This research has been conducted in order to develop a short-range ensemble numerical weather prediction system over southern Africa using the Conformal-Cubic Atmospheric Model (CCAM). An ensemble prediction system (EPS) combines several individual weather model setups into an average forecast system where each member contributes to the final weather forecast. Four different EPSs were configured and rainfall forecasts simulated for seven days ahead for the summer months of January and February, 2009 and 2010, for high (15 km) and low (50 km) resolution over the southern African domain. Statistical analysis was performed on the forecasts so as to determine which EPS was the most skilful at simulating rainfall. Measurements that were used to determine the skill of the EPSs were: reliability diagrams, relative operating characteristics, the Brier skill score and the root mean square error. The results show that the largest ensemble is consistently the most skilful for all forecasts for both the high and the low resolution cases. The higher resolution forecasts were also seen to be more skilful than the forecasts made at the low resolution. These findings conclude that the largest ensemble at high resolution is the best system to predict rainfall over southern Africa using the CCAM. / Dissertation (MSc)--University of Pretoria, 2014. / gm2014 / Geography, Geoinformatics and Meteorology / unrestricted

Open file dissertation.pdf to read abstract / Dissertation (MSc)--University of Pretoria, 2007. / Geography, Geoinformatics and Meteorology / Unrestricted

The complex and coupled physical processes taking place in the atmosphere, ocean and land surface are described in Global Circulation Models (GCMs). These models have become the main tools to simulate climate variability and project future climate change. GCMs have the potential to give physically reliable estimates of climate change at global, continental or regional scales, but their projections are currently of too course horizontal resolution to capture the smaller scale features of climate and climate change. This situation stems from the fact that GCM simulations, which are effectively three-dimensional simulations of the coupled atmosphere-ocean-land system, are computationally extremely expensive. Therefore, downscaling techniques are utilised to do perform simulations over preselected areas that are of sufficiently detailed to represent the climate features at the meso-scale. Dynamic regional climate models (RCMs), based on the same laws of physics as GCMs but applied at high resolution over areas of interest, have become the main tools to project regional climate change. The research presented here utilises the Conformal-Cubic Atmospheric Model (CCAM), a variable-resolution global atmospheric model that can be applied in stretched-grid mode to function as a regional climate model. As is the case with RCMs, CCAM has the potential to improve climate simulations along rough topography and coastal areas when applied at high spatial resolution, whilst side-stepping the lateral boundary condition problems experienced by typical limited-area RCMs. CCAM has been developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia. The objective in the study is to test capability of a regional climate model, CCAM, to realistically simulate cumulus convection at different spatial scales over regions with steep topography, such as the eastern escarpment of South Africa. Since both GCMs and RCMs are known to have large biases and shortcomings in simulating rainfall over the steep eastern escarpment of southern Africa and in particular Lesotho, the paper “Model simulations of rainfall over southern Africa and its eastern escarpment” (Chapter 3) has a focus on verifying model performance over this region. In the paper the CCAM simulations include six 200 km resolution Atmospheric Model Intercomparison Project (AMIP) simulations that are forced with sea surface temperatures and one 50 km resolution National Centre for Environmental Prediction (NCEP) reanalysis simulation that is forced with sea surface temperatures and synoptic scale atmospheric forcings. These simulations are verified against rain gauge data sets and satellite rainfall estimates. The results reveal that at these resolutions the model is capable of simulating the key synoptic-scale features of southern African rainfall patterns. However, rainfall totals are often drastically overestimated. A key aspect of model performance is the representation of the diurnal cycle in convection. For the case of South Africa, the realistic representation of the complex patterns of rainfall over regions of steep topography is also of particular importance. At a larger spatial scale, the model also needs to be capable of representing the west-east rainfall gradient found over South Africa. The ability of CCAM to simulate the diurnal cycle in rainfall as well as the complex spatial patterns of rainfall over eastern South Africa is analysed in “High Resolution Rainfall Modelling over the Eastern Escarpment of South Africa” (Chapter 4). The simulations described in the paper have been performed at 8km resolutions in the horizontal and span a thirty-year long period. These are the highest resolution climate simulations obtained to date for the southern African region, and were obtained through the downscaling reanalysis data of the European Centre for Medium-range Weather Forecasting (ECMWF). The simulations provide a test of the robustness of the CCAM convective rainfall parameterisations when applied at high spatial resolution, in particular in representing the complex rainfall patterns of the eastern escarpment of South Africa. / M (Geography and Environmental Management), North-West University, Potchefstroom Campus, 2015

The complex and coupled physical processes taking place in the atmosphere, ocean and land surface are described in Global Circulation Models (GCMs). These models have become the main tools to simulate climate variability and project future climate change. GCMs have the potential to give physically reliable estimates of climate change at global, continental or regional scales, but their projections are currently of too course horizontal resolution to capture the smaller scale features of climate and climate change. This situation stems from the fact that GCM simulations, which are effectively three-dimensional simulations of the coupled atmosphere-ocean-land system, are computationally extremely expensive. Therefore, downscaling techniques are utilised to do perform simulations over preselected areas that are of sufficiently detailed to represent the climate features at the meso-scale. Dynamic regional climate models (RCMs), based on the same laws of physics as GCMs but applied at high resolution over areas of interest, have become the main tools to project regional climate change. The research presented here utilises the Conformal-Cubic Atmospheric Model (CCAM), a variable-resolution global atmospheric model that can be applied in stretched-grid mode to function as a regional climate model. As is the case with RCMs, CCAM has the potential to improve climate simulations along rough topography and coastal areas when applied at high spatial resolution, whilst side-stepping the lateral boundary condition problems experienced by typical limited-area RCMs. CCAM has been developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia. The objective in the study is to test capability of a regional climate model, CCAM, to realistically simulate cumulus convection at different spatial scales over regions with steep topography, such as the eastern escarpment of South Africa. Since both GCMs and RCMs are known to have large biases and shortcomings in simulating rainfall over the steep eastern escarpment of southern Africa and in particular Lesotho, the paper “Model simulations of rainfall over southern Africa and its eastern escarpment” (Chapter 3) has a focus on verifying model performance over this region. In the paper the CCAM simulations include six 200 km resolution Atmospheric Model Intercomparison Project (AMIP) simulations that are forced with sea surface temperatures and one 50 km resolution National Centre for Environmental Prediction (NCEP) reanalysis simulation that is forced with sea surface temperatures and synoptic scale atmospheric forcings. These simulations are verified against rain gauge data sets and satellite rainfall estimates. The results reveal that at these resolutions the model is capable of simulating the key synoptic-scale features of southern African rainfall patterns. However, rainfall totals are often drastically overestimated. A key aspect of model performance is the representation of the diurnal cycle in convection. For the case of South Africa, the realistic representation of the complex patterns of rainfall over regions of steep topography is also of particular importance. At a larger spatial scale, the model also needs to be capable of representing the west-east rainfall gradient found over South Africa. The ability of CCAM to simulate the diurnal cycle in rainfall as well as the complex spatial patterns of rainfall over eastern South Africa is analysed in “High Resolution Rainfall Modelling over the Eastern Escarpment of South Africa” (Chapter 4). The simulations described in the paper have been performed at 8km resolutions in the horizontal and span a thirty-year long period. These are the highest resolution climate simulations obtained to date for the southern African region, and were obtained through the downscaling reanalysis data of the European Centre for Medium-range Weather Forecasting (ECMWF). The simulations provide a test of the robustness of the CCAM convective rainfall parameterisations when applied at high spatial resolution, in particular in representing the complex rainfall patterns of the eastern escarpment of South Africa. / M (Geography and Environmental Management), North-West University, Potchefstroom Campus, 2015

In 2006 a Numerical Weather Prediction (NWP) model known as the Unified Model (UM) from the United Kingdom Meteorological Office (UK Met Office) was installed at the South African Weather Service (SAWS). Since then it has been used operationally at SAWS, replacing the Eta model that was previously used. The research documented in this dissertation was inspired by the need to verify the performance of the UM in simulating and predicting weather over South Africa. To achieve this aim, three model configurations of the UM were compared against each other and against observations. Verification of rainfall as well as minimum and maximum temperature for the year 2008 was therefore done to achieve this. 2008 is the first year since installation, where all the configurations of the UM used in the study are present. For rainfall verification the model was subjectively verified using the eyeball verification for the entire domain of South Africa, followed by objective verification of categorical forecasts for rainfall regions grouped according to standardized monthly rainfall totals obtained by cluster analysis and finally objective verification using continuous variables for selected stations over South Africa. Minimum and maximum temperatures were subjectively verified using the eyeball verification for the entire domain of South Africa, followed by objective verification of continuous variables for selected stations over South Africa, grouped according to different heights above mean sea level (AMSL). Both the subjective and objective verification of the three model configurations of the UM (for both rainfall as well as the minimum and maximum temperatures) suggests that 12km UM simulation with DA gives better and reliable results than the 12km and 15km UM simulations without DA. It was further shown that although there was no significant difference between the model outputs from the 12km and the 15km UM without DA, the 15km UM simulation without DA, proved to me more reliable and accurate than the 12km UM simulation without DA in simulating minimum and maximum temperatures over South Africa, on the other hand the 12km UM simulation without DA is more reliable and accurate than the 15km UM simulation without DA in simulating rainfall over South Africa. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Geography, Geoinformatics and Meteorology / unrestricted

L'augmentation artificielle de la couche stratosphérique d'aérosol de sulfate a été proposée comme méthode pour réduire le réchauffement climatique causé par les émissions anthropiques de gaz à effet de serre. Dans cette thèse, nous présentons un modèle global atmosphère-aérosol nouvellement développé, évaluons sa performance par rapport aux observations et l'appliquons pour étudier l'efficacité et les impacts de cette forme possible d'ingénierie climatique. Nous trouvons que l'effet de refroidissement réalisable par unité de masse de soufre injectée peut diminuer de façon plus drastique qu'estimé précédemment pour des taux d'injection élevés et que des injections à plus haute altitude ou dans des régions plus grandes n'entraînent pas un refroidissement plus fort. L'efficacité de la méthode pourrait donc être plutôt limitée, tout au moins dans les cas d'injections tropicales de dioxyde de soufre que nous avons modélisées. Par ailleurs, il existe plusieurs effets secondaires potentiellement nocifs, tels que le chauffage stratosphérique dû à l'absorption de rayonnement par l'aérosol provoquant de fortes perturbations dans la dynamique atmosphérique, la composition chimique de la stratosphère et les nuages hauts. Enfin, nous trouvons que les effets radiatifs de l'injection d'aérosol stratosphérique et de l'éclaircissement des nuages marins, une autre technique de géo-ingénierie proposée, seraient largement additifs et complémentaires lors de leur application parallèle. Cela pourrait permettre de concevoir un port-folio d'approches pour atteindre des objectifs climatiques spécifiques et réduire les effets secondaires indésirables de l'ingénierie climatique. / The enhancement of the stratospheric sulphate aerosol layer has been proposed as a method to abate the global warming caused by anthropogenic greenhouse gas emissions. In this thesis we present a newly developed global atmosphere-aerosol model, evaluate its performance against observations, and apply it to study the effectiveness and impacts of this possible form of climate engineering. We find that the achievable cooling effect per injected sulphur mass unit may decrease more drastically for larger injections than previously estimated and that injections at higher altitude or over larger areas do not result in a stronger cooling. The effectiveness of the method may therefore be rather limited, at least when using tropical injections of sulphur dioxide as in our model experiments. In addition, there are several potentially harmful side effects, such as stratospheric heating due to absorption of radiation by the aerosol causing strong perturbations in atmospheric dynamics, composition, and high-level clouds. Furthermore, we find that the radiative effects of stratospheric aerosol injection and marine cloud brightening, another proposed geoengineering technique, would be largely additive and complementary when applying them together. This might allow the design of portfolio approaches to achieve specific climate goals and reduce unintended side effects of climate engineering.

Dust-climate interaction over the Middle East and North Africa (MENA) has long been studied, as it is the "dustiest" region on earth. However, the quantitative and qualitative understanding of the role of dust direct radiative effect on MENA climate is still rudimentary. The present dissertation investigates dust direct radiative effect on MENA climate during summer with a special emphasis on the sensitivity of climate response to dust shortwave absorption, which is one of the most uncertain components of dust direct radiative effect. Simulations are conducted with and without dust radiative effect, to differentiate the effect of dust on climate. To elucidate the sensitivity of climate response to dust shortwave absorption, simulations with dust assume three different cases of dust shortwave absorption, representing dust as a very efficient, standard and inefficient shortwave absorber. The non-uniformly distributed dust perturb circulations at various scales. Therefore, the present study takes advantage of the high spatial resolution capabilities of an Atmospheric General Circulation Model (AGCM), High Resolution Atmospheric Model (HiRAM), which incorporates global and regional circulations. AMIP-style global high-resolution simulations are conducted at a spatial resolution of 25 km. A significant response in the strength and position of the local Hadley circulation is predicted in response to meridionally asymmetric distribution of dust and the corresponding radiative effects. Significant responses are also found in regional circulation features such as African Easterly Jet and West African Monsoon circulation. Consistent with these dynamic responses at various scales, the tropical rainbelt across MENA strengthens and shifts northward. Similarly, the temperature under rainbelt cools and that over subtropical deserts warms. Inter-comparison of various dust shortwave absorption cases shows that the response of the MENA tropical rainbelt is extremely sensitive to the strength of shortwave absorption. Further analyses reveal that the sensitivity of the rainbelt stems from the sensitivity of the multi-scale circulations that define the rainbelt. Importantly, the summer precipitation over the semi-arid strip south of Sahara, including Sahel, increases in response to dust radiative effect. The maximum response and sensitivity are predicted over this region. The sensitivity of the responses over Sahel, especially that of precipitation, is comparable to the mean state. Locally, the precipitation increase reaches up to 50% of the mean, while dust is assumed to be a very efficient absorber. As the region is characterized by the "Sahel drought", the predicted precipitation sensitivity to the dust loading over this region has a wide-range of socioeconomic implications. The present study, therefore, suggests the importance of reducing uncertainty in dust shortwave absorption for a better simulation and interpretation of the MENA climate in general, and of Sahel in particular.

Cluster computer systems assembled from commodity off-the-shelf components have emerged as a viable and cost-effective alternative to high-end custom parallel computer systems.In this thesis, we investigate how scalable performance can be achieved for database systems on clusters. In this context we specfically considered database query processing for evaluation of botlenecks and suggest optimization techniques for obtaining scalable application performance. First we systematically demonstrated that in a large cluster with high disk bandwidth, the processing capability and the I/O bus bandwidth are the two major performance bottlenecks in database systems. To identify and assess bottlenecks, we developed a Petri net model of parallel query execution on a cluster. Once identified and assessed,we address the above two performance bottlenecks by offoading certain application related tasks to the processor in the network interface card. Offoading application tasks to the processor in the network interface cards shifts the bottleneck from cluster processor to I/O bus. Further, we propose a hardware scheme,network attached disk ,and a software scheme to achieve a balanced utilization of re-sources like host processor, I/O bus, and processor in the network interface card. The proposed schemes result in a speedup of upto 1.47 compared to the base scheme, and ensures scalable performance upto 64 processors. Encouraged by the benefits of offloading application tasks to network processors, we explore the possibilities of performing the bloom filter operations in network processors. We combine offloading bloom filter operations with the proposed hardware schemes to achieve upto 50% reduction in execution time. The later part of the thesis provides introductory experiments conducted in Community At-mospheric Model(CAM), a large scale parallel application used for global weather and climate prediction. CAM is a communication intensive application that involves collective communication of large messages. In our limited experiment, we identified CAM to see the effect of compression techniques and offloading techniques (as formulated for database) on the performance of communication intensive applications. Due to time constraint, we considered only the possibility of compression technique for improving the application performance. However, offloading technique could be taken as a full-fledged research problem for further investigation In our experiment, we found compression of messages reduces the message latencies, and hence improves the execution time and scalability of the application. Without using compression techniques, performance measured on 64 processor cluster resulted in a speed up of only 15.6. While lossless compression retains the accuracy and correctness of the program, it does not result in high compression. We therefore propose lossy compression technique which can achieve a higher compression, yet retain the accuracy and numerical stability of the application while achieving a scalable performance. This leads to speedup of 31.7 on 64 processors compared to a speedup of 15.6 without message compression. We establish that the accuracy within prescribed limit of variation and numerical stability of CAM is retained under lossy compression.

La vapeur d'eau est une composante clé du système climatique. Sa distribution et sa variabilité sont des sources d'incertitude dans les modèles climatiques. L'utilisation d'observations et des réanalyses des Contenus Intégrés en Vapeur d'Eau (CIVE) peut faciliter leur évaluation. Dans ce travail, des données CIVE-GPS retraitées du réseau mondial ont été utilisées pour la période 1995-2010. Afin d'évaluer les incertitudes et les inhomogénéités dans les séries GPS, une comparaison globale avec les données de réanalyse ERA-Interim a été faite. Un bon accord général a été trouvé sur les moyennes, la variabilité et les tendances ait été trouvé. Des interruptions et inhomogénéités ont été constatées dans les séries GPS, ainsi que les problèmes de représentativité dans les zones côtières et de topographie complexe. Dans ERA-Interim, des tendances trop fortes ont été constatées dans certaines régions. ERA-Interim a aussi été comparé avec d'autres réanalyses (MERRA-2, ERA-20C, 20CR), et des différences ont été trouvées dans les tendances de les CIVE sur l'Afrique, l'Australie et l'Antarctique. Enfin, les jeux de données CIVE-GPS et CIVE-ERA-Interim ont été utilisés pour évaluer quatre configurations du modèle de circulation générale atmosphérique LMDZ avec deux physiques, libres et guidées avec les vents d'ERA-Interim. Il a été trouvé que la nouvelle physique est plus humide aux latitudes tropicales. Sans guidage, pour les deux physiques, le modèle présente des difficultés à reproduire les tendances et la variabilité obtenues par GPS et par ERA-Interim. Cela confirme l'importance de la dynamique à grande échelle pour les tendances et la variabilité des CIVE. / Water vapour is a key component of the Earth’s climate system, and its distribution and variability are sources of uncertainty in climate models. The use of long-term integrated water vapour (IWV) observations and reanalyses can help in their assessment. This work pioneered the use of reprocessed GPS IWV data for 1995-2010, converted from estimates of Zenith Total Delay. The conversion was assessed, with the goal of producing a high quality long-term IWV data set. Due to uncertainties in the GPS observations and homogeneity concerns, a global comparison with ERA-Interim reanalysis data was made. Although a general good agreement in means, variability and trends was found, issues in both data sets were highlighted. In GPS, gaps and inhomogeneities in the time series were evidenced, as well as representativeness differences in coastal areas and regions of complex topography. In ERA-Interim, too strong trends in certain regions were found. ERA-Interim was also compared with other reanalyses (MERRA-2, ERA-20C, 20CR), and differences were found in the IWV trends over Africa, Australia, and Antarctica. Finally, GPS and ERA-Interim IWV were used to assess four configurations of the LMDZ atmospheric general circulation model with two different physics and with or without nudging towards ERA-Interim wind fields. Impact of the model physics on the IWV mean was found, with the new physics being moister at tropical latitudes. Overall, the model free runs in both physics have difficulty reproducing the trends and variability observed in ERA-Interim and GPS. This is improved with the nudging, which confirms the importance of large-scale dynamics on IWV trends and variability.

Au cours de l’automne 2012, un réseau d’observation très complémentaire a été déployé dans la région des Cévennes pour la période d’observation spéciale (SOP) du projet HyMeX. Ce réseau d’observation a été spécifiquement élaboré afin d’étudier la structure et l’hétérogénéité des précipitations et en particulier, l’impact du relief sur cette structure. Dans un premier temps, l’analyse de la distribution des gouttes de pluie (DSD) au sol et le long d’un profil vertical à partir des observations nous permet de décrire précisément la structure des précipitations le long d’un gradient topographique. Afin de comprendre l’influence du relief sur cette structure, nous nous concentrons sur les processus microphysique associés à la structure des précipitations. Pour ce faire, nous définissons trois régimes de pluie et étudions l’évolution verticale de la DSD le long du gradient topographique. Les variations en nombre ou en taille dans la DSD peuvent être associées à différents processus microphysiques ou dynamiques. Pour finir, nous estimons la capacité d’un modèle paramétré de l’atmosphère tel que WRF à représenter la structure des précipitations et les processus associés dans une zone de montagne. / During Fall 2012, a complementary observational network has been deployed in the Cévennes region (South of France) for the Special Observation Period (SOP) of the HyMeX project. This network has been specifically designed to study the structure and heterogeneity of precipitations and, in particulat, the impact of orography on this structure. Firstly, the analysis of the Drop Size Distribution (DSD) at the ground et along a vertical profile from ground observations allow us to describe precisely the rainfall structure along a topographical gradient. In order to understand the influence of a relief on this structure, we focus our study on the microphysical process associated with the structure of precipitations. To do so, we define three rainfall regime et study the vertical evolution of the DSD along the topographical gradient. Variations in number and size of the DSD can be associated with different microphysical or dynamical process. Finally, we estimate the capacity of a bulk atmospheric model such as WRF to represent the rainfall structure and associated mechanisms above a mountainous area.