1,721,033 research outputs found
Investigations in combinatorial and solid phase organic chemistry
No full textThe insecticide industry faces increasing challenges from insect resistance to growing concerns over environmental issues. Synthesis of analogues of the insect moulting hormone, 20-hydroxyecdysone by solid phase methods was targeted as potential entry into new insecticide chemistry. The synthetic route utilised readily available 20-hydroxyecdysone that was converted by oxidative cleavage into poststerone that underwent solid phase attachment and either reductive amination or hydrazone formation. In the course of the reductive amination, a stereoselective reduction of the C20 ketone was used to generate dihydropoststerone. Synthesis of acetals on the 2,3 A-ring diol was carried out. Little biological activity over the native poststerone was identified. Intermolecular radical additions to arenes are generally inefficient due to the large number of side products generated. Synthesis of three precursors for intramolecular radical addition of aryl radicals to arenes was accomplished. Two precursors were protected as model benzyl ethers and the third was immobilised on polystyrene resin. Each precursor underwent radical cyclisation and the results showed that solid phase immobilisation favoured intramolecular cyclisation over radical quench. The use of high resolution Magic Angle Spinning (MAS) NMR to follow solid phase reactions was demonstrated. The preparation of resins loaded with the Wang linker and a short resin-based synthesis of alkenes was monitored by NMR.</p
The North Atlantic Ocean and climate change in the UK and northern Europe
No full textThis article considers the changes in the North Atlantic Ocean since the commencement of the RAPID array in 2004, which measures the Meridional Overturning Circulation. Emphasis is placed on the heat and freshwater transports associated with this circulation and its influence on the climate of the UK
Mean sea level trends around the English Channel over the 20th century and their wider context
No full textThis paper provides estimates of rates of change in mean sea level around the English Channel, based on an extensive new hourly sea level data set for the south coast of the UK, derived from data archaeology. Mean sea level trends are found to vary by between 0.8 and 2.3 mm/yr around the Channel. The rates of mean sea level change are calculated by removing the coherent part of the sea level variability from the time series of annual mean sea level before fitting linear trends. The improvement in accuracy gained by using this approach is assessed by comparing trends with those calculated using the more traditional method, in which linear trends are fitted directly to the original records. Removal of the coherent part of the sea level variability allows more precise trends to be calculated from records spanning 30 years. With the traditional approach 50 years is required to obtain the same level of accuracy. Rates of vertical land movement are approximated by subtracting the mean sea level trends from the most recent regional estimate of change in sea level due to oceanographic processes only. These estimated rates are compared to measurements from geological data and advanced geodetic techniques. There is good agreement around most of the UK. However, the rates estimated from the sea level records imply that the geological data suggest too much submergence along the western and central parts of the UK south coast. Lastly, the paper evaluates whether the high rates of mean sea level rise of the last decade are unusual compared to trends observed at other periods in the historical record and finds that they are not.<br/
Assessing changes in extreme sea levels: Application to the English Channel, 1900–2006
No full textA recently extended and spatially rich English Channel sea level dataset has been used to evaluate changes in extreme still water levels throughout the 20th century. Sea level records from 18 tide gauges have been rigorously checked for errors and split into mean sea level, tidal and non-tidal components. These components and the interaction between surge and tide have been analysed separately for significant trends before determining changes in extreme sea level. Mean sea level is rising at 0.8–2.3 mm/year, depending on location. There is a small increase (0.1–0.3 mm/year) in the annual mean high water of astronomical tidal origin, relative to mean sea level, and an increase (0.2–0.6 mm/year) in annual mean tidal range. There is considerable intra- and inter-decadal variability in surge intensity with the strongest intensity in the late 1950s. Storm surges show a statistically significant weak negative correlation to the winter North Atlantic Oscillation index throughout the Channel and a stronger significant positive correlation at the boundary with the southern North Sea. Tide–surge interactions increase eastward along the English Channel, but no significant long-term changes in the distribution of tide–surge interaction are evident. In conclusion, extreme sea levels increased at all of the 18 sites, but at rates not statistically different from that observed in mean sea level
A comparison of the main methods for estimating probabilities of extreme still water levels
No full textSea-level return periods are estimated at 18 sites around the English Channel using: (i) the annual maxima method; (ii) the r-largest method; (iii) the joint probability method; and (iv) the revised joint probability method. Tests are undertaken to determine how sensitive these four methods are to three factors which may significantly influence the results; (a) the treatment of the long-term trends in extreme sea level; (b) the relative magnitudes of the tidal and non-tidal components of sea level; and (c) the frequency, length and completeness of the available data. Results show that unless sea-level records with lengths of at least 50 years are used, the way in which the long-term trends is handled in the different methods can lead to significant differences in the estimated return levels. The direct methods (i.e. methods i and ii) underestimate the long (> 20 years) period return levels when the astronomical tidal variations of sea level (relative to a mean of zero) are about twice that of the non-tidal variations. The performance of each of the four methods is assessed using prediction errors (the difference between the return periods of the observed maximum level at each site and the corresponding data range). Finally, return periods, estimated using the four methods, are compared with estimates from the spatial revised joint probability method along the UK south coast and are found to be significantly larger at most sites along this coast, due to the comparatively short records originally used to calibrate the model in this area. The revised joint probability method is found to have the lowest prediction errors at most sites analysed and this method is recommended for application wherever possible. However, no method can compensate for poor data
Modelling of tide and surge elevations in the Solent and surrounding waters: the importance of tide–surge interactions
No full textA regional two-dimensional hydrodynamic model using the MIKE-21 software and data from a pre-operational forecasting system of the English Channel is described and applied to the Solent–Southampton Water estuarine system. The regional model was able to predict surge heights with a root mean squared error (RMSE) accuracy of 0.09 m during a three month hindcast in the winter of 2009, comparing closely with accuracy assessments from other independent systems. However, consistent underprediction of tidal harmonic constituent amplitudes was present throughout the region leading to errors in the prediction of the total water level elevations. Despite the complex nature of the Solent tidal regime, interpolation of tidal elevations from harmonic analysis at fixed tide gauge locations was shown to be effective in reducing this uncertainty at gauged and un-gauged sites. The degree to which tide–surge interactions were taking place was examined. Of particular interest was the quantification of the sensitivity of the predicted surge to the levels of uncertainty expected in the prediction of the tide within a complex nearshore region such as the Solent. The tide–surge interaction during three surge events was shown to be greatest in the Western Solent and Southampton Waters regions, where the tidal uncertainty was greatest. Interaction between the tide and surge resulted in a change of up to 0.3 m (11%) in the predicted total peak water level when the surge was added to the harmonic analysis-based tidal prediction. Despite the significant effect of removing the tide–surge interactions, tests indicated that the error in tidal range expected in the regional models tidal prediction altered the prediction of the surge only enough to induce changes in peak total water elevations by up to 0.03 m during an event on 10th March 2008. The findings suggest that the current tidal predictions in complex estuarine systems, such as the Solent, are of high enough quality to reproduce the majority of the tide–surge interactions taking place and that the error in the surge due to uncertainties in the predicted tide are expected to be relatively small
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