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P. D. Tyson

Abstract

Horizontal velocity spectra of the mountain wind show that maximum turbulent energy is generated by waves the order of 10 km in length and a period of 1 hr, thereby filling the spectral gap between purely micrometeorological fluctuations on the one hand, and mesometeorological on the other.

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O. Miron and P. D. Tyson

Abstract

Circulation anomaly fields for wet and dry spells on monthly, seasonal and near-decadal scales are presented to show that at all these scales, both at the surface and at 500 mb, wet conditions are associated with negative anomalies of geopotential height over the continent and positive anomalies over the oceans to the southwest and south, and vice versa during dry spells. The changes in the pressure anomaly fields are shown to produce distinctive adjustments in the atmospheric fields of motion responsible for rain-producing conditions.

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T. G. J. Dyer and P. D. Tyson

Abstract

By fitting a trigonometrical regression model to a regionally averaged time series for the period 1910–72, and by extrapolation to the year 2000, an estimate of future extended wet and dry spells has been made for the summer rainfall region of South Africa. It is suggested that the periods 1972–81 and 1991–2000 will experience above normal rainfall and the period 1981–90 will be drier than normal. Results to date show that mean regional rainfall for the hydrological years 1972–73 to 1975–76 has conformed to the suggested pattern.

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P. D. Tyson and R. A. Preston-Whyte

Abstract

Observations are presented to show the occurrence of boundary layer oscillations in the form of widespread and large-scale topographically-induced mountain-plain and plain-mountain winds over the deeply dissected eastern plateau slopes of Natal. By night, extensive northwesterly advection of cool air occurs between the Drakensberg escarpment zone and the coast; by day, the flow is reversed and southeasterly inland advection is characteristic of clear, fine conditions. The distinctive temporal and spatial characteristics of regional winds correspond in a near-ideal manner to those expected in a thermodynamically direct circulation caused by the diurnal cycle of insolation.

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P. D. Tyson, M. Garstang, and R. Swap

Abstract

Kinematic air parcel trajectory analysis is used to determine patterns of horizontal air transport in 2000 km × 2000 km areas over southern Africa. From these, composite zonal and meridional transport fields are derived for the subcontinent to estimate the extent to which recirculation of air and aerosols may take place in the lower troposphere between the surface and 500 hPa. The nature and degree of recirculation beneath the persistent 500-hPa absolutely stable layer is demonstrated, and transport by recirculation in discrete streams is shown to constitute 44% of the total transport over the region.

From a determination of air volume fluxes and estimates of aerosol concentrations, the total mass flux of aerosols by direct transport and by recirculation in conditions during which semipermanent, subtropical, continental anticyclones prevail is estimated to be about 51 Mton yr−1 in the surface-to-hPa layer. Recirculation comprises approximately 22 Mton yr−1 of this amount. Of the recirculated transport, about 5 Mton yr−1 is recirculated to the west in easterly transport and 17 Mton yr−1 to the east in westerly transport.

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N. J. Tapper, P. D. Tyson, I. F. Owens, and W. J. Hastie

Abstract

An energy balance model is presented and tested against surface temperature fields observed over Christchurch under clear winter conditions. The fit between observed and simulated fields is shown to be close by night and slightly less so by day. Using the model the spatial variation of net radiation, sensible, latent and substrate heat fluxes within the Christchurch urban heat island is illustrated for average winter cloudless days and nights.

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