Observational trends, physical reasons and modelling results suggest an increase in extreme precipitation with climate warming. In particular, sub-daily precipitation extremes are expected to increase heavily raising concerns about the future impacts of flash floods in urban environments and for small or steep river catchments. In order to quantify the potential risk of flash floods in the future, impact studies often require site-specific sub-daily estimates of precipitation extremes. But in their current stage, most Regional Climate Models (RCMs) are only able to provide areal averaged projections at ca. 12.5km resolution and simulated sub-daily precipitation extremes tend to be heavily biased. As a result, statistical downscaling methods are needed to provide site-specific more reliable projections of sub-daily precipitation extremes. In this thesis, a statistical downscaling method was developed to project site-specific future hourly precipitation extremes over the UK. Circulation patterns (CPs) were classified using a fuzzy rules based approach to categorize extreme hourly precipitation events according to their corresponding atmospheric conditions. In a next step, an analogue day method was applied to find the most similar day in the past by comparing the RCM simulated daily precipitation and temperature with the observations for each CP. The daily maximum hourly precipitation record on the most similar day was extracted and perturbed based on precipitation duration-temperature relationships conditioned on CPs. Within the field of statistical downscaling techniques, the applied method is best described as a hybrid of the analogue and the regression-based method. It was shown that the method is capable of reproducing observed extreme hourly precipitation over different validation periods. Projections based on the applied statistical downscaling method indicate increases in UK hourly extremes but with high variations depending on the twelve different stations, the two future time periods, the two emission scenarios and the four different GCM-driven RCMs.

This thesis presents the studies of terrestrial planetary atmospheric circula- tion regimes using simplified GCMs with different levels of complexity. Two different versions of the simplified GCM PUMA (Portable University Model for the Atmosphere) are used — PUMA-S with Newtonian cooling scheme and PUMA-G with a semi-grey two-band radiative transfer scheme and dry convective adjustment. A series of controlled experiments are conducted by varying planetary rotation rate and imposed equator-to-pole temperature dif- ference using PUMAS-S, and by varying rotation rate, planetary obliquity, and the ratio of optical depth in long-wave (thermal radiation) band to that in short-wave (stellar radiation) band using PUMA-G. These defining parame- ters are further combined with each other into dimensionless forms to establish parameter spaces, in which the occurences of different circulation regimes are mapped and classified. For the PUMA-S experiments, very coherent trends when varying planetary rotation rate (thermal Rossby number) is found. It is demonstrated that the GRW mechanism is mainly responsible for the equato- rial super-rotation observed in our experiments. Regular baroclinic waves are obtained at intermediate values of thermal Rossby number and depend strongly on the strength of radiative and frictional damping. Global atmospheric ener- getics in terms of Lorenz energy cycle and meridional heat transport efficiency also exhibits strong dependence on planetary rotation rate from our PUMA-S experiments. Theories of geostrophic turbulence (especially the recently in- troduced zonostrophic turbulence) and jet formation are examined using the PUMA-S experiments. For the PUMA-G experiments, Similar trends are ob- served with respect to varying planetary rotation rate, while new regimes like strongly subrotating atmospheres are found when varying obliquity in PUMA- G. Tidally-locked planets are also studied by modifying the incoming stellar irradiation in PUMA-G. It is found that atmospheric optical depth in the long- wave band plays an important role in setting the heat transport efficiency from day-side to night-side. These results provide significant insights into the ter- restrial planetary atmospheric circulation dynamics and the inference of circu- lation regimes of extrasolar planets. Future studies will focus on the effect of seasonal/diurnal cycle, the parametrisation of eddy heat transport efficiency, as well as the modification of the two-band semi-grey radiative transfer scheme to incorporate pressure broadening effects

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This thesis examines the spatial and temporal expression of Holocene glacier fluctuations in southeast Iceland. The study uses geomorphological evidence to reconstruct the former extent of Lambatungnajökull -a non-surging, valley glacier flowing from the eastern flank of the Vatnajökull ice-cap. Lichenometry is used to date recent glacial landforms and decode the pattern of glacier fluctuations over the last 300 years. Tephrochronology is used to date older features (<10,000 yr). The results show a pattern of fluctuations driven by climatic change. During the Lateglacial-Early Holocene Period the glacier terminus was situated at the present-day coastline. The ice-margin has retreated c. 20 km during the last 10,000 years. At least four periods of glacier re-advance have been identified, at c. 5000,3000,1600 and 170 years BP. Overall, the cumulative ice recession since c. 10 ka BP represents an ELA rise of c. 400 m which equates to an increase in mean air temperature of at least 2°C, assuming constant precipitation levels. Since the late 181" century, Lambatungnajökull has been in overall retreat. Moraines dated using two different lichenometric techniques indicate that the most extensive period of glacier expansion during historical time culminated shortly before c. AD 1795, probably in the 1780s. Recession over the last 200 years has been interrupted by re-advances in the 1850s, 1870s, and c. AD 1890. In the 20`h century, most notably in the 1930s and 1940s, Lambatungnajökull receded more rapidly than at any time during the previous 150 years. However, brief cold spells (-5 yrs), centred around the years AD 1918 and AD 1964, temporarily halted glacier recession. Lambatungnajökull has only retreated slightly over the last 20 years. The degree and nature of glacier retreat since 1930 compares well with similar-sized glaciers in southern Iceland. Furthermore, the pattern of glacier fluctuations over the last 150 years reflects the temperature oscillations recorded at nearby meteorological stations. Much of the climatic variation experienced in southern Iceland, and the glacier fluctuations that result, can be explained by secular changes in the North Atlantic Oscillation. A shift to more zonal atmospheric circulation and a weaker Icelandic Low - resulting in a greater frequency of negative NAO anomalies - may have been responsible for the cooling and associated glacier advances of the 18`h and 190' centuries. One implication of this work relates to the exact timing of the Little Ice Age in the Northeast Atlantic. The advanced position of glaciers during the late 18`" century suggests that this period represented the culmination of the Little Ice Age in Iceland. This contrasts with the current consensus that the Little Ice Age 'glacier maximum' in southern Iceland was during the late 19`h century. Other implications concern lichen-dating and its wider applications. Firstly, this research shows that the 'growth' curve of yellow-green Rhizocarpon lichens over the last 270 years in southeast Iceland is not linear. Although growth rates appear constant for periods of several decades, the growth 'curve' is exponential overall, with larger (older) lichens apparently growing more slowly than smaller lichens. Secondly, growth rates of Rhizocarpon Section Rhizocarpon in Iceland vary as a function of climate, with growth in the moist, maritime, southeast being c. 40% faster than in the cooler and drier northwest. Thirdly, this growth rate relationship - across the Northeast Atlantic region as a whole - can best be expressed in terms of climatic 'oceanicity' (r2 = 0.95). This latter relationship could be used to estimate lichen growth rates in areas where dating curves cannot be constructed. Finally, these findings suggest that lichen growth rates are likely to have varied in response to climatic change. In Iceland, slow-growing lichens, such as Rhizocarpon, probably grow more rapidly now - since the climatic amelioration of the 1920s and 30s - than they did in the cooler and drier periods of the 18's and 19'" centuries

Since the turn of the century record temperatures have been observed in at least 20 different countries across Europe. Isolated hot days are not often an issue; most devastation occurs when hot temperatures persist over many days. For example, the 2003 heatwave over Europe caused 40,000 deaths over a four week period at a cost of e 13.1 million to the agriculture sector. It is clear that accurate models for the risks associated with heatwaves are important to decision makers and planners who wish to reduce the number of people affected by these extreme events. Extreme value theory provides a statistical framework for modelling extreme events. Extreme value models for temperature data tend to focus solely on the intensity, overlooking how long periods of hot weather will last and what the spatial extent of the event will be. For heatwaves, it is vital to explicitly model extremal dependence in time and space. An aim of this thesis is to develop extreme value methods that can accurately capture the temporal evolution of heatwaves. Specifically, this is the first to use a broad class of asymptotically motivated dependence structures that can provide accurate inferences for different types of extremal dependence and over different orders of lagged dependence. This flexibility ensures that these models are less likely to dramatically under or over-estimate the risks of heatwave events. Climate change is now widely regarded as a driving force behind increased global temperatures. Extending the extreme value heatwave models to include covariate structure permits answers to critical questions such as: How will a 1°C warming in the global temperature increase the chance of a 2003 style event? The 2009 heatwave over Australia highlighted issues posed when multiple cities are affected simultaneously. Both Adelaide and Melbourne observed record temperatures during the same event which led to 374 deaths and 2000 people being treated for heat related illness. It is not enough for heatwave models to account for temporal dependence, they also need to explicitly model spatial dependence. Large-scale climatic phenomena such as the El Nino-Southern Oscillation are known to affect the temperatures across Australia. This thesis develops new spatial extreme value methods that account for covariates, which are shown to model the 2009 event well. A novel suite of spatial and temporal risk measures is designed to better understand whether these covariates have an effect on the spatial extent and duration of heatwaves. This provides important information for decision makers that is not available using current methodology.

This thesis comprises experimental investigations regarding the microphysical processes occurring within mesospheric clouds on Earth and Mars. CO2 trapping in amorphous ice was investigated with relevance to Earth’s mesosphere as a possible temporary sink of gas phase CO2. Experimental limits to the trapping process were evaluated and extrapolated to mesospheric conditions. This process was shown to be only plausible under extreme conditions. Metal deposition on low temperature ice was also investigated with relevance to PMCs. The reactivity of Mg and K was evaluated experimentally and with electronic structure calculations. A secondary Meteoric smoke Particle (MSP) formation process was hypothesised, with calculations suggesting this will impact the distribution of MSPs in the mesosphere during cloud season. In regards to the Martian mesosphere, microphysical processes that influence CO2 cloud formation have been investigated using a novel experimental system. Mass distributions of CO2 nucleation on nanoparticles were used to determine variables critical to modelling CO2 cloud formation. It is predicted here that lower temperatures than previously thought would be required to form CO2 clouds in the mesosphere. The crystal structure of CO2 ice under Martian conditions has also been experimentally investigated, with a cubic crystal structure determined (CO2-I). Temperature dependent parameters of CO2-I were applied to rates of nucleation and sedimentation for CO2 ice particles.

A set of in situ meteorological and radiometric measurements is acquired to facilitate the investigation of skin sea surface temperature (SSST) variability. The Tasco THI-500L radiometer produces SSST accurate to 0.5K when deployed in-shore and calibrated every 82 hours. Radiometer performance is enhanced if the instrument is insulated from, and characterised for, the effect of solar heating, allowing operational deployment to improve the availability of in situ SSST. Two statistical tests for similarity of populations are evaluated as quantifiers of sea surface temperature variability. Violations of the parametric requirements of the analysis of variance F-Test produce unreliable results. A dependable measure of SSST variability is generated using the non-parametric Mann-Whitney U-Test. Applying this test to imaging radiometer measurements shows that U depends on the variation between radiometric images in the physical factors that govern the ocean-atmosphere heat fluxes. The implications of this dependency for satellite SSST validation and climate modelling are considered. Analysis over metre- to kilometre- scales produces three trends of SSST variability against measurement interaction time. No dependency of these trends on meterological conditions is found. Applying the U-Test to large-scale ATSR-1 ASST data of the Tropical Pacific Ocean finds less variability than in small-scale SSST. Large-scale SSST are shown to be inhomogeneous with co-incident bulk sea surface temperature (BSST), implying that the use of ATSR-1 SSST in climate models will impact on the model results. Forcing a model of the Tropical Pacific Ocean separately with SSST and BSST shows a divergence in model output for both seasonal and ENSO SST variability. The BSST output is 0.7K warmer than the SSST output in the central equatorial Pacific during the Spring warming period.

Mixed-phase clouds contain both liquid and ice particles. They have important roles in weather and climate and such clouds are thought to be responsible for a large proportion of precipitation. Their lifetime and precipitation rates are sensitive to the concentration of ice. This project focuses upon the formation of ice within clouds containing liquid droplets colder than 273 K. A new bench-top instrument has been developed to study ice nucleation in liquid droplets. Pure water droplets of sizes relevant to clouds in the lower atmosphere do not freeze homogeneously until temperatures below ~237 K are reached. However, literature measurements of nucleation rates are scattered over two kelvin and there is uncertainty over the actual mechanism of ice formation in small droplets. The freezing of droplets with diameters equivalent to ~4 – 17 μm has been observed. It was found that ice nucleation rates in the smallest droplets of this size range were consistent with nucleation due to the droplet surface, but that surface nucleation does not occur at fast enough rates to be significant in the majority of tropospheric clouds. Water droplets can be frozen at higher temperatures than relevant for homogeneous freezing due to the presence of a class of aerosol particles called ice nuclei. Field observations of ice crystal residues have shown that mineral dust particles are an important group of ice nuclei, and the ice nucleating ability of seven of the most common minerals found in atmospheric dust has been described. In comparison to the other minerals, it was found that the mineral K-feldspar is much more efficient at nucleating ice. To relate this result to the atmosphere, a global chemical and aerosol transport modelling study was performed. This study concluded that dust containing feldspar emitted from desert regions reaches all locations around the globe. At temperatures below ~255 K, the modelled concentration of feldspar is sufficient to explain field observations of ice nuclei concentrations.

This thesis aims to assess and develop objective and robust approaches to evaluate regional climate model (RCM) historical skill using performance metrics and to provide guidance to relevant groups as to how best utilise these metrics. Performance metrics are quantitative, scalar measures of the numerical distance, or ’error’, between historical model simulations and observations. Model evaluation practice tends to involve ad hoc approaches with little consideration to the underlying sensitivity of the method to small changes in approach. The main questions that arise are to what degree are the outputs, and subsequent applications, of these performance metrics robust? ENSEMBLES and CORDEX RCMs covering Europe are used with E-OBS observational data to assess historical and future simulation characteristics using a range of performance metrics. Metric sensitivity is found in some cases to be low, such as differences between variable types, with extreme indices often producing redundant information. In other cases sensitivity is large, particularly for temporal statistics, but not for spatial pattern statistics. Assessments made over a single decade are found to be robust with respect to the full 40-year time period. Two applications of metrics are considered: metric combinations and exploration of the stationarity of historical RCM bias characteristics. The sensitivity of metric combination procedure is found to be low with respect to the combination method and potentially high for the type of metric included, but remains uncertain for the number of metrics included. Stationarity of biases appears to be highly dependent on the potential for underlying causes of model bias to change substantially in the future, such as the case of surface albedo in the Alps. It is concluded that performance metrics and their applications can and should be considered more systematically using a range of redundancy and stationarity tests as indicators of historical and future robustness.

Dust aerosol is abundant and important in the Earth system due to its influence on the radiation balance, the hydrological cycle, human health and ecosystems. Reducing the currently large uncertainties in dust effects requires realistic simulations of the lifecycle of dust aerosol which depends on the time, location and amount of dust emission. Emission estimates from models show a large variety, the reduction of which requires a systematic evaluation of dust-emitting winds. Different processes are known, but their relative importance was previously poorly quantified. This work investigates dust-emitting winds in North Africa based on single meteorological processes which helps guiding future model development. Based on 32 years of ERA-Interim data and a dust model, the emission amounts associated with nocturnal low-level jets (NLLJs), atmospheric depressions and mobile, long-lived cyclones are estimated climatologically for the first time. The results highlight NLLJs as an important driver for dust emission, particularly in the Bodélé Depression during winter. Associated maxima in mid-morning emission underline the importance of temporally high-resolved winds for dust modelling. ERA-Interim systematically underestimates NLLJ core wind speeds, likely due to artificially increased mixing in stable boundary layers. Derived emission frequencies over the Bodélé Depression agree well with observations, but differ elsewhere. Atmospheric depressions, often in the form of heat lows and lee depressions, occur frequently and coincide with the majority of dust emission. Few depressions develop into mobile and long-lived cyclones which coincide with particularly intense events. The largest emission fractions associated with cyclones are found in northeast Africa during spring, primarily at day with a small emission reduction by soil moisture. Smaller West African areas show similar fractions, likely associated with nearsurface signatures of African Easterly Waves. Comparing results derived from ERA-Interim against the Earth system model of the UK Met Office shows considerable disagreement in NLLJ core wind speeds and dust emissions. In depth analysis underlines the urgency for model development that improves the synoptic-scale conditions and the stable boundary layer. Such model improvements hold the potential to advance the scientific understanding of dust aerosol in the Earth system.