Master and Bachelor theses

These are suggested themes for a Master thesis in our group:

Extratropical cyclones in zonally varying flows

Exit and entrance regions of jet streams are known to be amenable to the formation of extratropical cyclones affecting mid-latitudes frequently during winter time. Their frontal development strongly depends on whether they form in the jet entrance (Shapiro-Keyser type) or the jet exit region (Norwegian type).

In this masterthesis we want to investigate the occurrence of cyclogenesis and frontal evolution in such regions with an idealized numerical simulation using the weather prediction model COSMO. Fundamental conceptual approaches used in atmospheric dynamics, e.g. baroclinic energy conversion and dispersion, momentum conservation, potential vorticity concepts and Q-vector forcings in frontal regions will be applied.  How is cyclonic growth initiated in these regions? How sensitive is cyclogenesis to the strength of the Jet? How do baroclinic wave trains develop and how do their individual cyclones interact?

Requirements: A favor for fluid/atmospheric flow dynamics and conceptual thinking is helpful.  Enthusiasm for computer work and programming will be very helpful, however programming skills are not required. A desire to learn new plotting/programming techniques is helpful.

Supervision: Lukas Papritz und Sebastian Schemm

Foehn Online-Trajectories and Microphysical Characterisation of Foehn Warming

The origin of Foehn air in the northern Alpine valleys is a long-standing debate: according to the thermodynamic Foehn theory the air parcels originate from near-surface levels in the Po valley and Ticino and then ascend along the Alpine southern slope before descending into the valleys, e.g. into the Rhine Valley. More recent studies, on the other hand, strongly indicate that the Foehn air in the northern Foehn valleys comes from levels above 1500 m before descending.

In this study a detailed investigation of a recent Foehn case is planned.The Foehn event will first be simulated with the COSMO weather prediction model and studied in an Eulerian way, i.e. by looking at wind, temperature and humidity fields. Then, air mass trajectories are calculated to follow the air parcels from south of the Alps, over the Alpine passes and finally to the Foehn valleys. This Lagrangian analysis constitutes the core of the Master thesis and is based on a sophisticated new tool: online trajectories. In contrast to 'traditional' trajectories, the online trajectories are directly calculated in the COSMO model and take advantage of the full space and time resolution of the model. This allows to look at the 'Foehn origin debate' in a completely new way. Typical questions to look at will be: Do the air parcels ascend on the Alpine south side, and if yes how much? How long do they need to reach the northern Foehn valleys?

In a second part, thermodynamic evolution of air parcels (temperature, potential temperature, latent heating, relative humidity) are studied in greater detail. The basic motivation for this is the warming of the Foehn air as it crosses the Alps: in the Rhine valley, for instance, it can be many degrees Celsius warmer than in Ticino. Why? The thermodynamic Foehn theory associates this warming with latent heat release in precipitation over Ticino. However, there are many reasons why this can't be the definite answer. Based on the online trajectories, this study aims to quantify the warming due to microphysical processes in individual air parcels. Again, the high time resolution of the online trajectories and the multitude of microphysical fields available from COSMO allow a substantially refined analysis of the Foehn warming.

Requirements: Interest in Alpine meteorology (Foehn), in numerical simulation of weather (COSMO) and air streams (online trajectories). Enthusiasm and flair for computer work and programming will be very helpful.

Supervision:  Annette Miltenberger und Michael Sprenger

Polar cyclones in the seas around Antarctica

During summer season dry, cold katabatic winds from Antarctica transport high potential vorticity air over the open seas of the Weddell and the Ross Sea. They are known to favor the genesis of polar cyclones at the sea-ice edge.

The focus of this master thesis is to study the genesis of polar cyclones in the Southern Ocean, with a focus on Weddell and Ross Sea. It is lead by the question, how moisture uptake enhances their baroclinic development on their track along the coast of Antarctica and how they enhance local precipitation. These investigations will be done in the framework of one or more case studies using ERA-Interim reanalysis data and highly resolved simulations using COSMO. Both Lagrangian and Eulerian methods will be applied. 

Requirements: A favor for conceptual thinking and a general interest in dynamical aspects of the atmospheric flow is desired. Enthusiasm for computer work and programming will be very helpful, however programming skills are not required. A desire to learn new plotting/programming techniques is helpful.

Supervision: Lukas Papritz und Sebastian Schemm

The use of polarimetric weather radar data in the Alps

MeteoSwiss is currently installing a new generation of polarimetric weather radars in Switzerland.The amazing world of radar polarimetry provides a lot of interesting additional information about the type of precipitation. This allows improving the standard measurements by adding more details about the shape, orientation, size distribution and the type of the hydrometeors.

In this study the focus is on the analysis of the measured I/Q data of the horizontal and the vertical channel, that is the raw measurements of a polarimetric radar: the goal will be the validation of the algorithm that computes from the I/Q data the reflectivity in both channels as well as the retrieval of the Doppler velocity information and other polarimetric moments (ZDR, rho_HV, Phi_DP).

The polarimetric moments recorded during interesting strong convective thunderstorms or during stratiform precipitation events shall be analyzed to extract useful information. In addition the results may be applied to improve the algorithm to estimate the precipitation at ground level.
Working close to both the forecasters and the scientists of the radar, satellite and nowcasting team will be a unique opportunity to get scientific support and at the same time feedback from the end users.

Requirements: The candidate should have good programming skills, and a strong interest in data processing, radar technology and precipitation.

Supervision: Marco Boscacci and Urs Germann (MeteoSwiss) [Contact at IACETH: Michael Sprenger and Heini Wernli]

Working place: Radar/Satellite/Nowcasting Group, MeteoSwiss, Locarno-Monti

The importance of different microphysical heating rates for the PV evolution in a warm conveyor belt

Warm conveyor belts (WCBs) are well-defined air streams in extratropical cyclones. They originate in the boundary layer of the warm sector and strongly ascend ahead of the surface cold front. Due to the strong ascent, clouds and precipitation form.
During cloud formation many different microphysical processes lead to the release of latent heat, like e.g. condensation of water vapour, freezing of cloud droplets, depositional growth of ice and snow or freezing of rain. Each of these heating rates has the potential to modify the potential vorticity (PV). In a recent study it has been shown, that condensation of water vapour and ice phase processes, mainly the depositional growth of snow, contribute most to the latent heat release in the ascending WCB and therefore also strongly modify the PV.

In this study, sensitivity experiments with the numerical weather prediction model COSMO will be performed. In order to assess the importance of the different microphysical heating rates for the PV evolution in a WCB, the latent heat release due to certain microphysical processes (e.g. due to depositional growth of ice) will be switched off in the COSMO model. It is then possible to investigate how the WCB ascent is modified and how the PV evolution is changed due to the lack of certain heating processes. The changed PV evolution can subsequently have a strong impact on the large-scale circulation pattern in the upper troposphere.

Requirements: Basic knowledge and interest in atmospheric dynamics and cloud microphysics is required for this study. Enthusiasm for computer work and programming will be very helpful

Supervision:Hanna Joos and Heini Wernli

Lagrangian investigation of European temperature extremes

Continental temperature extremes, i.e., very hot days in summer and cold days in winter have a large impact on human health, energy consumption and ecosystems.
They can have a duration of days to weeks and are associated with an anomalous continental-scale circulation pattern. Hot extremes, for instance, occur very frequently in association with a persistent high-pressure system, i.e., an atmospheric blocking. The evolution of the extremes is likely associated with anomalies in advection (e.g., cold air outbreaks from the Arctic) and/or radiation (e.g., intense solar heating during high-pressure conditions). It is the aim of this master thesis to investigate the dynamical and physical processes leading to European temperature extremes with a Lagrangian approach.

ERA-Interim will be used to calculate backward trajectories from hot and cold extreme events in different regions in Europe. This technique will provide information about the pathway of the involved air masses (the advection component) and about changes in potential temperature (due to radiation and turbulence). In addition, it will be possible to assess the evolution of "extremeness" along the trajectories, i.e., to quantify how long an air mass is associated with an extreme and how fast the temperature of an air mass can change from "normal" to "extreme".

Requirements: Computer-based analysis plays also a key role in this project. Enthusiasm for meteorology, atmospheric physics and computer work is very helpful.

Supervision: Heini Wernli and Stephan Pfahl

Characterization of an extreme precipitation and flood event in Switzerland

In this case study the characterization of an extreme precipitation and flood event in Switzerland is investigated with the aid of the COSMO model and the ECMWF reanalysis ERA-Interim dataset.

The aim of this thesis is to identify important precursor signals of extreme precipitation for (e.g., upper-level cutoff lows, high CAPE, moisture advection from the subtropics). The ECMWF dataset is used for a detailed synoptic analysis of the event and the high-resolution model COSMO is used to perform sensitivity experiments with modified initial conditions. It will allow us to determine more precisely which parameters were important to trigger this event.

Requirements: Computer-based modeling and analysis is essential for this project. Enthusiasm for mesoscale meteorology is helpful.

Supervision: Nicolas Piaget

Quantification of the downstream impact of extratropical transition during the hurricaneseason 2011

During extratropical transition (ET) a tropical cyclone (TC) subsequently looses
its tropical characteristics and transforms into an asymmetric baroclinic extratropical cyclone. This is often accompanied by a substantial reconfiguration of the midlatitude flow in downstream regions. Thus high impact weather (HIW) may occur in Europe as a remote response to ET near the North American East Coast.

After rather few Atlantic ET events within the last years, 2011 exhibited various tropical/extratropical interactions and ETs in the North Atlantic basin which all presumably lead to HIW in Europe. For example, during the ETs of Hurricanes Irene and Katia late summer heat waves occurred in Central Europe terminated by heavy thunderstorms ahead of narrow PV streamers. Downstream of the ETs of Ophelia and Philippe a severe flooding event has been triggered on the Alpine North side (Lütschinen) during 7-10 October 2011, that involved a marked atmospheric river.

In this Master thesis project a recently developed tool will be used to quantify the impact of ET on the European HIW in 2011 in one or more selected case studies. Therefore the TC is removed from the initial conditions of a COSMO run using a PV inversion technique. This allows for investigating if the heat wave and/or flooding still occurs in Europe, when the TC is not present. A major technical task will be the visualisation and investigation of COSMO model data or – dependent on the student's skills - the setup of COSMO sensitivity experiments. It is also thinkable to supervise more than one thesis, each dealing with a particular case or period.

Requirements: Computer-based modeling and analysis is essential for this project, and enthusiasm for dynamical meteorology is helpful.

Supervision: Christian Grams and Heini Wernli

Simulating the isotopic composition of snow from a US winter storm

Stable isotopes of precipitation potentially provide useful information on microphysical and dynamical processes in storms. In this thesis, we explore this potential with the help of the newly developed limited-area model COSMOiso. The isotopic composition of the snow is simulated that fell during the Megalopolitan Snowstorm in the New York area in February 1983. Results from the model are compared to observations by Gedzelman et al. (1989). The model is then used for a detailed synoptic analysis of the storm and the related variations in the isotopic composition of the precipitation. In this way, it can be evaluated how different processes, e.g., large scale air mass advection and microphysical conditions during cloud formation, contribute to the isotopic variability. Ultimately, this research aims at reversing these relationships and using the isotopic information as additional constraints on the (simulated) physical processes in the storm.

Requirements: Computer-based modeling and analysis is essential for this project, and enthusiasm for dynamical meteorology is helpful.

Supervision: Stephan Pfahl

Detection of the planetary boundary layer by remote sensing instruments

The troposphere can be divided into the free troposphere (FT) and the planetary boundary layer (PBL) that is also called mixing layer. PBL is the lowest part of the troposphere, which is directly influenced by the earth surface, its roughness and topography, responding with moisture, energy and chemical fluxes within less than one hour. The boundary layer thickness, underlying a diurnal and annual cycle, varies from tens to hundreds of meter during periods with high stable stratification to four kilometers or more. The study of mass and energy exchanges within the PBL is important for the weather forecast and dispersion models.

A measurement campaign has shown that several remote sensing instruments can be used to detect the PBL height even in mountainous environments. Several of these instruments (windprofiler, radiometer, ceilometer, lidar) are operationally used by MeteoSwiss at several measuring sites. The algorithms to derive the PBL height from these instruments has to be improved and comparison between the various results have to be performed. Finally a comparison with the PBL height calculated by the weather forecast model COSMO2 should allow to modify/validate the COSMO model.

Description of the tasks :
• Improve the PBL height detection methods in order to have operational algorithms for various remote sensing instruments.
• Scientific analysis of the limits of each algorithms/measuring method to detect the PBL height and comparison of the various instruments.
• Comparison between the experimental PBL height detection with COSMO model results.
• characterize the PBL height on the Swiss plateau for various meteorological conditions.

Requirements: The candidate should have good programming skills, preferably in Matlab, and a strong interest in data processing, measurement techniques and meteorology.

Supervision: Martin Collaud (MeteoSwiss) [Contact at IACETH: Michael Sprenger and Heini Wernli]

Working Place: the aerological station of Payerne

Climatology of diabatic Rossby waves

Diabatic Rossby waves (DRWs) are shallow extratropical cyclones at lower tropospheric levels that maintain themself by continuous condensational heating. Necessary conditions for the DRW mechanism to operate are the presence of a baroclinic zone and sufficient moisture availability. The quantity that characterizes a DRW is a positive anomaly of potential vorticity (PV) at low levels. The continous moist-diabatic PV production (by condensational heating) at the DRW's front side results in a rapid propagation speed of the DRW. Studies of strongly intensifying extratropical cyclones revealed that DRWs are often involved in causing extreme weather events like strong wind and heavy precipitation.

In this study the global ERAinterim reanalysis data set will be used to create a climatology of DRWs. An analysis tool for DRW tracking that is already available will be applied to estimate the climatological frequency and the geographical distribution of the DRW occurrence. It will be examined how many of them undergo a strong pressure drop leading to an intense extratropical cyclone. In addition, an investigation of the intensification mechanism, which involves the interaction of the low-level PV vortex with (pre-existing?) tropopause-level troughs will also be performed.

Requirements: Basic knowledge and interest in atmospheric dynamics is required for this study. Enthusiasm and flair for computer work and programming will be very helpful.

Supervision: Maxi Böttcher and Heini Wernli

Clear air turbulence

Clear air turbulence (CAT) near the tropopause has relevance for aviation but also for cross-tropopause transport of chemical constituents. In this project, we intend to study the flow situations associated with CAT at different scales and to assess how well the COSMO NWP model is able to simulate these CAT events.

Several different aspects of CAT could be studied in this Master thesis:

1) a climatology of CAT events over Europe, based on COSMO analysis. The events are characterised with respect to their spatial and temporal structure and a link is made to the synoptic- and mesoscale flow, e.g. the position of CAT relative to the jet stream has to be studied. Further questions: Are the events somehow linked to the Alps? What is the life cycle of turbulent events?

2) based on Pilot report reports (PIREPs) of CAT over the US the following questions should be studied: Which particular synoptic- and mesoscale flow patterns are found near turbulence outbreaks? And more specifically, how do Rossby wave breaking events correlate with turbulence events? How do turbulence events based on reports over the US correlate to tropopause characteristics (e.g., tropopause height, strength, gradient of height) and where are they located relative to the tropopause (altitude above or below)?

3) predictability of CAT in the COSMO NWP model. To this aim, CAT events detected from Flight Data Monitoring (FDM) onboard SWISS airplanes will be used and compared to COSMO deduced CAT indices, including a very sophisticated CAT indicator developed at NCAR. A statistical analysis will be performed to assess the quality of the COSMO forecasts and to improve the CAT forecasts. Furthermore, the CAT forecasts from COSMO simulations with different lead times will be compared and thus the predictability be assessed.

Collaboration: The project is in collaboration with Dr. Robert Sharman from NCAR (National Center for Atmospheric Research) in Colorado, US; CAT events over Europe will be provided by

Requirements: Basic knowledge of dynamics is required for this project. Computer-based analysis plays also a key role in this project. Enthusiasm for synoptic meteorology and computer work is very helpful. The analysis will be done with Fortran and Matlab; some background would be helpful, but is not mandatory.

Supervision: Michael Sprenger, Vanessa Stauch (MeteoSwiss), Bob Sharman (NCAR)