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With the development of ensemble forecasting, operational meteorologists are faced with large amounts of constantly updating complex information which they must quickly interpret to issue forecasts and warnings. In this thesis a novel clustering technique is introduced that reduces ensemble forecasts to a few representative forecast trajectories. Clustering is performed using k-medoids with the distance metric defined by the Fractions Skill Score (FSS) of the gradient in 850hPa wet-bulb potential temperature to group ensemble members with similar frontal features. The number of clusters is selected using lead-time-coherence of the clusters over a window of interest when clustering is most distinct. Members nearest to the centre of each cluster during this window of interest are chosen as representative members to be viewed by forecasters. Clustering is found to be more coherent during low predictability events when ensemble spread is large. The clustering method was compared to an alternative that uses the FSS of large-scale rain rate and it was found that while similar, results are not interchangeable. The gradient of wet-bulb potential temperature had higher time-coherence and therefore was judged preferable. The method was evaluated during the Met Office winter testbed of 2021-22, and representative members found were found to correspond well to forecasters judgement of the distinct scenarios in the ensemble, hence providing a useful reduction in the data that needs to be considered in issuing forecasts. The method draws attention to low predictability events that appear across several forecasts. While this method has been created to fill a need with ensemble forecasting, it is anticipated that it can be used in many other areas of research such as identifying circulation patterns, seasonal and climate forecast trajectories, and exploring different meteorological phenomena by modifying variable choice and other parameters. The method is also planned for use at the Met Office.

Extreme weather events are often the result of slow-moving, large-scale wave patterns. Greater understanding of these large-scale modes of variability would allow us to anticipate how quasi-stationary modes might depend on changes to the background state in future climate. In this thesis the Empirical Normal Mode (ENM) technique, a technique for extracting dynamical modes of variability from atmospheric timeseries data, is utilised to examine the dependence of mode structure and frequency on jet latitude. The ENM methodology is extended to include the lower troposphere spanned by isentropic surfaces that can intersect the ground. This involves careful accounting of the terms in large amplitude pseudomomentum and pseudoenergy associated with this region of the atmosphere - terms that contribute to the "boundary wave activity" in the limit of small amplitude. In the third chapter, the implementation of the technique itself is validated by testing the characteristic 'intrinsic' phase speed of the ENMs against an empirical phase speed derived from the modes' principal component timeseries, using a set of idealised model experiments simulated using the Reading IGCM2.2. It is found that the phase speed matching conditions are met for the dominant freely propagating baroclinic modes, validating the approach to the calculation of wave activity and some approximations used in deriving relevant wave activity norms. In the fourth chapter, a new series of idealised experiments are devised that possess a jetstream with controllable latitude such that the change in behaviour of the modes of variability with a shift in jet latitude may be examined. The initial and relaxation temperature field in thermal wind balance with a prescribed zonal wind field with jet latitudes ranging from ∼ 40° to ∼ 65° is found, and a sloping tropopause is added in order to maintain baroclinicity. Subsequently, in the fifth chapter, the ENM structures of these experiments are found, and the change in the phase speed of the modes as the jet latitude changes is explored. A quasi-stationary branch of modes is identified which is associated with the most perturbation energy (for each zonal wavenumber) and therefore can propagate most strongly westwards against the background state westerly flow. As the jet is shifted polewards, the wavelength of the most energetic modes remains approximately the same, but they shift to lower zonal wavenumbers due to reduction in the latitude circle circumference.