Microphysical and dynamical interactions between aerosols and clouds are associated with some of the largest uncertainties in projections of future climate. Many possible aerosol effects on clouds have been suggested, but large uncertainties remain. In order to improve model projections of future climate, it is essential that we improve our quantitative understanding of anthropogenic aerosol effects. Several studies investigating interactions between satellite-observed aerosol and cloud properties have been published in recent years. However, the observed relationships are not necessarily due to aerosol effects on clouds. They may be due to cloud and precipitation effects on aerosol, meteorological covariation, observational data errors or methodological errors. An analysis of methodological errors arising through climatological spatial gradients is performed. For region sizes larger than 4°×4°, commonly used in the literature, spurious spatial variations in retrieved cloud and aerosol properties are found to introduce widespread significant errors to calculations of aerosol-cloud relationships. Small scale analysis prior to error-weighted aggregation to larger region sizes is recommended. Appropriate ways of quantifying relationships between aerosol optical depth (τ) and cloud properties are considered, and results are presented for three satellite datasets. There is much disagreement in observed relationships between τ and liquid cloud droplet number concentration and between τ and liquid cloud droplet effective radius, particularly over land. However, all three satellite datasets are in agreement about strong positive relationships between τ and cloud top height and between τ and cloud fraction (f_c). Using reanalysis τ data, which are less affected by retrieval artifacts, it is suggested that a large part of the observed f_c-τ signal may be due to cloud contamination of τ. General circulation model simulations further demonstrate that positive f_c-τ relationships may primarily arise due to covariation with relative humidity, and that negative f_c-τ relationships may arise due to scavenging of aerosol by precipitation. A new method of investigating the contribution of meteorological covariation to the observed relationships is introduced. Extratropical cyclone storm-centric composites of retrieved aerosol and cloud properties are investigated. A storm-centric description of the synoptics is found to be capable of explaining spurious f_c-τ relationships, although the spurious relationships explained are considerably smaller than observed relationships.

The Diviner Lunar Radiometer is a nine-channel radiometer on board the Lunar Reconnaissance Orbiter, launched in June 2009 and currently orbiting the Moon. Diviner is the first orbiting multi-spectral instrument to observe the lunar surface in the mid- and far- infrared, therefore this thesis details the first steps towards utilising observations in this new spectral region. The author, as part of the Diviner instrument team, contributed to the pre-launch testing and calibration of Diviner. Specifically, the transmission profiles of the B2 and B3 filters, which could not be measured after integration into the instrument due to their long wavelength passbands, were determined. Further investigations of the far-infrared mesh filters were also conducted, as the spectral responses of similar mesh filters were found to have broadened after integration into the Mars Climate Sounder instrument, the pre-cursor instrument to Diviner. To test if this would occur in Diviner also, a new setup was made to approximately re-create the instruments' optical layout. No such spectral broadening was measured in the B1, B2 or B3 filters. The linearities of Diviner's detectors were also confirmed through analysis of the pre-flight calibration data. Laboratory emission spectra taken under ambient conditions differ from those taken in a lunar environment, which induces a temperature gradient in soil and mineral samples. Due to a lack of spectra measured under such conditions, which are directly comparable to Diviner‟s measurements, a new chamber for performing high-resolution emission measurements in a simulated lunar environment was built and calibrated. This setup induces a temperature gradient in each sample by heating it from below while it is surrounded by a cooled radiation shield, all enclosed in a very low pressure vacuum chamber. Lunar analogue minerals have been measured, first in reflectance, and then in simulated lunar conditions, forming the basis of a new lunar spectral library. The uses of the mid- and far-infrared have been described herein, utilising calibrated Diviner observations to: [1] map the temperatures of the lunar poles; [2] investigate regions containing higher than average abundances of rocks; and [3] determine the composition of regions of the lunar surface by combining observations with the new spectral library made during this project.

Research during the last two decades has established that variability of the winter polar stratospheric vortex can significantly influence the troposphere, affecting the likelihood of extreme weather events and the skill of long-range weather forecasts. This influence is particularly strong following the rapid breakdown of the vortex in events known as sudden stratospheric warmings (SSWs). This thesis addresses some outstanding issues in our understanding of the dynamics of this stratospheric variability and its influence on the troposphere. First, a geometrical method is developed to characterise two-dimensional polar vortex variability. This method is also able to identify types of SSW in which the vortex is displaced from the pole and those in which it is split in two; known as displaced and split vortex events. It shown to capture vortex variability at least as well as previous methods, but has the advantage of being easily applicable to climate model simulations. This method is subsequently applied to 13 stratosphere-resolving climate models. Almost all models show split vortex events as barotropic and displaced vortex events as baroclinic; a difference also seen in observational reanalysis data. This supports the idea that split vortex events are caused by a resonant excitation of the barotropic mode. Models show consistent differences in the surface response to split and displaced vortex events which do not project stongly onto the annular mode. However, these differences are approximately co-located with lower stratospheric anomalies, suggesting that a local adjustment to stratospheric potential vorticity anomalies is the mechanism behind the different surface responses. Finally, the predictability of the polar stratosphere and its influence on the troposphere is assessed in a stratosphere-resolving seasonal forecast system. Little skill is found in the prediction of the strength of the Northern Hemisphere vortex at lead times beyond one month. However, much greater skill is found for the Southern Hemisphere vortex during austral spring. This allows for forecasts of interannual changes in ozone depletion to be inferred at lead times much beyond previous forecasts. It is further demonstrated that this stratospheric skill descends with time and leads to an enhanced surface skill at lead times of more than three months.

This thesis describes an investigation of, and improvements to, the microphysical modelling of aerosols in the Oxford-Rutherford Appleton Laboratory Aerosol and Clouds retrieval (ORAC), which is used to obtain aerosol properties from measurements by the Advanced Along Track Scanning Radiometer (AATSR). Modelling decisions determine the light scattering properties of the aerosol classes which in turn alter the retrieved aerosol properties: aerosol optical depth, and effective radius. The maritime, mineral dust, urban, and biomass burning aerosol classes were first investigated, and then improvements implemented. Major additions to the scheme include the ability to model non-spherical dust as spheroids, soot as fractal aggregates, and to coat spherical particles with an extra layer of differing refractive index (whose thickness can be modified by ambient relative humidity where necessary). Output from aerosol retrievals containing these new models is presented. Modelling of marine aerosol was found to be adequate, but an improvement in the relative humidity assumptions led to an average 5 % increase in aerosol optical depth (AOD). Modelling of mineral dust aerosols has been dramatically altered by the addition of non-spherical dust and hygroscopic particles, leading to increases in measured AOD of over 100 % during dust events, compared to the previous model. Measurement of biomass burning aerosol has been tested with an `ageing' aerosol scheme, leading to increases in over-land measured AOD of 0.14 (~50 % increase). With such significant changes in AOD, representation of aerosol light scattering properties is seen to be important factor in the accuracy of the ORAC scheme. Finally, a method of optimising the placement of detectors in an aerosol measurement device is presented.

Recent spectroscopic observations of transiting exoplanets have permitted the derivation of the thermal structure and molecular abundances of H₂O, CO, CO₂, CH₄, metallic oxides and alkali metals in these extreme atmospheres. Here, for the first time, a fully-fledged retrieval algorithm has been applied to exoplanet spectra to determine the thermal structure and composition. The development of a suite of radiative transfer and retrieval tools for exoplanet atmospheres is described, building upon an optimal estimation retrieval algorithm extensively used in solar system studies. Firstly, the collection of molecular line lists and the pre-tabulation of the absorption coefficients (k-distribution tables) for high temperature application are discussed. Secondly, the best-fit spectra for hot Jupiters are demonstrated and discussed case by case. Available sets of primary and secondary transit observations of exoplanets are used to retrieve atmospheric properties from these spectra, quantifying the limits of our knowledge of exoplanetary atmospheres based on the current quality of the data. The contribution functions and the vertical sensitivity to the molecules are fully utilised to interpret these spectra, probing the structure and composition of the atmosphere. Finally, the retrievals provide our best estimates of the thermal and compositional structure to date, using the covariance matrices to properly assess the degeneracy between different parameters and the uncertainties on derived quantities for the first time. This sheds light on the range of diverse interpretations offered by other authors so far, and allows us to scrutinise further atmospheric features by maximising the capability of the current retrieval algorithm and to demonstrate the need for broadband spectroscopy from future missions.

Numerical simulations of atmospheric and oceanic flows are fundamentally limited by a lack of model resolution. This thesis describes the application of unstructured mesh finite element methods to geophysical fluid dynamics simulations. These methods permit the mesh resolution to be concentrated in regions of relatively increased dynamical importance. Dynamic mesh adaptivity can further be used to maintain an optimised mesh even as the flow develops. Hence unstructured dynamic mesh adaptive methods have the potential to enable efficient simulations of high Reynolds number flows in complex geometries. In this thesis, the thermally driven rotating annulus is used to test these numerical methods. This system is a classic laboratory scale analogue for large scale geophysical flows. The thermally driven rotating annulus has a long history of experimental and numerical research, and hence it is ideally suited for the validation of new numerical methods. For geophysical systems there is a leading order balance between the Coriolis and buoyancy accelerations and the pressure gradient acceleration: geostrophic and hydrostatic balance. It is essential that any numerical model for these systems is able to represent these balances accurately. In this thesis a balanced pressure decomposition method is described, whereby the pressure is decomposed into a ``balanced'' component associated with the Coriolis and buoyancy accelerations, and a ``residual'' component associated with other forcings and that enforces incompressibility. It is demonstrated that this method can be used to enable a more accurate representation of geostrophic and hydrostatic balance in finite element modelling. Furthermore, when applying dynamic mesh adaptivity, there is a further potential for imbalance injection by the mesh optimisation procedure. This issue is tested in the context of shallow-water ocean modelling. For the linearised system on an $f$-plane, and with a steady balance permitting numerical discretisation, an interpolant is formulated that guarantees that a steady and balanced state remains steady and in balance after interpolation onto an arbitrary target mesh. The application of unstructured dynamic mesh adaptive methods to the thermally driven rotating annulus is presented. Fixed structured mesh finite element simulations are conducted, and compared against a finite difference model and against experiment. Further dynamic mesh adaptive simulations are then conducted, and compared against the structured mesh simulations. These tests are used to identify weaknesses in the application of dynamic mesh adaptivity to geophysical systems. The simulations are extended to a more challenging system: the thermally driven rotating annulus at high Taylor number and with sloping base and lid topography. Analysis of the high Taylor number simulations reveals a direct energy transfer from the eddies to the mean flow, confirming the results of previous experimental work.

A number of recent studies have found a strong link between peak global warming due to anthropogenic carbon dioxide and cumulative carbon emissions from the start of the industrial revolution. This thesis builds on this work by using a simple climate model to apply the concept of cumulative emissions to emission floors, by comparing cumulative emissions with other types of emissions target, and by extending the work to apply to noncarbon dioxide (CO₂) greenhouse gases and short-lived climate forcers (SLCFs). Though peak global warming correlates well with cumulative carbon emissions, the link to emissions over shorter periods or in the years 2020 or 2050 is shown to be weaker. It is also shown that the introduction of emissions floors does not reduce the importance of cumulative emissions, but may make some warming targets unachievable. For pathways that give a most likely warming up to about 4°C, cumulative emissions from pre-industrial times to year 2200 correlate strongly with most likely resultant peak warming in the simple model used, regardless of the type of emissions floor used. The maximum rate of CO2- induced warming is not determined by cumulative emissions but is shown to be limited by the peak rate of CO₂ emissions. A simple model of non-CO₂ greenhouse gases is also developed and used to investigate SLCFs. It is shown that emissions of SLCFs today have little impact on peak warming, and that delaying near-term reductions in SLCFs would not have a significant impact on peak warming. Only once CO₂ emissions are falling do SLCF emissions have a significant impact on peak warming. A global climate policy framework is presented as an example of how the work in this thesis could be used in policy. Future work is also discussed, particularly verification of these results in a more complex model.

No description available.

The ice giants, Uranus and Neptune, are the two outermost planets in our solar system. With only one satellite flyby each in the late 1980’s, the ice giants are arguably the least understood of the planets orbiting the Sun. A better understanding of these planets’ atmospheres will not only help satisfy the natural scientific curiosity we have about these distant spheres of gas, but also might provide insight into the dynamics and meteorology of our own planet’s atmosphere. Two new ground-based, near-infrared datasets of the ice giants are studied. Both datasets provide data in a portion of the electromagnetic spectrum that provides good constraint on the size of small scattering particles in the atmospheres’ clouds and haze layers. The broad extent of both telescopes’ spectral coverage allows characterisation of these small particles for a wide range of wavelengths. Both datasets also provide coverage of the 825 nm collision-induced hydrogen-absorption feature, allowing us to disentangle the latitudinal variation of CH4 abundance from the height and vertical extent of clouds in the upper troposphere. A two-cloud model is successfully fitted to IRTF SpeX Uranus data, parameterising both clouds with base altitude, fractional scale height, and total opacity. An optically thick, vertically thin cloud with a base pressure of 1.6 bar, tallest in the midlatitudes, shows strong preference for scattering particles of 1.35 μm radii. Above this cloud lies an optically thin, vertically extended haze extending upward from 1.0 bar and consistent with particles of 0.10 μm radii. An equatorial enrichment of methane abundance and a lower cloud of constant vertical thickness was shown to exist using two independent methods of analysis. Data from Palomar SWIFT of three different latitude regions.

The Composite Infrared Spectrometer (CIRS) instrument onboard the Cassini spacecraft will be used to probe the atmosphere and surface of Saturn's giant moon Titan. This thesis describes an investigation of the capabilities of CIRS as a remote sounding instrument. To enable infrared spectra to be computed, a radiative transfer code has been adapted for Titan's atmosphere. The atmospheric model, including gases and aerosol particles, was refined by comparison of synthetic spectra with results from the IRIS instrument of the Voyager 1 spacecraft. Characteristics of the instrument have been deduced from laboratory measurements. The size and shape of the field of view was found for the mid-infrared detectors. A Fourier code was developed to transform the raw data (interferograms). Blackbody spectra taken with the flight instrument were analysed to calculate the noise equivalent radiance for the detectors of all three focal planes. Finally, the data regarding instrument performance was used in combination with the predictive radiative transfer code to consider in detail the extent to which gaseous bands and other spectral features will be observable for a variety of limb and nadir viewing modes. Current observing strategies are reviewed and recommendations for scientific emphasis in the light of the actual instrument performance are made.