Browsing by Author "Lennard, Christopher James"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Open Access
    The causes of avian extinction and rarity
    (1997) Lennard, Christopher James; Hockey, Phil A R
    Biological extinction rates have escalated by as much as 1000 times the background extinction rate over the last 1500 years, causing concern over the long-term survival of many species. Avian extinctions since 1600 have been well documented relative to other taxa, as have current levels of avian threat. This study analyses avian extinctions post-1600 and current threats in an attempt to develop some predictive capacity about which avian taxa should be awarded the highest conservation priority. Analyses performed include examinations of the causes of avian extinction and threat, geographical location of extinct and threatened species, prehistoric and historical extinction rates, endemicity, migration, bird body size and phylogenetic diversity. An analysis dealing with historical and phylogenetic aspects of endangered and critically threatened species was performed, from which the world's most threatened species were identified. Factors which were the primary cause of historical extinctions are generally not the primary factors threatening today's extant avifauna. Whilst introduced predators and exploitation were primary causes of historical extinctions, habitat destruction poses the greatest threat to extant birds. Species predisposed to extinction typically have restricted ranges, and, compounded by habitat loss, these ranges are becoming more restricted. This has resulted in mainland-dwelling species becoming as prone to extinction as island-dwelling species have been historically. Introduced predators, however, do still threaten many of the world's most threatened species and their potential effects are highlighted in the phylogenetic analysis. Already, many extinctions may be inevitable over the next 25 years as a result of habitat loss. The magnitude of extinctions across all animal and plant species in the next few decades could be comparable with that of previous mass extinctions unless immediate conservation action is taken. However, future conservation efforts will have to be prioritized, and this study is intended as a contribution towards such a prioritization exercise.
  • Thumbnail Image
    Item
    Open Access
    An exploration of South Africa's wind climate using station records and reanalysis data
    (2016) Argent, Brendan; Hewitson, Bruce; Lennard, Christopher James; Hahmann, Andrea
    Sparse information about the wind climate of South Africa behooves an exploration of the drivers of surface wind speed, especially in the context of wind resource assessment. This work quantifies the coupling between the synoptic circulation states and station-scale flows to develop a process-based regionalisation of wind regimes over the country .A thorough inspection of available South African Weather Service (SAWS) wind records is conducted and a quality control procedure is applied. The procedure reveals a large proportion of the data are missing and existing data contain numerous errors such that only107 of the original 960 stations passed the quality control criteria. However, data from these107 stations only overlap temporally 2% of the time, which makes the data inappropriate fora regionalisation procedure. To ameliorate this, a method for incorporating bias-corrected time series data from a reanalysis data set is developed. Data from the 0.3◦ resolution hourly Climate Forecast System Reanalysis (CFSR) be-tween 1989-2010 is selected to improve the temporal coverage of the station data. The raw CFSR data overestimates wind speeds and underestimates the temporal variability and long-term trends. A bias correction method based on the wind speed and direction, time of day and month of the year is developed which successfully removes the mean error on wind speed and direction and improves the correlation with station records. This is achieved without disrupting spatial correlation patterns. Corrected and extended wind time series from each station site are used for the regionalisation. The regionalisation uses a self-organising map (SOM) to define the archetypal synoptic circulation patterns in the reanalysis data set and the influence of these on the local wind climate is quantified. 12 representative atmospheric states are defined by the SOM that are consistent with the existing literature and capture the major synoptic circulation states. A hierarchical clustering is then used to define wind climate regions based on the coupling between these circulation states and the extended station data. Six relatively cohesive spatial wind-climate groupings are identified that are physically consistent with the driving synoptic environment and are characteristic in terms of terrain and response to synoptic drivers. This process-based regionalisation facilitates a future assessment of potential changes in the wind climate of South Africa as a result of a warming world.
  • Thumbnail Image
    Item
    Open Access
    Identification and simulation of extreme precipitation using a computationally inexpensive methodology
    (2008) Lennard, Christopher James
    An examination of characteristics extreme precipitation in the greater Cape Town region is undertaken. Thereafter, an investigation into the characteristics of these changes is made using two approaches. The first is an empirical methodology to explore the historical attributes of extreme events and the second a numerical method. These are used to demonstrate an approach to produce high resolution forecasts of extreme precipitation if computational resources are scarce. Initially, changes in the characteristics of extreme precipitation in the greater Cape Town region is documented. Then self organizing maps are used to identify archetypal synoptic circulations that are associated with extreme precipitation over the region. Thereafter, days whose synoptic state matched those of the synoptic archetypes are simulated at a resolution of one kilometer to capture regional topographic modification of extreme precipitation. Following this, the simulated precipitation is validated against observed data and the model performance is assessed. These approaches were tested over Cape Town, South Africa which has complex topography where extreme rainfall is not well predicted. As this methodology is computationally relatively inexpensive, it has applicability to regions of the world where these resources are limited, more especially Africa where the state of climate science is poor. An analysis of historical station data from three locations in the greater Cape Town region showed mixed trends in extreme rainfall where extreme rainfall was taken as that in the 90th percentile. One station, located in the lee of topography, showed a statistically significant increase in the intensity of extreme rainfall and another, at a relatively topography-free location, a significant decrease. The third station showed no significant trend. Decadal changes in monthly precipitation show a shift in the start and end of the extreme rainfall season to starting later in winter and continuing into the early spring. The station with the significant increase in extreme rainfall intensity also showed an increase in 99th percentile rainfall intensity. Synoptic states associated with extreme rainfall in the greater Cape Town region were then examined. These were identified as mid-latitude cyclones with centers at relatively low latitudes. They were characterized by strong pressure gradients at the surface and in the upper air high as well as high regional humidities. Precipitation characteristics of the frontal systems ranged from precipitation that fell over a number of days in relatively low daily amounts to very heavy precipitation that fell in one day. Over the twenty-three year test period examined, there are changes
  • No Thumbnail Available
    Item
    Open Access
    The causes of avian extinction and rarity
    (1997) Lennard, Christopher James; Hockey, Phil
    Biological extinction rates have escalated by as much as 1000 times the background extinction rate over the last 1500 years, causing concern over the long-term survival of many species. Avian extinctions since 1600 have been well documented relative to other taxa, as have current levels of avian threat. This study analyses avian extinctions post-1600 and current threats in an attempt to develop some predictive capacity about which avian taxa should be awarded the highest conservation priority. Analyses performed include examinations of the causes of avian extinction and threat, geographical location of extinct and threatened species, prehistoric and historical extinction rates, endemicity, migration, bird body size and phylogenetic diversity. An analysis dealing with historical and phylogenetic aspects of endangered and critically threatened species was performed, from which the world's most threatened species were identified. Factors which were the primary cause of historical extinctions are generally not the primary factors threatening today's extant avifauna. Whilst introduced predators and exploitation were primary causes of historical extinctions, habitat destruction poses the greatest threat to extant birds. Species predisposed to extinction typically have restricted ranges, and, compounded by habitat loss, these ranges are becoming more restricted. This has resulted in mainland-dwelling species becoming as prone to extinction as island-dwelling species have been historically. Introduced predators, however, do still threaten many of the world's most threatened species and their potential effects are highlighted in the phylogenetic analysis. Already, many extinctions may be inevitable over the next 25 years as a result of habitat loss. The magnitude of extinctions across all animal and plant species in the next few decades could be comparable with that of previous mass extinctions unless immediate conservation action is taken. However, future conservation efforts will have to be prioritized, and this study is intended as a contribution towards such a prioritization exercise.