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General circulation model simulations of Southern African regional climate.Joubert, Alec Michael. January 1994 (has links)
Dissertation submitted to the Faculty of Science,
University of the Witwatersraild, Johannesburg
for the Degree of Master of Science. / Six general circulation model simulations of present-day southern African
climate are assessed, Each of these models are early-generation equilibrium
climate models linked to simple mixed-slab oceans. Simulations of surface
air temperature over the subcontinent are sensitive to the grid-scale
parameterisation of convection in summer. At high latitudes, large simulation
errors are caused by errors in the specification of sea-ice albedo feedbacks.
Increased spatial resolution and the inclusion of a gravity wave drag term in
the momentum equations results in a markedly-improved simulated mean sea
level pressure distribution. Tho models successfully simulate the pattern of
rainfall seasonality over the Subcontinent, although grid-point simulation of
precipitation is unreliable. Treatment of convection, cloud radiative feedbacks
and the oceans by this generation of models is simplistic, and consequently
there is a large degree of uncertainty associated with predictions of future
climate under doubled-carbon dioxide conditions. For this reason, more
reliable estimates of future conditions will be achieved using only those
models which reproduce present climate most accurately. Early-generation
general circulation models suggest a warming of 4°C to 5°C for the southern
African region as a whole throughout the year. Over the subcontinent,
warming is expected to be least in the tropics, and greatest in the dry
subtropical regions in winter. Estimated changes in mean sea level pressure
indicate a southward shlft of all pressure systems, with a weakening of the
subtropical high pressure belt and mid-latitude westerlies. Little agreement
exists between the models concerning predictions of regional precipitation
change. However, broad scale changes in precipitation patterns are in
accordance with predicted circulation changes over the subcontinent.
Generally wetter conditions may be expected in the tropics throughout the
year and over the summer rainfall region during summer. Decreased winter
rainfall may be expected over the winter rainfall region of the south-western
Cape. However, estimated precipitation changes are grid-point specific and
therefore must riot be over-interpreted. The present climate validation has
resulted in more reliable estimates of future conditions for the southern
African region. This approach should be extended to recent slrnulations which
include more comprehensive treatment of important physical processes. / Andrew Chakane 2018
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Modelling present and future climates over Southern Africa.Joubert, Alec Michael January 1997 (has links)
Thesis submitted to the Faculty of Science, Department of Geography and Environmental Studies, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the Degree of Doctor of Philosophy / The representation of contemporary southern African climate by a wide range of general
circulation models used in climate studies is evaluated. In addition, projections of regional
climate change by the models are interpreted in terms of their present climate performance.
Projections of regional climate change by two different types of climate models are
considered. First, projections of the equilibrium response to an instantaneous doubling of
atmospheric carbon dioxide using atmospheric models linked to simple mixed-layer oceans
are assessed. Second, projections of the transient response to gradually-increasing
anthropogenic forcing by fully-coupled ocean-atmosphere general circulation models are
considered.
All of the mixed-layer models considered have been developed since 1990 and are more
recent and generally higher-resolution versions of the models considered previously for
southern Africa. The improved resolution and model physics result in a general
improvement in the representation of several features of circulation around southern
Africa. Specifically, these include the meridional pressure gradient, the zonal wind profile,
the intensity and seasonal location of the circumpolar trough and the subtropical
anticyclones, as well as planetary wave structure at 500 hPa. Atmospheric models forced
by observed sea-surface temperatures simulate the large-scale circulation adjustments
around southern Africa known to accompany periods of above- and below-average rainfall
over the subcontinent. Fully-coupled models simulate the observed features of intra- and
intra- annual variability in mean sea-level pressure, although the simulated variability is
weaker than observed. Summer rainfall totals throughout southern Africa are overestimated
by all of the models, although the pattern of rainfall seasonality over the
subcontinent as a whole is well-reproduced. The inclusion of sulphate aerosols in addition
to greenhouse gases does not result in a statistically significant improvement in the
simulation of contemporary temperature variability over southern Africa.
Warming projected by fully-coupled models is smaller than projections by mixed-layer
models due to the fact that the transient response of the fully-coupled system and not an
equilibrium response of an atmospheric model linked to a mixed-layer ocean is simulated.
The inclusion of sulphate aerosols results in a reduction in the magnitude and rate of
warming over southern Africa. Projected changes in the diurnal temperature range are
seasonally-dependent, with increases in summer and autumn and decreases in winter.
Simulated changes in mean sea-level pressure are small but similar in magnitude to
observed anomalies associated with extended wet and dry spells over the subcontinent. No change in rainfall seasonality over southern Africa is expected. Nonetheless, little
confidence exists in projected changes in total rainfall. While both types of model simulate
a 10-15% decrease in summer rainfall on average, projected changes are smaller than the simulation errors and little inter-model consensus in terms of the sign of projected changes
exists. No change in the location or intensity of anticyclonic circulation and divergence at
700 hPa in winter is expected. While fully-coupled models provide a more comprehensive
treatment of the global climate system and the process of climate change, there is no
evidence to conclude that current fully-coupled models should be used to the exclusion of
mixed-layer models when developing regional climate change scenarios for southern
Africa. / Andrew Chakane 2018
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Simulating sea-surface temperature effects on Southern African rainfall using a mesoscale numerical modelCrimp, Steven Jeffrey January 1996 (has links)
Dissertation submitted to the Faculty of Science, University of the Witwatersrand, for
completion of the Degree of' Master of Science / The atmospheric response of the Colorado State University Regional Atmospheric
Modelling System (RAMS) to sea-surface temperature anomaliesis investigated. A period
of four days was chosen from 21 to 24 January 1981, where focus was placed on the
development and dissipation of a tropical-temperate trough across Southern Africa.
Previous experimenting this mesoscalenumerical model have detemined the kinematic,
moisture, and thermodynamic nature of these synoptic features. The research in this
dissertation focuses specifically on the sensitivity of the numerical model's simulated
responses to positive sea-surface temperature anomalies. Three separate experiments were devised, in which positive anomalous temperatures were added to the ocean surface north of Madagascar (in the tropical Indian Ocean), at the region of the Agulhas Current retroflection, and along the tropical African west coast (in the Northern Benguela and Angola currents). The circulation aspects of each sensitivity test were investigated through the comparison of simulated variables such as vapour and cloud mixing ratios, temperature, streamlines and vertical velocity, with the same variables created by a control simulation.
The results indicate that for the first sensitivity test, (the Madagascar anomaly),
cyclogenesis was initiated over the area of modified sea temperatures which resulted in a
marginal decrease in continental precipitation. The second sensitivity test (over the
Agulhas retroflection) produced a much smaller simulated response to the addition of
anomalously warm sea temperatures than the tropical Indian Ocean anomaly. Instability
and precipitation values increased over the anomalously warm retroflection region, and
were slowly transferred along the westerly wave perturbation and the South African east
coast. The third sensitivity experiment showed a predominantly localised simulated
increase in precipitation over Gabon and the Congo, with the slow southward progression
of other simulated circulation differences taking place. The small perturbations in each of
the simulated meteorological responses are consistent with the expected climate response
to anomalously warm sea-surface temperatures in those areas. / AC 2018
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