as applied climatology; this represents the important goal of all studies in climatology–one that affects modes of living. Here, three chapters are devoted towards the application of climatological research that might have useful application for operational purposes in industrial, manufacturing, agricultural, technological and environmental affairs. Please click here to explore the components of this work.https://digitalcommons.usu.edu/modern_climatology/1014/thumbnail.jp

The global monsoon system: research and forecast

The main objective of this workshop was to provide a forum for discussion between researchers and forecasters on the current status of monsoon forecasting and on priorities and opportunities for monsoon research. WMO hopes that through this series of quadrennial workshops, the following goals can be accomplished: (a) to update forecasters on the latest reseach findings and forecasting technology; (b) to update researchers on monsoon analysis and forecasting; (c) to identify basic and applied research priorities and opportunities; (d) to identify opportunities and priorities for acquiring observations; (e) to discuss the approach of a web-based training document in order to update forecasters on developments of direct relevance to monsoon forecasting

Optical studies of the mesospheric region

A three-field photometer has been employed at the University of Adelaide's Buckland Park field site to collect optical observations of the 557.7nm OI and 730nm OH airglow emissions. Data have been collected on an almost continuous basis since May 1995 through to May 2000, with observations made whenever the moon was not up. Techniques and analysis procedures have been developed which allow routine extraction of the parameters of gravity waves observed each night. A cross-spectral analysis was performed on processed data from the photometer to identify short period (less than 3 hours) wave activity on nights where the impact of clouds on the data was minimal. The resulting wave parameters are analysed for seasonal variability and used to build up a climatology of wave parameters over the 5 years of observation. No consistent seasonal variation was observed, although there was a strong eastward perference to the wave's propagation direction. Implications of this finding are discussed. A co-located MF radar has been operating in spaced antenna mode providing wind data concurrent with the optical observations for most of the acquisition period. When available the wind data allowed calculation of the intrinsic parameters for waves identified in the optical data. The seasonal variablility of these parameters was investigated. An evaluation of energy and momentum fluxes estimated using the method of Swenson et al (1998b) was carried out. Approximations made in this method were found to be inappropriate for the waves detected by the photometer, and a refined procedure was therefore developed. This gave more realistic results, although large number of physically unreasonable momentum flux measurements were reported. Possible reasons for these were explored, and the need for further investigations emphasised. The five year dataset also allowed investigation of the long-term behaviour of the airglow. Both the intensity and variance were analysed using the Lomb-Scargle method across the complete dataset to identify the dominant periods present. Following similar treatment, the MF spaced antenna winds were compared with the optical results; this utilised a complex spectrum extension to the basic Lomb algorithm. Seasonally related periodicities of two years, one year, one half of a year and one third of a year were observed in the optical data, along with a possible signature of a five and a half year period potentially linked to the eleven year solar cycle. The radar data did not have stong signatures of the one third of a year periodicity although the presence of an five and a half year periodicity could not be ruled out. Gravity wave activity, as measured by the optical intensity variance, reached a maximum during autumn with a secondary maximum occurring in spring. The annual variability of the wave spectrum detected by the photometer was also studied which showed a falloff in the wave energy at short periods (less than thirty minutes) during autumn and spring. This suggested that the enhanced wave activity at these times consisted mainly of waves with periods greater than thirty minutes.Thesis (Ph.D.) -- University of Adelaide, Dept. of Physics and Mathematical Physics, 2000