Radio occultation measurements from global navigation satellite systems, such as the Global Positioning System (GPS), represent a new source of numerical weather prediction information. Conventionally, radio occultation measurements are inverted using an Abel integral transform to obtain a profile of refractivity, and subsequently pressure, on geometric height levels via the hydrostatic relation. Although accurate temperature/water vapour retrievals are possible with a background estimate of water vapour/temperature, it is not possible to retrieve these quantities simultaneously using this method. Also, using this method, refractivity retrieval errors are introduced by assuming no horizontal structure local to ray periapsis (the spherical symmetry assumption), and the method is subject to 'first-guess' errors when the hydrostatic relation is initialised. Results from an investigation into how departures from spherical symmetry affect the performance of the Abelian inverse method are presented. It is shown that realistic horizontal humidity inhomogeneities can compensate for or reinforce horizontal temperature inhomogeneities, and therefore are important to consider in both the forward and inverse modelling. Using the results from this experiment, regression coefficients are fitted in an effort to predict temperature and refractivity retrieval errors from the horizontal temperature and humidity structure local to the measurement. The largest contribution to the predicted errors is shown to be from the local parabolic component of the horizontal structure, but is found to predict only a small fraction of the total error. A non-linear optimal estimation inverse method is presented with which it is possible to retrieve simultaneously profiles of temperature, humidity and surface pressure. Using this method, the measurements are assimilated with a priori information utilising error estimates of the a priori information and the measurements. The method implemented is validated using an ensemble of numerical simulations. Real observations from the GPS/MET pilot experiment are used to retrieve profiles of temperature, humidity and surface pressure which are validated using collocated ECMWF and NMC model analyses, radiosondes, and the Abelian inversion results. Temperature and refractivity comparisons between the optimal estimate and results from the Abelian inverse method show good agreement at high latitudes for all altitudes, resolving small-scale structure not shown by the model analyses. At low latitudes there is good agreement above the tropopause, below which a temperature bias ensues between the Abelian inversion and all other correlative data. Retrieved values for water vapour and surface pressure compare well with model analyses and collocated radiosondes. Biases between the UKMO and the ECMWF model analyses are consistent with known differences between the two models at the time of the dataset.

由於數據、模型等的尺度依賴性，尺度不匹配可能導致錯誤的結論。尤其在相互作用的地理過程研究，尺度適宜性對認知地理過程起著至關重要的作用。本文結合珠三角地區的氣象場和空氣品質過程模擬，對地理過程研究中的尺度適宜性問題進行了研究。 / 首先，基於尺度概念的定義，本文提出了地理過程研究中的四組尺度適宜性問題：多地理過程層、維度層、類型層和組成成分層。分析了其可操作級別以及評估流程和指標。 / 第二，在WRF模型中引入多解析度地形數據以研究數據與模型在氣象過程模擬的尺度適宜性。結果表明由於DEM數據與模型的尺度不適宜，可能對氣象變數的模擬帶來較大偏差。而3和30秒解析度的DEM數據與1千米解析度的模型可以較好地模擬香港地區的氣象過程。本案例有助於通過考慮尺度適宜性來提高氣象場的模擬能力。 / 第三，針對模擬模型與地理過程的尺度適宜性，應用WRF和CALMET模擬了香港地區的氣象過程。結果表明CALMET可以更好地模擬香港的氣象過程，但是WRF與CALMET模擬結果的差異在空間分佈上是異質的，即複雜地表條件的區域CALMET模擬改進明顯。多尺度模擬的結果也表明了跨尺度地理過程的相互影響，有助於多尺度地理過程模擬與認知。 / 第四，設計並研發了面向空氣品質過程的多尺度虛擬地理環境系統。本系統應用LAMP架構，支援模擬知識的管理、跨平臺及分散式平行計算，亦考慮到模擬的尺度依賴性，將有助於多尺度空氣品質過程的模擬和認知。應用該系統，對多尺度SO₂濃度過程進行模擬並分析了香港當地排放源的分擔率。較低的本地分擔率表明香港政府仍需要加強區域合作來治理空氣污染問題。 / 本論文的研究不僅加強對地理過程研究中的多尺度以及尺度適宜性問題的認知，有助於分解和研究複雜的尺度適宜性問題，研究案例和原型系統亦將會對香港和珠三角地區的空氣品質過程理解和管理作出有益貢獻。 / Although multiscale data and models are taken into account to study geography, due to their scale dependence, the scale mismatching may cause adverse results. Thus, scale compatibility is becoming crucial to decode geographic process, especially, for the interactive geographic processes. This thesis focused on scale compatibility in geographic process with a case study of multiscale meteorological and air quality simulation in Hong Kong Special Administrative Region (HKSAR) and the Pearl River Delta (PRD) region. / Firstly, based on the conceptual definition of scale, this dissertation identifies four groups of scale compatibility in geographic process research: multiple process level, dimension level, type level and component level. By illustrating the different operational levels from the abstract to operational, the author proposes a procedure to implement scale compatibility with potential criteria. / Secondly, scale compatibility in the reproduction of meteorological process is investigated between multiscale digital elevation model (DEM) data and the Weather Research and Forecasting Model (WRF). The experiments show that: DEM data with 3 and 30 arc sec resolutions are relatively more compatible with the WRF model of 1 km resolution to reproduce the meteorological field over Hong Kong; and uncertainty arising from scale mismatching between DEM data and the model may account for 38% of the variance in certain meteorological variables (e.g., temperature). This case study not only helps to improve meteorological simulations by taking the issue of scale compatibility into account, but also explains the significance and implementation of scale compatibility in geographic process research. / Thirdly, this thesis utilizes multiscale meteorological models to study the scale compatibility between dynamic models and interested geographic process. We conduct validation through three steps: daily statistics, spatial comparison and time series. The results support the idea that CALifornia METeorological model (CALMET) is more compatible to reproduce the meteorology process in Hong Kong; however, the discrepancy between the WRF and CALMET is spatially heterogeneous, with larger improvement over the area of complex topography and land use. The results also give evidence about the cross-scale interaction to interpret multi-scale geographic process. / Fourthly, applying the above findings, this dissertation presents a multiscale Virtual Geographic Environments (VGE) system to integrate geographical analysis and multiscale models in a cross-platform and parallel manner. With database system and Linux-Apache-MySQL-Perl (LAMP) architecture, users can manage and retrieve modeling concerning both data and model parameterization to help them reach a consensus on the simulation results and share modeling knowledge. Scale compatibility among data, models and analysis is also considered in the system design. Aided with high-resolved and regulable emission inventory, such multiscale system enables the practical application for various scenarios. As a case study, the VGE is applied to simulate and analyze the SO₂ concentration process and local contribution in HKSAR. / Achievements of this dissertation should greatly contribute to a better cognition of multiscale issues and scale compatibility concerning geographic process, and the VGE is expected to contribute to better understanding and management of air quality for both HKSAR and PRD. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Zhang, Chunxiao. / Thesis (Ph.D.) Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 135-157). / Abstracts also in Chinese.

Thesis (M. S.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2010. / Committee Chair: Peter J. Webster; Committee Member: Judith A. Curry; Committee Member: Robert X. Black. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis submitted for the degree of Doctor of Philosophy in the University of the Witwatersrand, Johannesburg / Andrew Chakane 2020

Gap, outflow, or Squamish wind, is the cold low level seaward flow of air through fjords which dissect the coastal mountain barrier of northwestern North America. These flows, occurring mainly during winter, can be strong, threatening safety, economic activity and comfort. Howe Sound gap winds were studied using a combination of observations and several types of models. Observations of winds in Howe Sound showed that gap wind strength varied considerably along the channel, across the channel and vertically. Generally, winds increase down the channel, are strongest along the eastern side, and are below 1000 m depth. Observations were unable to answer all questions about gap winds due to data sparseness, particularly in the vertical direction. Therefore, several modelling approaches were used. The modelling began with a complete 3-dimensional quasi-Boussinesq model (CSURAMS) and ended with the creation and testing of models which are conceptually simpler, and more easily interpreted and manipulated. A gap wind simulation made using RAMS was shown to be mostly successful by statistical evaluation compared to other mesoscale simulations, and by visual inspection of the fields. The RAMS output, which has very high temporal and spatial resolution, provided much additional information about the details of gap flow. In particular, RAMS results suggested a close analogy between gap wind and hydraulic channel flow, with hydraulic features such as supercritical flow and hydraulic jumps apparent. These findings imply gap wind flow could potentially be represented by much simpler models. The simplest possible models containing pressure gradient, advection and friction but not incorporating hydraulic effects, were created, tested, and found lacking. A hydraulic model, which in addition incorporates varying gap wind height and channel geometry, was created and shown to successfully simulate gap winds. Force balance analysis from RAMS and the hydraulic model showed that pressure gradient and advection are the most important forces, followed by friction which becomes an important force in fast supercritical flow. The sensitivity of gap wind speed to various parameters was found from sensitivity tests using the hydraulic model. Results indicated that gap wind speed increases with increasing boundary layer height and speed at the head of channel, and increasing synoptic pressure gradient. Gap wind speed decreases with increasing friction, and increasing boundary layer height at the seaward channel end. Increasing temperature dilterences between the cold gap wind air and the warmer air aloft was found to increase the variability of the flow — higher maximum but lower mean wind speeds.

Gap, outflow, or Squamish wind, is the cold low level seaward flow of air through fjords which dissect the coastal mountain barrier of northwestern North America. These flows, occurring mainly during winter, can be strong, threatening safety, economic activity and comfort. Howe Sound gap winds were studied using a combination of observations and several types of models. Observations of winds in Howe Sound showed that gap wind strength varied considerably along the channel, across the channel and vertically. Generally, winds increase down the channel, are strongest along the eastern side, and are below 1000 m depth. Observations were unable to answer all questions about gap winds due to data sparseness, particularly in the vertical direction. Therefore, several modelling approaches were used. The modelling began with a complete 3-dimensional quasi-Boussinesq model (CSURAMS) and ended with the creation and testing of models which are conceptually simpler, and more easily interpreted and manipulated. A gap wind simulation made using RAMS was shown to be mostly successful by statistical evaluation compared to other mesoscale simulations, and by visual inspection of the fields. The RAMS output, which has very high temporal and spatial resolution, provided much additional information about the details of gap flow. In particular, RAMS results suggested a close analogy between gap wind and hydraulic channel flow, with hydraulic features such as supercritical flow and hydraulic jumps apparent. These findings imply gap wind flow could potentially be represented by much simpler models. The simplest possible models containing pressure gradient, advection and friction but not incorporating hydraulic effects, were created, tested, and found lacking. A hydraulic model, which in addition incorporates varying gap wind height and channel geometry, was created and shown to successfully simulate gap winds. Force balance analysis from RAMS and the hydraulic model showed that pressure gradient and advection are the most important forces, followed by friction which becomes an important force in fast supercritical flow. The sensitivity of gap wind speed to various parameters was found from sensitivity tests using the hydraulic model. Results indicated that gap wind speed increases with increasing boundary layer height and speed at the head of channel, and increasing synoptic pressure gradient. Gap wind speed decreases with increasing friction, and increasing boundary layer height at the seaward channel end. Increasing temperature dilterences between the cold gap wind air and the warmer air aloft was found to increase the variability of the flow — higher maximum but lower mean wind speeds. / Arts, Faculty of / Geography, Department of / Graduate

Das Institut für Meteorologie engagierte sich in den Forschungsschwerpunkten atmosphärische Grenzschicht, Wasserhaushalt in Einzugsgebieten von regionaler Skala im großräumigen Umfeld, Dynamik der mittleren und oberen Atmosphäre, Variabilität der Ionosphäre, Strahlungsprozesse in der Atmosphäre, Modellierung von Aerosol- und Wolkenphysik, natürliche Klimavariabilität unter Einschluss der Rolle des Ozeans und Anregung und Ausbreitung großskaliger Strömungssysteme in Ozean und Atmosphäre. Dabei kamen Fernerkundungsverfahren vom Boden und von Satelliten aus, in-situ-Experimente, konzeptionelle und Simulationsmodelle zum Einsatz. Besondere Bedeutung hatte die Entwicklung und Anwendung von Verfahren zur tomographischen Rekonstruktion von dreidimensionalen Feldern atmosphärischer Größen. Eine enge Zusammenarbeit erfolgte mit dem Institut für Troposphärenforschung Leipzig, dem Institut für Atmosphärenphysik in Kühlungsborn, dem Max-Planck-Institut für Meteorologie in Hamburg und dem Geoforschungszentrum Potsdam.

Information from the TRMM-CSH and TRMM-2A12 datasets is used to examine the four-dimensional latent heating (LH) structures over the Asian monsoon region between 1998 and 2006. High sea surface temperatures, ocean-land contrasts and complex terrain produce large precipitation and atmospheric heating rates whose spatial and temporal characteristics are relatively undocumented. Analyses identify interannual and intraseasonal LH variations, with a large fraction of the interannual variability induced by internal intraseasonal variability. Also, the analyses identify a spatial dipole of LH anomalies between the equatorial Indian Ocean and the Bay of Bengal regions occurring during the summer active and suppressed phases of the monsoon intraseasonal oscillation. Comparisons made between the TRMM-CSH and TRMM-2A12 datasets indicate significant differences in the shape of the vertical profile of LH. Comparison of TRMM-LH retrievals with sounding budget observations made during the South China Sea Monsoon experiment shows a high correspondence in the timing of positive LH episodes during rainy periods. Negative values of LH, associated with radiative cooling and with higher troposphere cooling from non-precipitating clouds, are not captured by any of the TRMM datasets. In summary, LH algorithms based on satellite information are capable of representing the spatial and temporal characteristics of the vertically integrated heating in the Asian monsoon region. The TRMM-CSH presents better performance than TRMM-2A12. However, the vertical distribution of atmospheric heating is not captured accurately throughout all different convective phases. It is suggested that satellite derived radiative heating/cooling products are needed to supplement the LH products in order to give an overall better depiction of atmospheric heating.

It has been proposed that the Asian-Australasian monsoon system is influenced by large-scale sea-surface temperature (SST) variability in the three tropical oceans although how this influence is manifested has remained a largely open question. Closure of this issue is important because it is needed to explain trends in monsoon precipitation and circulation that have occurred in the last 30 years. Using an atmospheric general circulation model, we run a series of experiments with different configurations of global SST relating to various epochs occurring during the last century to evaluate their influence on the monsoon. Comparisons of circulation fields show that a colder SST configuration generates a weaker large-scale monsoonal circulation. On the other hand, warmer SST states generate stronger large scale circulations with more vigorous centers of divergence and convergence. Warmer SST configurations are associated with positive anomalies of precipitation in the eastern Bay of Bengal, Eastern Indian Ocean and South East Asia. Cooler SST configurations are associated with negative anomalies of precipitation in the Arabian Sea and Indian peninsula, especially at the beginning of the summer. Since SST gradients determine, to a large degree, the low level flow, they are also going to influence the transport of atmospheric moisture. Comparison of vertically integrated moisture transport fields between the different experiments show that cold SST configuration favors an increased inter-hemispheric flow of moisture but decreases in the westerly moisture flow in to the Bay of Bengal and India. Warm SST configurations, on the other hand, strengthens westerly flow into the eastern Indian Ocean. An increasing availability of moisture in a region of stronger convergence constitutes a favorable environment for the production of monsoonal precipitation. African easterly waves (AEW) constitute an important component of the African and tropical Atlantic Ocean climate during the boreal summer. An understanding of this component is essential since AEW are closely related with tropical Atlantic storm activity. We adopt an idealized modeling approach using the WRF model initialized with ERA-40 reanalysis data to study the mechanisms that trigger the formation and maintenance of AEW. The model domain includes the African continent, central and eastern Atlantic Ocean and the western Indian Ocean. Experiments are designed to test the relative importance of the thermal effect of the eastern African topography and the influence of the cross-equatorial pressure gradient, induced by the sea surface temperature (SST) on the origins and maintenance of AEW. Topography and SST variation are selectively added and removed. The control experiment shows that the model reproduces many of the mean features observed during the boreal summer. Westward propagating disturbances of 3-8 day period that originate between 30 and 40E at the surface levels and in the mid troposphere are well depicted. In addition, the model provides a reasonable representation of the AEJ. When all topographic features are removed, there is a weakening of the AEJ over land and ocean, however, longitude-time sections of meridional velocity still exhibit westward propagating disturbances that reach the western African coast at the surface and at the jet level with the same 3-8 day period. Spectral analysis of meridional velocity show that the variability associated with AEWs is reduced over East Africa and West Africa at 850-hPa and is reduced west of 20E along the southern flank of the jet and over northern Africa at the jet level. Maximum amplitude of the disturbances occurs right at the coast. The spatial distribution of barotropic and baroclinic energy conversions explains the reduction in AEWs over land and the intensification of these features at the coast. When the zonal SST gradient is removed, a weaker AEJ displaces southward and a weaker monsoon flow ensues. Spectral analysis of meridional velocity displays a variance reduction in the 3-8 day band at the 850-hP a level in western and eastern Africa and at the coast. At the 650-hPa level significant changes are not observed at the latitude of the AEJ (15N), however, a decrease in the variance associated with AEW occurs at the southern flank of the jet. A southward displacement of the jet favors a weakening of the baroclinic energy conversions. Barotropic conversions also appear to be weaker when the SST gradient is removed. The present study suggests that orography plays an important role in determining the variability of meridional wind associated with AEW over Eastern Africa at the lower levels. Further, zonal SST gradients over the Atlantic favor intensification of waves when they reach the coast and the maintenance of disturbances across the Ocean. Also, results could suggest that SST gradients support genesis of AEW just off the coast of Africa.

Space weather and its effect on technological systems are important for scientific research. Developing an understanding of the behaviour, sources and effects of ionospheric events form a basis for improving space weather prediction. This thesis attempts to use co-located radars and instruments for the analysis of ionospheric events over South Africa. The HF Doppler radar, ionosonde, Global Positioning System (GPS) and GPS ionospheric scintillation monitor (GISTM) receivers are co-located in Hermanus (34.4°S, 19.2°E), one of the observatories for the space science directorate of the South African National Space Agency (SANSA). Data was obtained from these radars and instruments and analysed for ionospheric events. Only the Hermanus station was selected for this analysis, because it is currently the only South African station that hosts all the mentioned radars and instruments. Ionospheric events identified include wave-like structures, Doppler spread, sudden frequency deviations and ionospheric oscillations associated with geomagnetic pulsations. For the purpose of this work, ionospheric events are defined as any unusual structures observed on the received signal and inferred from observations made by the HF Doppler radar. They were identified by visual inspection of the Doppler shift spectrograms. The magnitude and nature of the events vary, depending on their source and were observed by all, some or one instrument. This study suggests that the inclusion of a wider data coverage and more stations in South Africa merit consideration, especially since plans are underway to host a co-located radar network similar to that in Hermanus at at least three additional observatory sites in South Africa. This study lays a foundation for multi-station co-located radar and instrument observation and analysis of ionospheric events which should enhance the accuracy of space weather and HF communication prediction.