Term 1
Wednesday 2nd November, 1pm (Harrison 209)
Flossie Brown
Three years ago I asked whether volcanic eruptions that inject sulphur dioxide into the stratosphere have the potential to alter large-scale circulation patterns, and my supervisor suggested we look at the quasi-biennial oscillation (QBO), which can affect weather and transport of chemical species. We conducted simulations of tropical volcanic eruptions using the UM-UKCA aerosol-climate model with an explicit representation of the QBO. Eruptions emitting 60 Tg of SO2 (i.e., 1815 Mt. Tambora-magnitude) and 15 Tg of SO2 (i.e., 1991 Mt. Pinatubo-magnitude) were simulated at the equator initiated during two different QBO states. Two and a half year ago, we showed that tropical eruptions delay the progression of the QBO phases, with the magnitude of the delay dependent on the initial wind shear in the lower stratosphere and a much longer delay when the shear is easterly than when it is westerly. Finally, after lots of thinking and some very awesome collaborations with clever people, we have concluded that the QBO response in our model is driven by vertical advection of momentum by the stronger tropical upwelling caused by heating due to the increased volcanic sulfate aerosol loading. Direct aerosol-induced warming with subsequent thermal wind adjustment, as proposed by previous studies, is found to only play a secondary role. To prove this, Professor Peter Haynes made a simple dynamical model, which I will attempt to describe in this talk. Using the most accurate language, we would say that the dependence of the magnitude of the response on the initial QBO state results from differences in the QBO secondary circulation; in the easterly shear zone of the QBO, the vertical component of the secondary circulation is upward and reinforces the anomalous upwelling driven by volcanic aerosol heating, whereas in the westerly shear zone the vertical component is downward and opposes the aerosol-induced upwelling. However, I hope some PowerPoint graphics can convey our results to a broader audience. I also hope to give some insight into what it’s like to do a project outside of the PhD and what I’ve learnt about collaborating with other scientists.
Wednesday 16th November (Harrison 215)
Lamia Alyami
Integration of Extended Kalman filter in an epidemiological model: A robust approach to estimate the COVID-19 outbreak in Saudi Arabia.
State estimation of the nonlinear problem is a challenging task. The extended Kalman filter (EKF) is an extension of the traditional Kalman filter to non-linear systems and is famous in the nonlinear theory estimation. The description of the EKF algorithm consists of two main steps, the prediction step and the correction step. The most powerful EKF can estimate the disturbing measurements, or hidden states and an imperfect mathematical model. We propose using the EKF to estimate our proposed epidemiological model SEIQRD, which is subjected to an ODE in order to enhance the estimation process. In this talk, I will present the SEIQRD model and then EKF estimates based on the SEIQRD model. Moreover, I will talk about the nested sampling method, which has been used to estimate the model parameters; I will follow this by talking about the numerical simulation results of the active and death cases of the Saudi Arabian data which was considered in order to evaluate the proposed method.
Wednesday 23rd November (Harrison 250)
Will Beckwith-Chandler
Solar prominences – or filaments when they are observed on the solar disc – are (relatively speaking) cool, dense clouds of plasma supported by the sun’s complex magnetic field. Prominences can be observed to oscillate in a variety of ways. Such oscillations in these structures have been linked to their eruptions: the end of their lifetime. Understanding these more could help us to better-predict eruptive solar events that can cause catastrophes on Earth. In this talk, we present an overview of these structures and their properties, before showing how we can analyse the oscillations of prominences both in terms of full-disc solar flux and in terms of image data from the Solar Dynamics Observatory satellite’s AIA (Atmospheric Imaging Assembly) instrument. In addition, we will talk about how we can use images taken from different wavelength channels to deduce regional properties such as the temperature and emission measure of the structure, and how these can link back into the oscillatory behaviour.
Wednesday 30th November, 1:30-2:30pm (Harrison 250)
Madhuparna Das
Exponential sums have important significance in Analytic Number Theory. It is interesting to study the non-trivial upper bound for the exponential sums over primes. Finally, we give examples of the application of such bounds for three primes theorem and the zero free region of the Riemann zeta function.
Wednesday 7th December (Harrison 250)
Azza Al Gatheem
Nonlinear evolution in two-dimensional incompressible MHD flows
Magnetic fields are present in astrophysical bodies such as the Sun. Our research examines the effect of magnetic fields on these systems, extending the work of Meshalkin & Sini (1961) and Manfroi & Young (2002). The results will be useful for understanding the effects of the magnetic field in more turbulent regimes, although this study is concerned with the instabilities associated with classical laminar flow. We focus on the role played by the magnetic field in modifying the stability properties of tachoclines. Numerical analysis is performed on a two-dimensional incompressible flow. In the absence of magnetic fields, nonlinear interactions of Rossby waves lead to the formation of strong zonal flows. Nevertheless, even a very weak field suppresses Kolmogorov flow generation. Thus, the magnetic field in the tachocline is significantly affected by this phenomenon.
Initially, a standard linear analysis is carried out, in which growth rate bounds, stability criteria, and MHD effects are derived, and instabilities are investigated associated with idealised vortex structures. The nonlinear development of the instabilities associated with the velocity profiles is then investigated numerically, focusing first on the changes to the nonlinear evolution resulting from investigating the differences caused by the critical Reynolds number before MHD effects. Lastly, we investigate the interaction between MHD and flow effects during nonlinear evolution.
Wednesday 14th December (Harrison 250)
Harris Sop Nkuiate
Evaluating probabilistic forecasts in the presence of observation error
Observation error causes proper scoring rules, which are used to measure the performance of probabilistic forecasts, to favour good forecast of the observation rather than the truth, and yield scores that vary with the quality of the observations. Proper scoring rules can therefore favour worse forecast of the truth and can mask real changes in forecast performance if observation quality varies over time.
Ferro (2017) proposed a framework to solve these problems by producing unbiased estimates of the score that would be obtained if the forecast could be verified against the truth. He then used the formalism to construct error-corrected scores for categorical predictands. This work is aimed at extending Ferro’s framework to continuous predictands such as temperature. In the case of continuous predictands, the problem of finding unbiased estimates given an error distribution reduces to solving a Fredholm integral equation of the first kind.
We proposed a general method for constructing unbiased scoring rules for continuous outcomes in the presence of additive Gaussian error and multiplicative Gamma error using the theory of integral equations. Also, an extensive analysis of the existence and uniqueness of solutions for these integral equations is presented. Furthermore, our error-corrected scores are compared with those proposed by Bessac and Naveau (2021); and the effects of observation error on the mean and ranking of forecasts is explored for the different scoring rules.