A model of winter wheat (Triticum aestivum L.) development (Weir et al., 1984) has been used along with field experiments to test the hypothesis that small amounts of warming, that may occur as a result of greenhouse gas-induced climate change, will cause marked reductions and affect the year-to-year variability in winter wheat developmental phase durations. The model has been run using long series historic climatic data from two sites in the UK, with systematic year round additions to the historic baseline and with seasonally varying changes to historic temperature. Changes from the present temperature predicted by GCMs for the future years of 2010, 2030 and 2050 have also been added to the historic data and the sensitivity of modelled development has been assessed. The sensitivity of the model output to different sowing dates and to different vernalization models have also been assessed. Field experiments were conducted on winter wheat over two growing seasons to assess the sensitivity of developmental phases to elevated temperatures produced by growing the wheat under synthetic covers. Differences in the early development of three varieties on two sowing dates was studied in the first season. A further study on the phenological development of cultivar Avalon, again using two sowing dates, was conducted in the second year. Model mean and year-to-year variability in duration from sowing to maturity were significantly reduced by arbitrary warming and increases in temperature expected from climate change scenarios. Model results indicated that a northern UK site (Edinburgh) is more sensitive to increases in temperature than a southern UK site (Oxford), due, in part, to the temperatures experienced over winter being closer to the base temperature for development. Model tests also revealed that the shape of the vernalization effectiveness curve has dramatic effects on the duration from sowing to double ridges which implies the need for correct vernalization model parameterization for different cultivars. The field experiments highlighted the differences between the vernalization responses of three UK varieties of winter wheat and also showed that warming produced in field conditions can have dramatic effects on shortening of the growing season, causing inevitable shortening of . the grain filling period. Testing the model, using temperature data and observations of developmental phases from the field experiment, indicated that the model predicted the duration to early phases of development (up to double ridges) more successfully than later phases. Anthesis was predicted too late causing maturity also to be predicted late in all situations. Durations to stages were predicted less well at later sowings rather than earlier sowings and_ in warming conditions rather than in controls. Manipulation of model thresholds, of thermal, photo-thermal and photo-vernal-thermal time for each developmental phase, to fit the observations in the uncovered treatments and then application of the altered model to the covered treatments resulted in a considerable improvement in the model's ability to predict phase durations under warmer conditions. The implications of the shortened winter wheat growing season length, due to climatic warming, on crop yields is discussed.

This thesis is concerned with developing atmospheric systems in sub-tropical and tropical regions. It deals , specifically, with Northern African depressions, which have the local name Khamsin,and which develop frequently during the spring months causing occasional sand storms in the area. I The results show the necessity and importance of using the time-dependent terms in computing the vertical motion (omega) field for this kind of phenomenon. This importance arises from the rapidity of the development associated with a significant diurnal variation of the dynamical fields.

Some of the dynamical aspects of baroclinic waves of intermediate wavelengths are investigated. A westerly jet, centred at 4S[degrees]N is considered. The flow is initially perturbed by a small-amplitude disturbance, having a zonal scale of 4000 km. This is the wavelength of the most unstable wave as determined by linear model integrations. Firstly, the primitive equations are solved over one life-cycle of the wave of maximum instability. It is found that intermediate wavelengths amplify through a dominating baroc1inic mechanism. The main sources of the eddy energy are the static stability component and zonal component of APE [Pearce (1978)]. It is therefore concluded that Pearce's theory of atmospheric energetics is a comprehensive formulation describing fully the two fundamental roles of eddies; namely the upward and poleward heat transfer.

An assessment and comparison are made of two separate methods of parametrizing the boundary-layer processes in a general circulation model. The first method is based on the use of eddy diffusivities. By using the framework of asymptotic similarity theory to describe the boundary layer as observed: and as predicted by single-column models, an objective assessment is made of formulations of the eddy diffusivities. Idealised neutral. conditions and diabatic conditions are examined separately. A formulation based on the specification of the mixing-length is recommended.

The synoptic scale structure of a type of winter time baroclinic disturbance affecting the western Mexico-U.S. region is investigated by means of an objective composite analysis technique. This technique allows selection of individual cases and is based on the use of correlations between individual map variables over a domain centred on an objectively defined origin in space and time. The selection procedure is applied to a total of 19 systems to produce composite structures covering approximately 5000x4500 Km2 in map space and 3 days in time. Composites based on (a) the 3 most strongly correlated systems within a three day period and (b) the 8 to 12 most strongly correlated systems at each time within the three day period, are used for detailed diagnostic estimations. These estimations reveal the importance of both barotropic and baroc1inic processes in controlling the evolution of the 'Cold-Low' as it affects the western Mexico-U.S. region, and suggest that these disturbances may be responsible for significant subtropical-mid1atitude exchanges of heat and momentum. Some characteristics of the composite wave structure are considered within the context of recent theoretical studies of downstream development.

Data from Meteosat thermal infrared (TIR) imagery are related to rainfall measured directly by gauges at meteorological stations in the Republic of Niger (West Africa), spanning between 0 and 14 °E and 12 and 18 ON. Quantitative physical parameters of 116 clouds from hourly TIR are then examined with the aim of establishing meaningful relations between them and ground observed rainfall.

Three dimensional aspects of symmetric instability are investigated using analytic and numerical techniques. The linear stability of a balanced flow with constant vertical shear is studied. In the inviscid case the fastest growing mode is the symmetric mode which is oriented parallel to the basic state isotherms. When the modes are tilted relative to the basic state isotherms the growth rate decreases. Modes with opposite tilts have similar growth rates but different structures; modes with cyclonic tilts have larger amplitude at the boundaries than those with anticyclonic tilts. When the flow is viscous the fastest growing modes are rotated anticyclonically relative to the basic state isotherms. This is attributed to the viscous damping being larger for the cyclonic modes due to their large amplitude near the boundaries. The tilt of the fastest growing mode depends on the viscosity. The larger growth of the anticyclonic modes is maintained into the nonlinear regime. The nonlinear development of the tilted modes proceeds in a similar manner to "nontilted" symmetric instability. The circulations can grow in the presence of structure along the length of the band. The two dimensional symmetrically unstable flow is perturbed with three dimensional disturbances. Growing disturbances retain their three dimensional structure. They can have horizontal tilts which vary with height and change with time. The fastest growth occurs when disturbances have anticyclonic tilts and are elongated in the along front direction. Unstable circulations can grow when the negative potential vorticity is confined in the along front direction. The scale of the region in this direction must exceed the wavelength of the eddies in the across front direction. The unstable circulations develop with the horizontal tilts and wavelengths observed when the unstable region is unconfined. Inertia-gravity waves are excited in the stable regions.

No description available.

Urban areas are highly sensitive to extreme events such as heatwaves. In order to understand how cities will respond to thermal stress it is critical to quantify not only their temporal temperature dynamics but also their spatial temperature variability. However, many cities lack weather station networks with a sufficient spatial distribution to characterise spatio-temporal intraurban temperature dynamics. One means by which spatially complete measurements of urban temperature may be derived is to employ satellite thermal Earth observed data. While some success has been achieved in understanding the temperature characteristics of cities using such data, relatively little work has been undertaken on establishing the use of long time-series Earth observed data as a supplement or alternative to screen-level air temperatures frequently utilised in urban climatological studies. In this thesis a software framework, centred around the use of a spatial database, is developed which can be used to gain an improved understanding of how satellite thermal Earth observed data can be used in the long timeseries analysis of urban temperature dynamics. The utility of the system is demonstrated by processing a 23 year time series (1985-2008) of 1,141 Advanced Very High Resolution Radiometer (AVHRR) images and hourly United Kingdom (UK) Met Office weather station measurements for the Greater London area. London was selected as the region of interest as it is the UK’s only megacity, and has been shown to exhibit both a significant urban heat island and a severe increase in population mortality during previous heatwave events. The software framework was employed to conduct two inter-related sets of analysis. First, the relationship over time between AVHRR estimated surface temperature (EST) and screen-level air temperature records is investigated and quantified. The resulting relationships are then used to produce an empirical model that can predict spatially complete summer-season air temperi atures for London. Cross-validation testing of the model at selected London weather stations showed model root mean square error (RMSE) ranging from 2.70 to 2.94°C and absolute errors in air temperature estimation of 0.45 to 1.67°C. A key finding of the thesis is that the minimal variation in prediction error between the different stations indicate a level of spatial robustness in the model across the urban surface, that is within the limits of the AVHRR EST precision. In addition, the model was used to estimate spatially averaged air temperatures over the Greater London area for selected summers, and showed a maximum error in air temperature prediction of 1.44°C. Furthermore, the prediction error for the heatwave summer of 2003 was 0.51°C, suggesting that such a model can successfully be used to estimate air temperatures for extreme heatwave summers. Such predictions are directly relevant to future assessments of urban population exposure to heatwaves, and it is envisaged that they could be used in conjunction with a population vulnerability index to create a spatially complete heatwave risk map for London. This work is then extended to investigate the utility of satellite estimated surface temperature measurements to characterise temporally and spatially intra-urban heatwave dynamics using the commonly employed urban heat island intensity metric (UHII). Analysis of the AVHRR EST found that the data are highly sensitive to local meteorological conditions, and that temporal aggregation at the monthly scale is required to provide robust data-sets for inter-year analysis of summer temperatures and generation of the UHII metric. Statistical testing of EST and air-temperature derived UHII for the heatwave summer of 2003 against other non-heatwave summers showed no significant increase in intensity at the 95% confidence level. This raises questions as to the applicability of the UHII metric to capture increases in urban temperatures during a heatwave event.

A study has been made of the day to day, seasonal and diurnal variations of the total, hard and soft components of cosmic ray intensity at sea-level. The results obtained have been compared with those of other workers and discussed in terms of temperature changes in the atmosphere. Evidence is also given of two new world-wide cosmic ray effects apparently connected with solar activity.