Variability and characterizations of wind events over the Cape Columbine region along the west coast of South Africa
Master Thesis
2012
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University of Cape Town
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Abstract
Wind data over Cape Columbine is investigated over a 38 year long time series to determine whether there are noticeable changes or variability through time. The long period dataset is collected using an Automatic Weather Station. Diurnal and seasonal to annual characterizations of wind together with occurrences of related events are established. Other linkages with synoptic to near-global (e.g., ENSO) scale circulations are investigated by examining relationships with other meteorological variables. This is achieved by complimenting long term data series with data from the new technologically advanced Automatic Weather Station (AWS). From the new AWS, several relationships and connections between different atmospheric variables were developed. These include the relationship between Air Temperature and Humidity, Wind Speed and Atmospheric Pressure, and Wind Direction and Rainfall. The AWS is able to monitor the passage of synoptic (e.g., cold fronts) and mesoscale (e.g., coastal low) weather systems. Passages of cold fronts over time could easily be detected by utilizing all the different variables as recorded by the AWS (Air Temperature, Wind Speed and Direction, Relative Humidity, Atmospheric Pressure and Rainfall). The most significant atmospheric system that passed over Cape Columbine over the 2009/10 summer was a deep low pressure system, where a significant amount of rainfall, atmospheric pressure depression and drop in temperature were witnessed. There were 17 upwelling events during this six months study period. Several of them were short-lived at 4 to 6 day durations, and the longest upwelling event occurred on the 24th December 2009 and lasted until January 14th 2010. An investigation into the variability in characteristics and behavior of wind signals at Cape Columbine was conducted. The study investigated wind behavior for the period 1957 to 1995 with data collected from a point source Automatic Weather Station. Apart from the characteristics of wind over time period, linkages with large scale circulations, frequency and duration of these wind events occurrences were studied. These were achieved by partitioning the study period into different time scales; namely, the diurnal, seasonal, inter-annual and the decadal. On the diurnal time scale, a strong variability in wind strength over the course of each day due to the land/sea breeze existed. Seasonally, the wind regime in Cape Columbine was found to be driven by the latitudinal shifting of the semi-permanent South Atlantic Anticyclone. However, there are pre-dominant southerly winds throughout the year. The long record showed the possible influence of ENSO and the Benguela NiƱo on interannual and decadal variations in the winds at Cape Columbine.
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Includes bibliographical references.
Reference:
Ramulifho, A. 2012. Variability and characterizations of wind events over the Cape Columbine region along the west coast of South Africa. University of Cape Town.