Langzeitmessungen des Windfeldes in der Mesopausenregion (~ 92km) an verschiedenen Stationen in den mittleren Breiten der Nordhemisphäre zeigen systematische zonale Variationen beim (zeitlich) gemittelten Zonal- und Meridionalwind und bei den Amplituden und Phasen der halbtägigen Gezeiten. Als eines der herausragenden Muster, die zonale Variationen in der unteren mittleren Atmosphäre anregen, wird der Einfluß der Geopotentialstörungen zur zonalen Wellenzahl 1 und 2, die mit dem winterlichen stratosphärischen Polarwirbel verbunden sind, auf das Windfeld in der oberen Mesosphäre / unteren Thermosphäre numerisch mit dem COMMA-Modell der mittleren Atmosphäre untersucht. Die Modellergebnisse zeigen eine gute Übereinstimmung der zonalen Variationen des mittleren Zonalwindes, die im Breitenbereich 52ÆN bis 56ÆN beobachtet werden und in der Größenordnung von 10 - 20 m/s liegen. Auch die halbtägigen Gezeitenamplituden und -phasen zeigen qualitative und quantitative Übereinstimmungen zwischen Beobachtungen und Modellergebnissen. / Long-term time series of wind field observations in the upper mesosphere / lower thermosphere region at different locations in the midlatitude region indicate longitudinal variability in the (time-) mean zonal and meridional wind and in the amplitudes and phases of the semidiurnal tide, too. Being one of the prominent patterns forcing zonal inhomogenities in the lower middle atmosphere, the influence of the zonal wavenumber 1 and wavenumber 2 disturbances connected with the winter Northern Hemisphere stratospheric polar vortex on the mesosphere- / lower thermosphere wind field is numerically investigated with the COMMA model. The model results show that the zonal variations through the stationary waves coincide with typical observed mean zonal wind differences between different stations along the midlatitude belt between 52ÆN and 56ÆN with values about 10- 20 m/s. Also, the amplitude and phase variations of the semidiurnal tide show qualitative and quantitative agreements between model results and observations.

The profitability from the production wind power energy is related to the quality of the wind speed forecasts. All wind predicting methods needs meteorological data, for the prevailing synoptic situation, as input. High quality input is wanted for a better result. In this study a new idea of a method for estimation of high resolution wind fields is examined. The idea is to use an existing database, containing simulations of high resolution wind fields, to estimate the actual wind by combining the simulations in a way fitting actual synoptic data. The simulations in the database have been produced by the mesoscale MIUU-model, which has been developed by Leif Enger at Uppsala University. The database contains simulations characterized by different geostrophic wind speeds and directions. There is also a separation into four seasons, where values which are typical for each season is put on meteorological parameters. Reanalysis data from NCEP-NCAR, containing 850 hPa geopotential heights describing actual synoptic situations, is used to calculate geostrophic wind speeds and directions. Three different geostrophic wind calculation methods, the triangle method, the small cross-method and the large cross-method, are tested. The calculated geostrophic wind is compared between the methods. The small cross-method is chosen and the main reason for that is the large amount of reanalysis information considered by this method and the use of a small calculation area. Measurements of the wind speed and direction are available from the tower at Utgrunden. The geostrophic wind speeds and directions are therefore calculated especially for the position of Utgrunden. This is done by a linear weighting of data, from several grid points close to Utgrunden, with respect to the distance to Utgrunden. Linear weighting is also used when estimating the wind speed for Utgrunden. The wind speed is estimated by weighting together MIUU-model simulations, for different geostrophic wind speeds and directions, so that they fit the geostrophic wind values calculated for Utgrunden. The calculated wind speed, measured wind speed and calculated geostrophic wind speed, for Utgrunden, are compared. The correspondence, between the calculated and measured wind speed, turns out to be quite good for many time periods. The diurnal variations in the measured wind speed are partly captured by calculated wind speed, but the diurnal variations tend to be larger in the measured wind speed then in the calculated. There are also cases where there are large differences between the measured and estimated wind speed. Many of these cases are probably cased by unusual weather situations. By considering additional parameters, as the temperature field, it is likely that these wind estimations can be improved. With more research it may be possible to produce high resolution wind fields with enough accuracy to be useful as inputs in wind prognostic systems. The advantage with such a method would be that accurate high resolution wind fields could be produced without the use of a time consuming numerical high resolution model. / Lönsamheten för produktion av vindkraft elektricitet bestäms delvis av förmågan att göra bra vindprognoser för nästkommande dygn. Alla metoder för vindprognostisering behöver meteorologisk indata som beskriver den rådande synoptiska situationen. Kvaliteten och upplösningen på dessa indata har stor betydelse för metodens resultat. I denna studie undersöks en alternativ metod för bestämning av högupplösta vind fält. Idén är att man ska försöka utnyttja en tillgänglig databas av högupplösta vindfält, producerade av den mesoskaliga MIUU – modellen som är utvecklad av Leif Enger på meteorologiska institutionen vid Uppsala Universitet. Tanken är att dessa vindfält ska kunna kombineras på ett sådant sätt att de överensstämmer med en given synoptisk situation. MIUU – modell körningarna, i databasen, är indelade i situationer karaktäriserade av olika värden på den geostrofiska vindstyrkan och vindriktningen. Körningarna är gjorda för fyra säsonger, för vilka typiska värden för säsongen är satta på styrande parametrar. För att kunna kombinera MIUU - modell körningarna beräknas den geostrofiska vinden från 850 hPa geopotential höjd återanalys data tillgänglig från NCEP-NCAR. Tre olika beräkningsmetoder för geostrofisk vind testas och jämförs. Den ”lilla korsmetoden” väljs för uppgiften beroende på att den utnyttjar en förhållandevis stor mängd återanalys data, för beräkning av geostrofisk vind, samt använder litet beräkningsområde. Automatiskt uppmätta värden över vindhastighet och vindriktning finns tillgängliga från en mast positionerad vid Utgrunden i Kalmar sund. Den geostrofiska vinden beräknas därför i Utgrundens position. Beräkningen utförs genom linjär viktning av data från de från Utgrunden sett fem närmaste gridpunkterna (i lilla korsmetodens gridfält). En linjär viktning används sedan även för att vikta ihop de MIUU – modell simulerade vindfälten så att de passar de beräknade värdena på geostrofisk vindhastighet och vindriktning. Jämförelser görs mellan den beräknade vinden, den uppmätta vinden samt den geostrofiska vinden, för Utgrunden. Korrelationen, mellan uppmätt och beräknad vind, visar sig vara ganska god periodvis. Den dagliga variationen i den uppmätta vindhastigheten fångas delvis av beräkningsmetoden, men dygnsvariationen är betydligt större i den uppmätta vinden än i den beräknade. Det noteras även att det finns situationer då det är stora skillnader mellan beräknad och uppmätt vind. Dessa situationer beror i många fall troligen på onormala vädersituationer. Studium av ytterliggare parametrar, som t.ex. temperaturfältet, skulle troligen leda till betydande förbättringar i vinduppskattningen. Ytterligare forskning och förbättring av metoden skulle kunna leda till produktion av högupplösta vindfält med tillräcklig kvalitet för användning i vindprognostiseringsmodeller. Fördelen skulle i så fall vara möjligheten att kunna producera högupplösta vindfält utan användning av tidskrävande numerisk modeller.

Accurate forecasts of tropical cyclone surface wind fields are essential for decisions involving evacuation preparation and damage potential. Towards addressing these actions, a comparison of the CFAN tropical cyclone surface wind field model with the H*Wind wind field reanalyzes is done to assess the accuracy of the CFAN algorithm and to determine potential limitations of its use. 16 tropical cyclones were assessed through correlation coefficient, mean bias, and root mean square error. The resolution of initial conditions to be ingested into the model was also analyzed, along with storm type and whether or not wind shear was a limiting factor. Results suggest that the CFAN wind model accurately predicts the H*Wind analyses in most regions of the TC. The center of circulation has the highest error due to the CFAN wind model treating the center of circulation as a point rather than having finite lateral extent. Results from the sensitivity analysis based on input resolution show that the minimum input resolution for the CFAN wind model to produce fine spatial resolutions with high fidelity is 0.25°. It is shown that the reproductions of weaker tropical cyclones have lower accuracy due to wind field asymmetries within these systems, while stronger TCs are better reproduced, as these systems are usually better organized. Finally, through the wind shear analysis, it is shown that the accuracy of reconstruction is not dependent on the magnitude of vertical wind shear.

The land-sea transition in coastal areas makes the meteorological conditions rather complex and the structure of the coastal boundary layer is important for many reasons. Two examples are air pollution dispersion and wind energy potential. The wind field off the coast when offshore flow is present has been studied. At the coast there is a sudden change in surface properties that will affect the wind field in the area. When warm air is advected out over the much colder sea we get a stable stratification whose structure depends on wind speed and temperature difference. The roughness length over the sea is much less than over land, which gives less friction over the sea and usually increased winds. This is the general situation. But with a stable stratification the wind speed decreases near the surface partly due to the much denser air. However, there may be a wind speed decrease a certain distance from the coast, after an initial wind speed maximum. This is due to the growth of a stable internal boundary layer that develops over the sea. What is new in this investigation is that the more stable the stratification over the sea is, the farther offshore the decrease in wind speed occurs, probably more than 30 km. Thus with a very stable stratification the wind speed off the coast in coastal areas seems to be increased instead of decreased. This investigation also includes an explanation of why this behavior seldom is seen in measurements. The wind field structure dependence of the IBL-height and the stability has been studied and an expression for the distance to the decrease in wind speed has been found. Also the prediction of a sea breeze circulation is studied as well as the affect a low-level jet has on the wind field near the coast. Measurements from three sites are used, the Baltic Sea near the Island of Gotland, around the strait of Öresund and outside the Atlantic coast near Duck in North Carolina, USA. These measurements are used in simulations with a 2-dimensional meso-γ-scale model as well as a lot of arbitrary simulations. All simulations correspond very well to the expression found and the simulated cases agree well with the measured ones. In the lowest 100 m of the marine atmosphere over the Baltic Sea, the stratification is probably stable more than half of the year. Thus the behavior of decreased wind speeds a certain distance offshore, after an initial increase, would be a very common phenomenon. It must be of great importance in extracting wind energy, to avoid these areas and layers with decreasing wind speed, which in turn have lower wind energy potential. In an area with often very stable stratification over the sea there can be an energy loss of more than 50 % in the lowest 50 m of the boundary layer. / Sammanfattning av ”Vindfältet i kustnära områden” Övergången mellan land och hav gör de meteorologiska förhållandena i kustnära områden komplicerade. Vetskap om egenskaperna hos gränsskiktet vid kusten är viktigt av flera orsaker. Några exempel är spridning av luftföroreningar, vindenergipotential samt det faktum att man ska kunna ge rätt vindprognos till dem som av nytta eller nöje befinner sig utanför kusten. Vindfältet utanför kusten vid frånlandsvind har studerats här. Vid kusten sker en plötslig ändring av ytans egenskaper. När varm luft strömmar ut över kallt hav fås en stabil skiktning som beror på vindhastighet och temperaturdifferensen mellan land och hav. Skrovlighetslängden är större över land än över hav. Det ger vanligtvis en ökning av vindhastigheten över havet då luften strömmar från land till hav. Men vid tillräckligt stabil skiktning över hav samt med instabil skiktning över land fås istället ett vindavtagande. Initialt ökar dock alltid vindhastigheten. Det beror på tillväxten av ett internt gränsskikt. Höjden på detta måste vara tillräcklig för att ett avtagande skall ske. Stabilare skiktning över hav ger längre sträcka till avtagandet i vindhastighet som kan ske mer än 30 km utanför kusten och det kan inte längre kallas för kustnära område. Varför detta beteende sällan upptäcks utreds också. Vindfältets beroende av interna gränsskiktets höjd samt stabiliteten har studerats och ett uttryck för att beräkna avståndet till avtagandet i vindhastighet har också tagits fram. Även möjligheten att förutspå en sjöbris har studerats liksom den effekt en low-level jet har på vindfältet nära kusten. Mätningar från tre platser används, Östersjön nära Gotland, över Öresund samt vid Atlantkusten nära Duck i North Carolina, USA. Dessa tre fall simuleras i en 2-dimensionell modell liksom en mängd godtyckliga simuleringar av olika fall. I de lägsta 100 metrarna av det marina gränsskiktet över Östersjön är skiktningen troligen stabil mer än halva året. Beteendet med initialt ökande vindhastighet följt av en dramatiskt avtagande en viss sträcka utanför kusten borde vara ett vanligt fenomen. Vid utvinning av vindenergi måste det vara av stort intresse att undvika dessa områden med lägre vindhastighet, som i sin tur har lägre vindenergipotential. I ett område där det ofta blir stabilt skiktat över havet kan energiförlusten bli på över 50 % i de lägsta 50 metrarna av gränsskiktet.

Am Geophysikalischen Observatorium Collm, das 1932 als experimentelle Basis des Geophysikalischen Institutes der Universität Leipzig für meteorologische, seismologische und geomagnetische Messungen von Professor LUDWIG WEICKMANN errichtet worden war, wurde 1956 in Vorbereitung des Internationalen Geophysikalischen Jahres (International Geophysical Year [IGY]) mit hochattnosphärischen Messungen begonnen. Seit 1959 liegt der Schwerpunkt auf Windmessungen im Höhenbereich der oberen Mesosphäre / unteren Thermosphäre (80 - 110 km). Die Meß- und Auswertemethode wurde in den vergangenen Jahrzehnten aus sehr bescheidenen Anfängen heraus theoretisch und experimentell so entwickelt, daß derzeit eine vollautomatische komplexe Apparatur zur quasi-kontinuierlichen Windmessung in drei Referenzpunkten über Mitteleuropa (gegenseitige Entfernung 200 km) zur Verfügung steht, die die Momentanwerte des Windes nach Richtung und Geschwindigkeit mißt, die zugehörige Höhe feststellt, Mittelwerte bildet, Grund- und Gezeitenwind voneinander trennt und Höhen-Wind-Profile über vorgebbare Zeitabschnitte rechnet, aus denen letztendlich Höhen-Zeit-Schnitte der Windfeldparameter konstruiert werden können. Die vorliegende Arbeit skizziert die einzelnen Etappen dieser Entwicklung, berichtet von Problemen und ihrer Lösung und gibt Beispiele von Windfeldanalysen aus dem Jahre 1992. / The Collm Geophysical Observatory was founded by Professor L. WEICKMANN in 1932 as an experimental base of Leipzig University's Geophysical Institute for meteorological, seismological and geomagnetic observations. In 1956 as a preparation for the Internal Geophysical Year (IGY) we began with high-atmosphere measurements, and since 1959 wind measurements in the height range of the upper mesosphere / lower thennosphere (80 - 110 km) have been emphasized. During the past decades the method of measuring and analysing was developped theoretically and experimentally from primitive Starts so far, that at present a fully automatic and complex equipment with quasi-continuous measurements of the wind at three reference points within Central Europe (mutual distance 200 km) is available. These devices measure the instantaneous data of the wind according to direction and velocity, ascertain the corresponding height, calculate averages, separate the tidal wind components from the prevailing wind, and compute height wind-profiles for adjustable periods of time, from which height-time cross section of the wind field parameters can be finally constructed. The following paper outlines the particular stages of this development, informs about problems and their solution, and offers examples of wind field analyses for 1992.

<p>Landfalling tropical cyclones bring tremendous coastal and inland hazard, which depends strongly on the evolution of the wind field after the landfall. This work investigates the inland evolution of the tropical cyclone wind field via idealized numerical simulation experiments and existing theories explaining the physics of storms over the ocean. The complicated landfall process is idealized as a transient response of a mature axisymmetric tropical cyclone to instantaneous surface forcings associated with landfall.</p><p><br></p><p>First, idealized landfall experiments are performed in the f-plane Bryan Cloud Model (CM1), where surface drag coefficient and evaporative fraction are individually or simultaneously modified systematically beneath an axisymmetric mature storm. Surface drying stabilizes the eyewall and consequently weakens the overturning circulation, thereby reducing inward angular momentum transport that slowly decays the low-level wind field only within the inner-core. In contrast, surface roughening first weakens the entire low-level wind field rapidly and enhances the overturning circulation dynamically despite the concurrent thermodynamic stabilization of the eyewall; thereafter, the storm gradually decays in a manner similar to drying. As a result, total precipitation temporarily increases with roughening but uniformly decreases with drying. Storm inner size and outer size decrease monotonically and rapidly with surface roughening, while the radius of maximum wind can increase with moderate surface drying.</p><p><br></p><p>Second, the extent to which existing intensity theory formed for tropical cyclones over the ocean can explain the intensity response to idealized landfall is explored in this work. Existing theoretical predictions for the equilibrium response and transient response of storm intensity are compared against the simulated response found in previous idealized simulations. The equilibrium and transient response of storm intensity to combined surface forcings can be reproduced by the product of their individual responses, in line with traditional potential intensity theory. The intensification theory of Emanuel (2012) is generalized for predicting the weakening process and found capable of reproducing the transient intensity decay. Specifically, the rapid initial decay of near-surface wind can be captured by how kinetic energy is instantaneously reduced by surface friction, where the decay is a function of surface roughness.</p><p><br></p><p>Third, existing structural theory and TC radial length scale formed or identified for storms over the ocean are tested against the idealized landfall experiment where surface is individually dried or roughened. The equilibrium storm radial length scale can predict the transient response of storm size to surface roughening throughout the decay evolution. For surface drying experiments, TC size scales with the intensity after around 12h. The E04 wind field model can generally capture the transient response of TC low-level wind field to individual surface drying and roughening, from radius of maximum wind speed to the outer region. The E04 prediction for these two types of experiments exhibits limited dependence on the subsidence cooling rate applied in the model.</p><p><br></p><p>Overall, though results are insufficient to explain the complicated wind field evolution of every real-world landfalling storm, it provides a fundamental understanding of how storm low-level wind fields respond to inland surface properties. This work also indicates the potential for existing theory to predict how tropical cyclone intensity evolves after landfall in the real world, which is essential for improving the forecasts on any timescale and the risk assessments.</p>

Langzeitmessungen des Windfeldes in der Mesopausenregion (~ 92km) an verschiedenen Stationen in den mittleren Breiten der Nordhemisphäre zeigen systematische zonale Variationen beim (zeitlich) gemittelten Zonal- und Meridionalwind und bei den Amplituden und Phasen der halbtägigen Gezeiten. Als eines der herausragenden Muster, die zonale Variationen in der unteren mittleren Atmosphäre anregen, wird der Einfluß der Geopotentialstörungen zur zonalen Wellenzahl 1 und 2, die mit dem winterlichen stratosphärischen Polarwirbel verbunden sind, auf das Windfeld in der oberen Mesosphäre / unteren Thermosphäre numerisch mit dem COMMA-Modell der mittleren Atmosphäre untersucht. Die Modellergebnisse zeigen eine gute Übereinstimmung der zonalen Variationen des mittleren Zonalwindes, die im Breitenbereich 52ÆN bis 56ÆN beobachtet werden und in der Größenordnung von 10 - 20 m/s liegen. Auch die halbtägigen Gezeitenamplituden und -phasen zeigen qualitative und quantitative Übereinstimmungen zwischen Beobachtungen und Modellergebnissen. / Long-term time series of wind field observations in the upper mesosphere / lower thermosphere region at different locations in the midlatitude region indicate longitudinal variability in the (time-) mean zonal and meridional wind and in the amplitudes and phases of the semidiurnal tide, too. Being one of the prominent patterns forcing zonal inhomogenities in the lower middle atmosphere, the influence of the zonal wavenumber 1 and wavenumber 2 disturbances connected with the winter Northern Hemisphere stratospheric polar vortex on the mesosphere- / lower thermosphere wind field is numerically investigated with the COMMA model. The model results show that the zonal variations through the stationary waves coincide with typical observed mean zonal wind differences between different stations along the midlatitude belt between 52ÆN and 56ÆN with values about 10- 20 m/s. Also, the amplitude and phase variations of the semidiurnal tide show qualitative and quantitative agreements between model results and observations.

In this thesis, three improved analysis techniques for scatterometer wind estimation are presented. These techniques build upon previous methods that help validate scatterometer data. This thesis examines the theory connecting the 1D and 2D kinetic energy spectra and uses QuikSCAT data to measure the 2D kinetic energy spectrum of ocean winds. The measured 2D kinetic energy spectrum is compared to the traditional 1D kinetic energy spectrum. The relationship between the 2D kinetic energy spectra and the 1D kinetic energy spectra confirms findings from previous studies that ocean winds modeled in 2D are isotropic and nondivergent. The 1D and 2D kinetic energy spectra also confirm the known conclusion that the zonal and meridional components of ocean winds are uncorrelated. Through simulation, the wind response function (WRF) is calculated for three different QuikSCAT processing algorithms. The WRF quantifies the contribution that the wind at each point of the surface makes to a given wind estimate. The spatial resolution of the different processing algorithms is estimated by their WRFs. The WRFs imply that the spatial resolution of ultrahigh resolution (UHR) processing is finer than the spatial resolution of conventional drop-in-the-bucket (DIB) processing; the spatial resolution of UHR processing is ~5-10 km while the spatial resolution of DIB slice processing is ~12-15 km and the spatial resolution of coarse resolution DIB egg processing is ~30 km. Simulation is used to analyze the effectiveness of various wind retrieval and ambiguity selection algorithms. To assist in the simulation, synthetic wind fields are created through extrapolating the 2D Fourier transform of a numerical weather prediction wind field. These synthetic wind fields are sufficiently realistic to evaluate ambiguity selection algorithms. The simulation employs the synthetic wind fields to compare wind estimation with and without direction interval retrieval (DIR) applied. Both UHR and DIB wind estimation processes are performed in the simulation and UHR winds are shown to resolve finer resolution wind features than DIB winds at the cost of being slightly noisier. DIR added to standard QuikSCAT UHR wind estimation drops the wind direction root-mean-squared error by ~10° to ~24.74° in the swath sweet spot.

Am Geophysikalischen Observatorium Collm, das 1932 als experimentelle Basis des Geophysikalischen Institutes der Universität Leipzig für meteorologische, seismologische und geomagnetische Messungen von Professor LUDWIG WEICKMANN errichtet worden war, wurde 1956 in Vorbereitung des Internationalen Geophysikalischen Jahres (International Geophysical Year [IGY]) mit hochattnosphärischen Messungen begonnen. Seit 1959 liegt der Schwerpunkt auf Windmessungen im Höhenbereich der oberen Mesosphäre / unteren Thermosphäre (80 - 110 km). Die Meß- und Auswertemethode wurde in den vergangenen Jahrzehnten aus sehr bescheidenen Anfängen heraus theoretisch und experimentell so entwickelt, daß derzeit eine vollautomatische komplexe Apparatur zur quasi-kontinuierlichen Windmessung in drei Referenzpunkten über Mitteleuropa (gegenseitige Entfernung 200 km) zur Verfügung steht, die die Momentanwerte des Windes nach Richtung und Geschwindigkeit mißt, die zugehörige Höhe feststellt, Mittelwerte bildet, Grund- und Gezeitenwind voneinander trennt und Höhen-Wind-Profile über vorgebbare Zeitabschnitte rechnet, aus denen letztendlich Höhen-Zeit-Schnitte der Windfeldparameter konstruiert werden können. Die vorliegende Arbeit skizziert die einzelnen Etappen dieser Entwicklung, berichtet von Problemen und ihrer Lösung und gibt Beispiele von Windfeldanalysen aus dem Jahre 1992. / The Collm Geophysical Observatory was founded by Professor L. WEICKMANN in 1932 as an experimental base of Leipzig University''s Geophysical Institute for meteorological, seismological and geomagnetic observations. In 1956 as a preparation for the Internal Geophysical Year (IGY) we began with high-atmosphere measurements, and since 1959 wind measurements in the height range of the upper mesosphere / lower thennosphere (80 - 110 km) have been emphasized. During the past decades the method of measuring and analysing was developped theoretically and experimentally from primitive Starts so far, that at present a fully automatic and complex equipment with quasi-continuous measurements of the wind at three reference points within Central Europe (mutual distance 200 km) is available. These devices measure the instantaneous data of the wind according to direction and velocity, ascertain the corresponding height, calculate averages, separate the tidal wind components from the prevailing wind, and compute height wind-profiles for adjustable periods of time, from which height-time cross section of the wind field parameters can be finally constructed. The following paper outlines the particular stages of this development, informs about problems and their solution, and offers examples of wind field analyses for 1992.

Thesis (MScEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Tropical cyclones are synoptic scale rotating storms capable of generating intense wind speeds and rainfall with potentially devastating social and economic consequences. In addition to abnormally high winds and rainfall, the associated storm surge and extreme waves can lead to severe coastal erosion, damage to coastal property and inundation. A good understanding of the risk exposure to these events is therefore of great importance to planners and designers of coastal infrastructure in vulnerable regions. Probabilistic approaches have been routinely adopted for the calculation of extreme tropical cyclone induced wind speeds, with significant developments in these techniques over the last few decades. While the application of these approaches has become widely adopted in regions such as the North Atlantic, North Pacific and South Pacific Oceans, relatively little attention has been paid to the South-West Indian Ocean. This thesis focusses on the quantification of the risk exposure to tropical cyclones over the South-West Indian Ocean, using current state-of-the-art techniques. The primary results of the thesis are extreme wind speed maps at various return periods of interest for engineering design. Best track data for the South-West Indian Ocean, as archived by the Joint Typhoon Warning Centre (JTWC), has been used as the primary dataset forming the basis of this study. These data provide estimates of the location and intensity of historical tropical cyclones at six hourly intervals. Location data are provided as estimates of longitude and latitude of the eye, while intensity data are provided as estimates of the maximum sustained surface (10 m elevation) wind speed and/or minimum central pressure. The modelling of tropical cyclone wind fields has been carried out using both the Holland (1980) and the Willoughby et al. (2006) parametric wind field models. Using the limited information available in the best track data as input to the model, surface wind fields which reasonably resemble those of actual storms have been generated. Both considered parametric wind field models have been shown to yield reasonable wind speeds and directions when compared with measurements. Of the two considered models the Willoughby et al. (2006) model has been shown to provide the best fit to historical wind speed measurements. Extreme value analyses of tropical cyclone induced wind speeds based on historical data alone have been shown to lead to potentially large errors, owing to the small sample size of the historical data. This highlights the need to augment the historical database through a probabilistic approach. Largely following the methods described in Powel et al. (2005) and Emanuel et al. (2006), a synthetic track model for the South-West Indian Ocean has been developed. The objective of the synthetic track model is to simulate thousands of years of tropical cyclone tracks, thereby circumventing errors induced by small sample sizes in the available historical best track data. The synthetic track model developed as part of this study is a Markov chain model, capable of simulating track propagation and intensity evolution along the track, from track genesis through to termination. The model is purely statistical, based on properties derived from the historical best track data. Adjustments have however been made to account for physical limitations such as those imposed by the equator and the maximum potential intensity which an event can attain. The statistical characteristics of synthetic tracks have been shown to agree well with those of the historical population. Applying the Willoughby et al. (2006) wind field model along synthetic tracks has enabled the simulation of 5 000 years of tropical cyclone induced wind speeds at any location of interest in the South-West Indian Ocean. Applying calculations on a 1 degree geographical grid, wind speed maps corresponding to return periods of 50, 100, 200 and 500 years have been generated for the South-West Indian Ocean. Extreme wind speeds along coastal regions provide valuable input for the design of coastal infrastructure in the region. / AFRIKAANSE OPSOMMING: Tropiese siklone is sinoptiese orde roterende storms wat in staat is om aansienlike windspoed en reënval, tot gevolg te hê met potensiële vernietigende sosiale en ekonomiese gevolge. Benewens die abnormale sterk winde en hoë reënval kan die verwante stormdeinings en vloedgolwe lei tot ernstige kus-erosie, skade aan kusfront-eiendom en oorstromings. ‘n Goeie begrip van die risiko-blootstelling aan hierdie gebeurtenisse is daarom van groot belang vir die beplanners en ontwerpers van kus-infrastruktuur in kwesbare gebiede. As gevolg van die beduidende ontwikkeling van probabilistiese benadering tot die berekening van windspoed wat veroorsaak word deur ekstreme tropiese siklone, word hierdie tegnieke huidiglike op ‘n roetine basis aangewend. Terwyl die toepassing van hierdie benaderings wyd aanvaar word in gebiede soos die Noord-Atlantiese, Noordelike- en Suidelike Stille Oseaan, word relatief min aandag gegee aan die Suid-Westelike Indiese Oseaan. Hierdie tesis fokus op die kwantifisering van die risiko-blootstelling aan tropiese siklone in die Suid-Westelike Indiese Oseaan met die gebruik van die huidige gevorderdste tegnieke. Die primêre resultaat van die tesis is uiterste wind spoed kaarte vir ‘n verskeindenheid herhaal periodes wat van belang in vir engenieursontwerp. Beste roete-ata vir die Suid-Westelike Indiese Oseaan, soos voorsien deur die Gesametlike Tifoon Waarskuwing Sentrum (JTWC), is gebruik as die primêre data stel wat die basis vorm van hierdie studie. Hierdie data gee die beste skattings van die ligging (lengte- en breedtegraad), en intensiteit (maksimum volgehoue oppervlak (10m hoogte) wind spoed en/of sentrale druk tekort) van historiese tropiese siklone teen ses-uurlikse intervalle. Die modelering van tropiese sikloon windvelde was uitgevoer met die gebruik van die Holland (1980) en die Willoughby et al. (2006) parametriese windveldmodelle. Met die gebruik van beperkte inligting wat beskikbaar is in die beste roete data as invoer vir die model, was oppervlak wind velde gegenereer wat ‘n billike ooreenstemming het met die van werklike storms. Beide tegnieke se parametriese windveldmodelle is al bewys om redelike akkurate windspoed en windrigtings te lewer in vergelyking met waargenome waardes. Van die twee modelle het die Willoughby et al. (2006) model se resultate die beste ooreenstemming gewys met historiese wind spoed metings. Dit is al uitgewys dat uiterste waarde-analises van tropiese sikloon veroorsaakte windspoed moontlik kan lei tot groot foute in die resultate as gevolg van die klein monster-grootte van die historiese data. Dit beklemtoon die noodsaaklikheid om die historiese databasis aan te vul met behulp van probabilistiese metodes. Die metodes soos beskryf deur Powel et al. (2005) en Emanuel et al. (2006) is hoofsaaklik gebruik om ‘n sintetiese roete-model vir die Suid-Westelike Oseaan te ontwikkel. Die doelwit van die sintetiese roete model is om duisende jare se tropiese sikloonroetes te produseer, en in effek foute te vermy as gevolg van die gebruik van klein monster groottes van die beskikbare historiese beste roete data. Die sintetiese roete model wat tydens hierdie studie ontwikkel is, is ‘n Markov kettingmodel wat in staat is om die roete verspreiding asook die evolusie van intensiteit saam die roete te simuleer vanaf die onstaan tot die beëindiging van die sikloon se roete. Die model is suiwer statisties en is gebasseer op die eienskappe soos afgelei vanaf die historiese beste roete data. Aanpassings is gemaak om rekening te hou van die fisiese beperkings soos die wat opgelê word deur die ewenaar en die maksimum potensiële intensiteit wat ‘n sikloon kan bereik. Dit is voorgelê dat die statistiese einskappe van die sintetiese roetes goed saamstem met die van die historiese populasie. Die toepassing van die Willoughby et al. (2006) wind veld model langs die sintetiese roetes het dit moontlik gemaak om 5000 jaar se windspoed, wat veroorsaak is deur tropiese siklone, te genereer by enige ligging wat van belang is in die Suid-Westelike Indiese Oseaan. Met berekeninge wat op ‘n 1 grade geografiese ruitnet gedoen is, is windspoedkaarte vir herhaal periodes van 50, 100, 200 en 500 jaar opgestel vir die Suid-Westelike Indiese Oseaan. Die uiterste wind spoed in kusgebiede gee waardevolle invoer vir die ontwerp van kus-infrastruktuur in die omgewing.