Inland Antarctic lakes are among the harshest environments in the world for life to inhabit. Ice cover causes low levels of light and temperature, and prevents mixing by wind, resulting in low nutrient levels and truncated food chains. Such ecosystems are widely regarded as sensitive indicators of climate change, and it is therefore useful to build up a strong physical and biological understanding of them. In 2003 an automatic probe (Palethorpe et al. 2004) was deployed on Crooked Lake, an ultra-oligotrophic freshwater lake in Eastern Antarctica which has been the subject of limnological studies since 1990. The probe measured several physical parameters in, above, and below the ice layer at temporal resolutions of up to one measurement every five minutes. A physics-based model was developed to simulate the growth and melt of the lake ice over time, considering all heat and radiation fluxes. Meteorological data were used as inputs to the model, with ice thickness the main output. The model fitted Crooked Lake ice thickness well, despite having narrow mechanistic constraints on parameter values. A number of simpler models were also developed which provided comparable goodness of fit, and illustrated that air temperature is the dominant variable in such systems. The issue of optimum complexity was addressed using model selection criteria, and some criteria selected a simple model over the physics-based model. However when both were subjected to long-term model runs with superimposed global warming scenarios, the simple model was shown to be unstable. In addition, a 1992-93 biological dataset was analysed. Populations were shown to exhibit a significant annual cycle, but no significant smaller-scale population oscillations, suggesting that higher sample rates were required to identify such phenomena. A prototype procedure was developed using simulated data to inform field sampling strategies, in the aim of identifying the population dynamics that are predicted by many plankton models.

The path delay between a GPS satellite and a ground based GPS receiver depends, after elimination of ionospheric effects using a combination of the two GPS frequencies, on the integral effect of the densities of dry air and water vapour along the signal path. The total delay in the signal from each satellite is known as the slant delay as the path is most likely to be non-azimuthal. The slant paths are then transferred into the vertical (or zenith) by an elevation mapping function, and this new parameter is known as the Zenith Total Delay or ZTD. ZTD gives a measure for the integrated tropospheric condition and is now widely accepted as a standard product from a network of dual frequency GPS receivers. With further calculation, taking into account surface pressure and temperature, we can then convert a portion of the ZTD into an estimate of the Integrated Water Vapour content of the atmosphere (IWV). As IWV may potentially change rapidly on a very short timescale, it is the speed at which IWV can be calculated which is of critical importance to short term meteorological forecasting. Often, rapid changes in IWV are associated with high humidity conditions caused by extreme weather events such as thunderstorms. Extreme weather events such as these are typically difficult to predict and track under current operational meteorological systems and, as they have the potential to cause great damage, it is in the interests to both the public and Met Services to significantly improve nowcasting wherever possible. As such the requirement for dense near real-time GPS networks for meteorological applications becomes apparent. Furthermore water vapour is one of the most important constituents of the atmosphere as moisture and latent heat are primarily transmitted through the water vapour phase. As well as this, water vapour is one of the most important greenhouse gases, and as such accurate monitoring of water vapour is of great importance to climatological research. This thesis assesses the quality of GPS water vapour estimates by comparison against a number of other remote sensing instruments to determine what the true value of the water vapour is and how well GPS water vapour estimates accurately represent real atmospheric fluctuations. Through these comparisons we can derive site specific bias corrections and thus, reconstruct a bias corrected time series of data for climate applications. Furthermore to ensure all biases associated with GPS processing changes are removed, a long time series of raw GPS data has been reprocessed under a consistent processing routine to again identify any climate trends in the data. Finally, this thesis addresses the question of whether near real-time GPS derived tropospheric estimates are of sufficient quality for climate applications without the need for time consuming reprocessing.

There is growing interest in quantifying the impact of climate change on extreme hydrologic events where failing to integrate the effect of climate change in rainfall estimation will underestimate the severity of the events and the adequacy of current hydraulic structure designs. The purpose of this study aims is to assess the rainfall trend and frequency analysis with impact from climate change in Peninsular Malaysia using statistical methods. The thesis consists of two sections, where the statistics of rainfall trend are assessed by Mann-Kendall (MK) test and non-stationary tests while the frequency analysis illustrates the changes in distribution functions that fit full series and sub-series of annual maximum rainfall. The study area is delineated into five regions according to their distance to the nearest coast (the different extents of the influence of monsoon to the study area) to examine the spatial characteristic of the rainfall series. The MK test has detected changes for each delineated region during different monsoon seasons. At the same time, the result of non-stationary tests reveal that changes in rainfall trend have developed around year 1995 in most of the stations (41% to 50% annual rainfall over the west coast regions; more than 50% of the short duration annual maximum rainfall in the central west region have shown non-stationarity). Among the regions, the short duration rainfall in central west region show most significant increasing trend by both the MK test and the non-stationary tests. Thus, year 1995 served as trend change-point to split full series data into two sub-series data and frequency analyses are performed on these data sets. From the outcomes of the frequency analysis using two sub-series data sets, the estimated quantiles from most of the regions have increased when the sub-series posterior to 1995 is used compared to full series data, implying an overall upward rainfall trend. The results also indicate that the combination of Generalised Extreme Value distribution function and L-moments for parameters estimation (GEV-LM) outperforms the other choices. The GEV-LM is able to fit well to all regions for short-duration rainfall and three regions for long-duration rainfall. This study demonstrates the importance of incorporating climate change in rainfall assessment. There are two-fold implications of this study. First, there is considerable variability of rainfall patterns due to climate change and hence, it is important to divide the study area into regions based on the results of the MK trend and non-stationary tests. Then, the best fitted distribution function and parameter estimation method combination for frequency analysis should be tested for every region. Second, it is important to appreciate the non-stationarity of rainfall series due to climate change and the impact on how frequency analysis shall be carried out. As the warming trends in Peninsular Malaysia started around year 1995, rainfall series have shown significance upward trend, while the results of the frequency analysis (estimated quantiles) reflects the changes in the rainfall characteristics as well. Hence, in this case, it is important to concern the non-stationarity in data to achieve better estimation performance using frequency analysis.

Thesis (Ph. D.)--University of Hong Kong, 1989.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

The launch of German geoscience satellite CHAMP inspired the increased interest in the study of ultra low frequency waves. In this work data from low Earth orbiting (LEO) German CHAMP satellite and South African ground-based magnetometer data were used to study geomagnetic pulsations, in particular continuous pulsations, Pc3, with periods in the range 10-45 seconds. Both Fast Fourier Transform (FFT) and Maximum Entropy Spectral Analysis (MESA) were used as analysis techniques to compute and compare spectra. We simulated a Pc3 oscillation using a sinusoidal function in order to test and establish appropriate parameters to use on the application of these analysis techniques. In this study the region chosen for a low latitude geomagnetic pulsations study excludes high current regions such as polar regions where field aligned currents occur. The structure of low-latitude pulsations was studied by comparing satellite and ground magnetic field measurements. The magnetic field measurements observed in the topside ionosphere by CHAMP were compared to Hermanus data for times when CHAMP crossed the ground station L-shell. The data were analysed for Pc3 pulsation activity using the MESA method to visualise field line resonance (FLR) in the vector magnetometer data. A number of discrete frequency oscillations for the fast mode wave were observed, one of which drives FLR at characteristic latitude as detected by both ground and satellite measurements. The toroidal mode frequency on CHAMP experiences a Doppler shift due to the rapid motion across the resonance region. Polarization hodograms in the resonance region show the expected 900 rotation of the field line resonant magnetic field components. We present first time ob- servations of toroidal standing Alfv´en mode oscillation with clearly L-dependent frequencies in the inner magnetosphere for L < 3. Our observations show FLR frequency continuously increasing as a function of decreasing latitude down to L = 1.6 and then decrease as a result of the larger plasma density of the upper ionosphere. The L-dependent frequency oscillations were observed in the presence of broadband compressional wave spectra. Our observation confirms the well-known magnetohydrodynamic (MHD) wave theoretical prediction of a compressional wave being the driver of the field line resonance, and that the Pc3 pulsations do not have a source with the same frequency structure. Keywords: ULF waves, Fast Fourier Transform, Maximum Entropy Spectral Analysis, CHAMP satellite, Geomagnetic pulsations, Pc3, Alfv´en wave, Field line resonance. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.

The estimation of the irradiance of sloping surfaces from standard meteorological measurements requires knowledge of the geometrical distribution of scattered radiation from the sky. Measurements of the radiance distribution of cloudless skies were made with a Linke-Feussner actinometer. When measurements of sky radiance N were expressed relative to the diffuse irradiance D of a horizontal surface, the angular distributions of N/D were remarkably independent of atmospheric turbidity. Standard distributions of N/D, drawn up for different solar zenith angles, were used to estimate the diffuse irradiance of slopes under cloudless skies. A new actinometer was designed for the measurement of the radiance of cloudy skies. A theoretical analysis of the energy budget of a thermopile in relation to the actinometer design is presented. Nine actinometers were used to measure mean distributions of radiance for partly cloudy and overcast skies. Results for overcast conditions indicated that the mean radiance near the horizon was larger than the value predicted by the 'Standard Overcast Sky' formula, but the increase in estimated irradiance of vertical surfaces was only about 2/16. A computer model was formulated for estimating the global irradiance of slopes using the new results for diffuse radiation. The model was applied to climatological mean radiation data from the Meteorological Office for Kew, Eskdalemuir, Aberporth and Lerwick.

The aim of this project is to show the capabilities of a RANS based numerical model in accurately analysing wind flow over real terrain regions, and assess its usage for wind energy applications. The main reasons this type of model is not widely used in the wind energy industry are due to the computational cost and the expertise required to operate such a model. These factors are assessed and various setups of the model are examined to consider the accuracy attained. The modelling process is also automated to reduce necessary user input in the process. The models performance is tested over a number of terrain types: Flat terrain (with surface roughness), an axisymmetric hill and a real terrain region (the Askervein hill). Primary consideration is given to velocity speed-up predictions which are paramount when considering the energy availability in the wind. A number of turbulence models have been tested for each terrain region to assess the improved accuracy obtained by using a more complicated CFD setup. The mesh discretisation has also been analysed for sensitivity to change, providing a comprehensive analysis of wind flow over Askervein. The CFD setup process is automated to reduce the time taken in setting up a model and increase the speed of providing a full wind field assessment for all wind directions, and allowing determination of average yearly values of velocity. This improves the access to such models for non-expert users and improves the availability of the model to wind energy developers siting farms in complex terrain regions. The model is shown to perform well for all terrain and roughness types. The turbulence properties are not well modelled, and that is a known limitation of this model type. The project demonstrates the advantages of CFD models for wind energy applications through the presented results and successful automation of the process.

Thesis (Ph.D.) -- University of Adelaide, Department of Economic Geology, 1976.