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Tropical stratosphere-troposphere exchange is examined by the use of water vapour data and trajectory calculations. Water vapour data from the Microwave Limb Sounder (MLS) instrument and two-dimensional trajectories calculated from the assimilated data of the United Kingdom Meteorological Office (UKMO) are used to examine the evolution of water vapour upon iseiltropes near the 68 hPa surface. Three-dimensional trajectories calculated from the assimilated data of the European Centre for Medium-Range Forecasts (ECMWF) are used to examine the flow of air across the tropopause. Combination of the MLS and UKIVIO data allows the construction of purelyisentropic evolutions of water vapour. On tirnescales less than those characteristic of diffusion and of methane oxidation, comparisons of these evolutions to the observed cases can be used to show where vertical motions must cross isentropic surfaces. As such, a comparison of monthly-averaged, water-vapour maps on isentropic surfaces near 68 hPa to monthly-averaged maps of the same data after ten-day iserltrol)icadvections is presented for most of the operation time of the 183 GHz antenna of the MLS instrument November 1992 to March 1993. The comparison shows that the two evolutions rarely match each other and thins that vertical airflows must be present over most of the tropical region, throughout the year, near 68 hPa. The use of three-dimensional trajectories allows for a more-direct and more-detailed analysis of tropical, cross-tropopause motion. Though the lesser reliability of vertical wind-data does mean that only those three-dimensional-trajectory results that were in basic agreement with the results of the two-dimensional case described above could be considered useful, such an agreement was generally found during this study. Three-dimensional trajectories are presented to show that though cross- trop opause motion does occur throughout the tropical region a persistent longitudinal-preference in the motion existed for all times investigated the winters of 1980, 1981 and 1993 and all seasons of 1992. Crossing frequencies are shown to be much higher over the mid-Pacific throughout the year and much lower over the Indonesian and Asian monsoons, both extrema being visible at the ECMWF model-levels at 110 and 90 hiPa. The peak crossing was stronger during the 1993 winter than in any other winter and stronger in autumn than in any other season of 1992. Furthermore, whenever the peak crossing showed an increase in magnitude it was accompanied by an increase in the concentration of particle crossings into the mid-Pacific. Taken together, the results of the two- and three- dimensional experiments also suggest that longitudinal variations in the 68 hPa, MLS water-vapour-concentrations occur not through differential uplift but by variations in the height of the tropopause.

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This thesis presents original research on the solar wind interactions with the Earth's magnetosphere, with main focus on processes occurring at the magnetopause. Such processes are often difficult to fully access observationally as their typical timescales are faster than the particle data cadence which is usually limited to the spin resolution, typically a few seconds, of the spacecraft. Here, a novel method is applied to data from the Cluster mission in order to acquire the pitch angle distribution of particles at 0.125 second per sample, the highest-temporal resolution available at the time of research described here. Two chapters of this thesis present results based on the exploitation of this high-temporal resolution dataset. The first study presents analysis of Cluster observations of the substructure of a flux transfer event (FTE), which reveals unprecedented details of the FTE. The work resolves previously unseen layers of the FTE, and provides evidence of recent reconnection in the outer layers (Varsani et al., 2014). The second study focuses on the microscale dynamics within Kelvin-Helmholtz waves at the dusk flank of the magnetopause. This analysis reveals the presence of boundary layer plasma travelling faster than the sheath (BPFTS), which appears to be the result of magnetic reconnection even though, contrary to earlier suggestions, there is no evidence of rolled-up KHI vortices. The final study conducts a survey of Cluster observations to compare the two techniques for identification of rolled-up KHI vortices: the low-density faster than sheath (LDFTS) and boundary plasma faster than sheath (BPFTS) methods. The results show that since BPFTS is more rigorously defined, based on the magnetopause transition parameter (TP), it eliminates ambiguity in the observation of the plasma depletion layer instead of the boundary layer. Furthermore, the flow directions identified by BPFTS method, are consistent with the expectations of the KHI growth criteria.

The remote marine boundary layer at the tropics represents a baseline condition for the Earth's atmosphere. Air in these regions has typically not encountered emissions for days to weeks and thus has been subject only to processing; that is degradation and reaction of species in the air. Understanding the most basic chemistry occurring in the remote marine boundary layer is fundamental to our overall understanding of the atmosphere as a whole. The source and manifestation of nitrogen oxides, NOx, in the remote marine boundary layer at the Cape Verde observatory and elsewhere has been intriguing. NOx observations could often not be reconciled easily with known chemistry. This, when coupled to the difficulty in measuring the very low concentrations present has led to various speculative theories which are thus-far unsubstantiated. By carefully characterising the NO2 photolysis/NO chemiluminescence technique for artefacts, interferences, and uncertainties the applicability and limitations of the technique were established. It was found that peroxyacyl nitrates cause an interference of ~5% which can lead to significant errors in NO2 determination, especially in cold or high altitude environments but are insignificant in the remote marine boundary layer at Cape Verde. Using two years of data collected at Cape Verde and a 0-D model of tropospheric chemistry the budget of NOx was estimated. Evidence that nitric acid deposited on aerosol, previously thought to be the terminating step in the cycle of NOx, is able to be released back to the atmosphere as NO2 and HONO has been shown. This `renoxification' process likely represents the dominant source of NOx in the tropical marine boundary layer, counter to previous theory of long range transport of reactive nitrogen species. The implication of these findings changes our fundamental understanding of NOx sources and sinks in the remote troposphere. Moreover, by understanding the limitations of NO2 measurement techniques allows for easier interpretation of sometimes puzzling data without the need for unprovable mechanisms. Additionally, a new technique for quantifying atmospheric HONO was developed by exploiting the limitations of photolytic NO2 measurement and photolysing differentially at two different wavelengths.

The atmosphere is an important pathway of particulate matter and their associated trace elements to the ocean and can play vital roles in marine bio-geochemical cycles. Factors that control trace element inputs from the atmosphere (e.g. solubility, sources) are not well understood, in particular for the South Atlantic Ocean. In this thesis, select major, trace and rare earth elements and the isotopes of two of the elements, lead (Pb) and neodymium (Nd), were investigated in potential sources of aerosols, and aerosol samples, from over the South Atlantic Ocean. Chapter 2 of this thesis presents new element concentration and isotopic compositions of potential source areas from South America and southern Africa, combined with previously published data to represent an up-to-date characterisation of natural and anthropogenic aerosol source areas in South America and southern Africa. Chapters 3 and 4 of this thesis present new element concentration and isotopic compositions of aerosol samples from across the 40°S transect of the South Atlantic Ocean (from the UK GEOTRACES GA10 transect). The results provide the first constraints on the elemental and isotopic composition of total and soluble fraction of aerosol samples across a zonal transect of the South Atlantic Ocean. By comparing the element and isotopic compositions of the aerosol samples to the potential source areas from South America we demonstrate that natural and anthropogenic sources from South America and southern Africa are major sources of atmospheric particulates, but of varied contribution, across the South Atlantic. Elemental and isotopic compositions of the aerosols suggest there is a strong impact of anthropogenic sources from South America on atmospheric particles delivered to the western South Atlantic and a strong influence of natural sources from South America on atmospheric particles delivered to the middle of the South Atlantic. We further demonstrate that there is an importance of anthropogenic southern African sources on aerosol composition over eastern South Atlantic as well as South American source inputs. In Chapter 4 we reveal that large ranges in trace element solubility (between < 5 and > 95%, generally above 40% across the ocean) in aerosols carried to the South Atlantic may be available for seawater dissolution and may have implications on marine biogeochemical cycles. Overall the present thesis underlines the important role that anthropogenic source areas can play on atmospheric composition, and provide for the first time measurements of major, trace and rare earth element availability to the surface ocean of the South Atlantic. The thesis also highlights the key role that multi-element concentrations and Pb and Nd isotopic compositions can play in deciphering the provenance of aerosols and element seawater supply.

The circulation induced by a steady uniform stress acting on the surface of a liquid in a closed basin of uniform depth is considered for Reynolds Numbers (surface current x depth) (dynamic viscosity) from 0-400. The main work is the numerical solution of the two—dimensional Navier Stokes equation for circulation in a vertical plane parallel to the wind stress. It is found that the circulation near the two ends of the basin is very different. At the windward end, it becomes vary slack, while at the leeward end, oscillations occur which decrease in amplitude from an end maximum (eddy) near the wall. These oscillations extend further from the wall, and increase in amplitude as the Reynolds Number it increased. At about Reynolds Number 500, they lead to reversed flow in the lee of the end eddy, and imply instability of the steady—state flow. At higher Reynolds Numbers, steady—state solutions still appear to be possible, but for finite basins the ' effects of the end walls and of the side walls will interact to give a. necessarily three—dimensional and complex streamline field.. The Numerical method of solution, which becomes progressively more difficult to apply as the Reynolds Number is increased, is basically the relaxation of finite difference approximations to the vorticity equation by the Liebmann process, using an electronic computer. The vorticity equation has also been solved in special cases by a series expansion. in the last Chapter on the grounds of similarity theory, and the rather sparse experimental evidence, the results are qualitatively applied to tho mean lake circulation.

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This thesis examines the behaviour of Indian monsoon depressions (MDs), synopticscale systems that frequent the monsoon trough throughout the boreal summer. The original contribution to knowledge is a substantial advancement of our understanding on the structure, variability, and dynamics of MDs. A feature-tracking algorithm is developed to emulate the subjective methods used by the India Meteorological Department, based on surface wind speed and surface pressure. This is applied to reanalysis data spanning from 1979 to the present day (2016), extracting 106 events. These are used to build a statistical composite, with which we interrogate the structure and variability subject to external forcings. It is found, for example, that MDs significantly intensify during La Ni˜na and active monsoon spells, and have different spatial structure over ocean than land. These events are also examined in satellite data, where we discover and characterise a bimodal, diurnal cycle in surface precipitation; the structure of cloud type with dominating deep convection south of the centre; and present the first composite picture of vertical hydrometeor structure in MDs. A propagation mechanism whereby the MD behaves as a vortex in the presence of a wall (i.e. the Himalayan massif) is presented and shown to perform better than competing theories at predicting the velocity and heading of MDs. It is also shown, using a case study in the Met Office Unified Model (MetUM), that varying antecedent soil moisture conditions can significantly change the length of a MD track but not its direction. The sensitivity of MDs to changes in horizontal resolution in the MetUM are also explored across seven case studies at eight resolutions. It is found that the intensity is typically slightly overpredicted, correlating with too great a latent heat release in the mid-troposphere. Spatial structure is shown to improve with resolution but improvement saturates beyond N512 (40 km) resolution.

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