This thesis examines tracer transport at mid- and high northern latitudes in the lower stratosphere during early 1992, using lidar observations of aerosol from the eruption of Mount Pinatubo and coordinate fields based on meteorological analyses. The errors and resolution of the measurement, the spatial and temporal coverage made by the observations and the specific behaviour of the constituent being measured are all important. The lidar observations used are of relatively high resolution, and offer good coverage of the mid- and high latitude stratosphere during early 1992. The suitability of the aerosol lidar data to tracer transport studies is discussed, in terms of aerosol microphysics and the properties of the specific measurement. The random error in the measurement is assessed as part of the analysis. In all the analyses, care is taken to avoid spatial averaging of the data, since that can lead to an overestimate of the mixing in the real atmosphere. The northern midlatitude aerosol distribution is found to be relatively stable by early 1992. Transport to high latitude is investigated, with some variability demonstrated as a function of time and potential temperature, covering the region from 520K down to 350K. This is the first time that such a large number of reasonably high resolution, mid- and high latitude observations have been analysed without averaging, to demonstrate irreversible winter-time poleward transport of a tracer to northern high latitude. The vortex edge data near 500K is examined further, using a novel statistical approach. The random error in the equivalent latitude coordinate is quantified for this region, and detailed aerosol tracer transport inferred.

The objective of this dissertation is to use the annual tree ring width and cellulose carbon and hydrogen stable compositions off three native British tree species: common beech (<I>Fagus silvaatica</I> L.), pedunculate oak (<I>Quercus robur</I> L.) and scots pine (<I>Pinus sylvestris</I> L.), for the period 1895 to 1994, to examine the responses of these trees to recent changes in climate and atmospheric CO<SUB>2</SUB>. The high frequency δ<SUP>13</SUP>C series of all three species display the most significant correlations with the climate parameters and, using simple regression models, it is also evident that this tree ring measure provides the most accurate climate reconstructions. Vapour pressure deficit, averaged for the months July to September, is the climate parameter most significantly correlated with the δ<SUP>13</SUP>C data. However, it is shown that this may not necssarity mean that the trees are responding dominantly to this parameter. Although it is apparent that the pine species δ<SUP>13</SUP>C series is the most responsive of the species to climate fluctuations, it is also noted that the combined δ<SUP>13</SUP>C signal from all three tree species displays an enhanced climate signal. Superimposed on the common climate signals are long term trends that are comparable for the δ<SUP>13</SUP>C series of all three species and the δD of the pine trees, but do not show any common signal for the other δD series or any of the ring width series. Converting the δ<SUP>13</SUP>C data to indicators of leaf gas exchange, it is evident that the rapid increasing trend in atmospheric CO<SUB>2</SUB> concentration ([CO<SUB>2</SUB>]), that began around 1930, coincides with a rising trend in leaf intrinsic water use efficiency but does not coincide with a change in the internal concentration of CO<SUB>2</SUB> in the leaf (c<SUB>i</SUB>). The results suggest that the increasing [CO<SUB>2</SUB>] induced all three tree species to increase assimilation rate and reduce stomatal conductance to such an extent that c<SUB>i</SUB> remained initially constant. As there are no consistent increases in ring widths during this period, the dominant response could be inferred to be stomatal conductance. However, preferential partitioning of photosynthates to other plant organs, especially roots, is a common response of C<SUB>3</SUB> plants to increased [CO<SUB>2</SUB>] and it is more likely that both assimilation rate and stomatal conductance have changed.

The UK Met Office’s Unified Model is used at a high global resolution to assess the impact of deep tropical convection on the tropical tropopause layer (TTL) and the transport of short-lived species. The modelled horizontal and vertical structure of organised convection, as well as the distribution of tropical tropopause temperatures and humidity, is shown to match closely with satellite data. Furthermore, the structure of the TTL is found to be similar for both weather and climate setups. In weather mode, monthly integrations are performed with idealised tracers of varying lifetimes. Novel diagnostics complement classic diagnostics to reveal how efficient rapid vertical transport can be in the model. These diagnostics, so-called ‘surface-to-TTL matrices’, utilise idealised tracers that are released regionally at the surface and are monitored at given regions and heights in the TTL. Deep convection is more frequent over the tropical warm pool compared to other tropical regions for all periods analysed. Convective ‘elevators’ rapidly lift air from 4-5km up to 12-14km. Nonetheless, the influx of tropospheric air entering the TTL (11-12km) is similar for all tropical regions with most convection stopping below ~14km. Above this height, regional tracer signatures associated with deep convection become clearer. The deepest convective elevators are seen to directly inject between 10-20% of surface tracer concentrations up to 15km. However, convection almost never reaches higher than this. The tropopause is coldest and driest between November and February, coinciding with the greatest upwelling over the tropical warm pool. From here, modelled tracers are lifted from the surface up to 15-17km where they are advected eastwards before descending over the East Pacific in accordance with the Walker circulation.

Condensation of water vapour carried by buoyant atmospheric plumes is responsible for cloud formation and leads to a release of latent heat. This release provides a secondary source of buoyancy away from the origin of the plume, and field observations suggest that the laterally entraining plume model does not capture the resulting behaviour correctly. To investigate this problem in the laboratory Bhat and Narasimha (1996) reproduced analogous off-source heating by using electrodes to heat an acidic jet in a deionised ambient. It was observed that the heating significantly disrupted the eddy structure of the jet and the mass flux was drastically reduced. This dissertation describes a numerical investigation of volumetrically heated jets. The numerical approach taken employs a method known as Monotone Integrated Large Eddy Simulation (MILES). It is a natural form of large eddy simulation that can capture inherently the correct flow of energy through the inertial range, and provides physical decay at the grid-scale cut-off. The method does not use an explicit turbulence model and hence makes no assumptions about the structure of the flow. The extent to which turbulent statistics can be recovered with such an approach is first explored, in the context of homogeneous turbulence. Open shear flows are then considered, first in a temporally evolving circular shear layer. Finally, spatially evolving jets with and without off-source heating are investigated. The results confirm observations made in experimental and numerical studies, but also provide further insight. Measurements, particularly of second-order correlations, help to clarify the understanding of this flow, and identify further requirements necessary for an analytic model to capture fully the phenomena observed.

Contrail formation and the occurrence of ice supersaturation in the atmosphere (which is needed for contrails to persist) is the focus of the thesis. The global observation and prediction in numerical models of ice supersaturation are crucial if the climate impact of contrails is to be fully understood. In this thesis the research presented uses observations and models to further understand the occurrence frequency and spatial extents of ISS regions. In chapter 2, in-situ observations of RHi made using radiosondes and commercial aircraft were used to investigate, respectively, the vertical and horizontal spatial extents of ISS regions. This showed that up to ~71-90% (vertical) and ~52% (horizontal) of ISS regions will be sub grid scale compared to the coarse resolutions of modelled and observed datasets. Chapter 3 shows the use of high-resolution radiosonde datasets to provide evidence of an s-shaped relationship between average RHi and the occurrence of ice supersaturation in thick (50 and 100hPa) vertical pressure layers. Chapter 4 shows the use of Lagrangian trajectory model coupled to a cirrus cloud parameterisation (Lagrangian-cirrus model) to represent ISS regions. Results showed up to a ~62-68% improvement in the prediction of RHi compared to ECMWF analyses. In chapter 5, the Lagrangian-cirrus model was coupled to the Aero2k emissions inventory to calculate contrail coverage (of 0.05-1.65%) over the North Atlantic Flight Corridor (NAFC). These results were used with the Edwards-Slingo Radiative Forcing (RF) model to calculate an RF impact due to contrail coverage of up to a net 400mWm^-2. Overall, the studies presented in this thesis remove some of the uncertainties in persistent contrail coverage calculations and RF estimates.

The issue of climate migration has been gIven the image of a mass phenomenon threatening peace and security. This suggests that this issue has been subject of the process called securitisation, in which non-traditional security issues are discussed and/or acted upon in terms of security and thereby drawn into the security domain. This PhD thesis examines in what manner, to what extent, and with what political consequences climate migration is securitised, and analyses what explains the course of the process. Classical literature on securitisation assumes that securitisation is a straightforward process, with specific outcomes. Instead, I argue that the securitisation of climate migration is not clear-cut, but takes on various forms and meanings. I conduct such an analysis of securitisation by means of a flexible, contextualised and interactive framework for analysis. In this framework, I integrate the four schools of thought on securitisation: the Copenhagen School, the Paris School, Critical Security Studies, and the Risk School. The framework allows for a flexible application of their theoretical insights. It assumes that a securitisation process can take on various meanings, shaped by the context in which it is situated and by interaction processes between the actors involved. Through an analysis of context and of dynamic processes of interaction that shape the securitisation process, it becomes apparent which insights of the four schools can be applied to explain the securitisation process at hand. The securitisation of climate migration is examined in a study of the UK's Foreign & Commonwealth Office (FCO)'s securitising move on climate migration and the response to this move by the Government of India. In this study, I apply the framework for analysis to trace the diverse and complex manner in which a securitisation process develops. I show how the FCO used a security narrative on climate migration as a strategy to convince other countries, such as India, to sign up to binding mitigation targets under the UN climate convention. The FCO hoped that countries would sign up to binding mitigation targets when knowing that a failure to address climate change could result in climate migration. I examine how India has reacted to such arguments and demonstrate that it has worked in a counterproductive way. The FCO's securitising move compounded a difficult negotiation environment on climate change, and it risks legitimatising India's strict border measures to halt Bangladeshi immigration. As a final step, I review how this (unsuccessful) securitisation process affects the UK's and India's positions in climate change negotiations, and analyse how the FCO is moving towards an economic prosperity narrative to increase the effectiveness of its climate change diplomacy.

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