During winter the ocean surface at the poles freezes over to form sea ice. Sea ice floats on the ocean surface and has a matrix structure caused by the rejection of salts during freezing. In the summer sea ice melts at its surface creating melt ponds. An accurate estimate of the fraction of the upper sea ice surface covered in melt ponds during the summer melt season is essential for a realistic estimate of the albedo for global climate models. I will present a melt-pond{sea-ice model that simulates the twodimensional (areal) evolution of melt ponds on an Arctic sea-ice surface. This advancements of this model compared to previous models are the inclusion of snow topography, a realistic hydraulic balance and calculation of drainage rates and the incorporation of a detailed one-dimensional thermodynamic model. Water transport across and through the sea-ice surface is described by the major hydraulic processes believed to be present. Thermodynamic processes are modelled using the mushy-layer equations in sea ice, heat diffusion equations in snow and using assumptions of turbulent heat flux in melt ponds, along with a three-layer two-stream radiation model. The model simulates a section of a sea ice floe considered to be in hydrostatic equilibrium, where edge effects such as the presence of leads are neglected and consists of a grid of cells, each of which can be in one of four possible configurations: snow covered ice; bare ice; melt pond covered ice or open water. Eventually, a cluster of adjacent cells each containing melt water may be considered to have formed a melt pond. Lateral and vertical melt water transport is described by Darcy's Law. The model is initialised with ice topographies that represent either first-year or multiyear sea ice, which are reconstructed from SHEBA ice thickness data using standard statistical methods. The roughness and thickness of the ice and snow surfaces were altered and the sensitivity of the model to the initial data was tested. First-year ice and multiyear ice simulations confirmed observed differences in individual pond size and depth. Sensitivity studies showed that pond fraction is most sensitive to mean initial snow depth in first-year ice simulations and reduction of ice permeability in all cases.

No description available.

After an accidental release of radioactivity to atmosphere, modelling assessments are needed to predict what the contamination levels are likely to be and what measures need to be taken to protect human health. These predictions will be imprecise due to lack of knowledge about the nature of the release and the weather, and also due to measurement inaccuracy. This thesis describes work to investigate this imprecision and to find better ways of including it in assessments and representing it in results. It starts by reviewing exposure pathways and the basic dose calculations in an emergency response assessment. The possible variability of key parameters in emergency dose calculations is considered, and ranges are developed for each. The imprecision typically associated with calculational endpoints is explored through a sensitivity study. This has been done using both a simple Gaussian atmospheric dispersion model and also real-time weather data in combination with a complex atmospheric dispersion model. The key parameters influencing assessment imprecision are identified. These are demonstrated to be factors relating to the release, arising from inevitable lack of knowledge in the early stages of an accident, and factors relating to meteorology and dispersion. An alternative improved approach to emergency response assessments is then outlined, which retains a simple and transparent assessment capability but which also indicates the imprecision associated with the results through incomplete knowledge. This tool uses input from real-time atmospheric dispersion and weather prediction tools. A prototype version of the tool has been created and this has been used to produce example results. The final stage of the thesis describes the use of the new tool to develop ways in which imprecise or uncertain information can be presented to decision makers. Alternative presentational techniques are demonstrated using example results.

Continuous measurements have been made of the air-earth current density, by the direct method, potential gradient, space charge density, by filtration, and the positive conductivity at Lanehead, situated in the N. Penninea, 440m above sea level. Wind speed, wind direction and rate of rainfall were also recorded, but the measurements used only qualitatively. The problems of making continuous measurements for long periods of time and under difficult weather conditions are discussed. The results can be divided into two types; fair weather and disturbed weather. Of the former, diurnal variations for the one-year period July 1967 to June 1968 were calculated. That of potential gradient shows a close similarity with the variation found on the Carnegie in the Pacific (TORRESON et al, 1946), contrary to expectations for a land station within the austausch region. The air-earth current, on the other hand, is found to depend on the variation in columnar resistance, and its variation is similar to that of the space charge density. The diurnal variation of columnar resistance is estimated, and the role of pollution in controlling this variation is discussed. For disturbed weather, a high positive correlation has been found between space charge density and potential gradient at the ground during steady precipitation. Both were mostly negative during the precipitation, i.e. of opposite sign to the precipitation current. Splashing and point discharge are ruled out as sources of the space charge, so that the results imply some charge separation process within a few tens of metres of the ground giving positive charge to the rain and negative to the air. For 42 periods of steady rain, with some of snow, relations of the form F = Ao + B were calculated and the frequency distributions of A and B are given. The importance of making continuous measurements of several electric parameters is stressed, together with the necessity of using statistical methods to eliminate the effects of unknown variables such as meteorological conditions. An atmospheric electric station has been set up at Lanehead, where air pollution is low, and an atmospheric electric climate for the station has been established.

Previous methods for estimating the charge transferred to earth by point discharge are reviewed. It is concluded that point discharge on trees may well make a large contribution to this transfer of charge but that no reliable method of measurement has previously been duvised. A laboratory study of the pulsed nature of the point discharge currents produced at metal points and natural points has been made. It is concluded from these measurements that the amplitude and repetition frequency of these current pulses can be used to give the total discharge current. A new method for measuring point discharge currents in trees has now been developed and the apparatus successfully calibrated. This takes the form of a capacitative electrode, attached to a branch at the top of a tree, which detects pulses of point discharge current in the tree. The electrode is connected to an electronic pulse detector which gives a measure of the point discharge current in the tree. Measurements of point discharge currents have been made in two trees over a period of three months in a plantation of conifers in Weardale. The potential gradient and the point discharge current through a metal point were measured outside the plantation over a period of eleven months. The charge transferred to earth by point discharge on one tree in three months was found to be 4.3 x 10(^-4)C whereas for the metal point in the same period the charge transferred was 72 x 10(^-4)C. It is concluded that this difference is a result of the higher potential gradients necessary to initiate point discharge on the tree compared with those for the metal point. The point discharge measurements for two trees, one on the edge and one inside the plantation, are compared and found to be of similar magnitudes. The charge per unit area transferred to earth by point discharge on all the trees in the plantation is estimated to be 270 C km(^-2) yr(^-1).

This research focuses on modelling the relationship between wind conditions, sea roughness and GNSS reflections received from Low Earth Orbit (LEO). The motivation for this study lies in the recent development of a GNSS reflections receiver platform for the UK-DMC satellite and the numerous advantages proposed GNSS Reflectometry can provide in Earth Observation and global disaster monitoring. The fIrst part of the thesis focuses on the simulation procedure of received GPS-R Delay-Doppler Map (DDM). Airborne GPS-R scatterometric model has been adapted into this space-borne application research. Aft~r deriving DDM simulations according to reflection scenario, the results of two-dimensional data-model fItting are presented and analysed. The sensitivity discussion of current GPS-R model suggests some limitations of the modelling method, especially under medium and high wind speed ranges. In the second part, we investigate the inversion scheme of DDMs for the purpose of extracting a statistical wave model empirically. The similar model structure of DDM simulation is used but the processing order is turned over. After deconvolution, DDMs are inversed back to spatial energy maps and spatial slope probability maps. Three inversion algorithms are developed and compared. Preliminary synthetic and real data experiments give evidence of the feasibility of the inversion methodology. Finally, in the third part of this research, a new geometric wave slope statistical model is discussed in the context of wave fIeld simulations. The sensitivity of obtained statistical model is discussed in terms of wind speed, wave direction and observing incident angle. This provides an alternative view point to look into the wave slope probability properties and compensate the traditional theoretic and empirical wave modelling methods. Key words: GNSS-Reflectometry, Delay-Doppler Map inversion, wind conditions, sea surface roughness, slope probability density function, statistical wave slope model.

No description available.

No description available.

No description available.

No description available.