African Stations Research Articles

States in temperature and rainfall variations

We report on the existence of three different states (or modes) in air temperature and rainfall variations. Meteorological records dating as far back as 1922 show that both temperature and rainfall variations in a number of East African stations are characterised by systematic switching between two of the three states. This implies that the prediction of droughts and floods may possibly be made on the basis of the latter states and the interstate switching sequences involved. A detailed analysis of the physical processes responsible for these three states is given in a subsequent paper.

Verification of FGGE Assimilations of the Tropical Wind Field: The Effect of Model and Data Bias

Abstract We examine the tropical wind field analyses produced by a recent assimilation of the Final FGGE II-b dataset. Our aim is to estimate the effects, on the tropical wind analyses, of biases in the data and biases in the assimilation system. The assimilation system was similar to that used operationally at ECMWF in the first half of 1985. The period studied is the first Special Observing Period (SOP-1). Important differences occur in the intensity of divergence at upper and lower levels in the western Pacific, as measured by cloud-track winds (SATOBs) and by rawinsondes (TEMPs). There appear to be important biases also in the SATOB estimates of the zonal flow at upper and lower levels in the eastern Pacific. There are substantial biases in the wind directions at some west African stations. The 6-hour forecasts which provide the background fields for the analyses show important underestimates of the mean intensity of the tropical divergence field, particularly in the equatorial western Pacific. The er...

The control of air pollution at South African coal-fired power stations

The control of air pollution at South African coal-fired power stations

On the Variations in Precipitable Water over Some West African Stations during the Special Observation Period of WAMEX

The precipitable water was computed from the upper air data for ten West African stations for the Special Observation Period (SOP) of the West African Monsoon Experiment (WAMEX), 14 July to 15 August 1979. The periodicity and the vertical distribution of precipitable water values were investigated. Comparisons of the precipitable water values showed that high positive and significant correlation existed between low-level and upper-level values for stations within the convective zone but at the coastal stations, very low but weakly significant (poor and nonsignificant at one station) correlations existed between the low-level and upper-level values. A period of 4.4 days in the precipitable water was obtained at one of the stations for which data were reliable. The relationship between this period and the period of some West African weather systems could not, however, be established with the data available.

The Predictability of Summer Drought Conditions in South African Grassland

ABSTRACT The feasibility of predicting total summer crop water use by grassland, based on early or pre-season soil moisture reserves, was investigated. Tests were based on the correlation over time between simulated soil moisture and related variables at six South African climate stations. A weather driven computer budget model was used to simulate daily soil moisture reserves. Results indicate that early prediction of seasonal crop water use is not feasible, but that deep soil moisture is stable enough over time to be a reliable monitoring variable for crop growing conditions. Moisture stored in the deep soil has good potential for monitoring more severe drought conditions.

A comparative study of VHF scintillation and spread F events over Natal and Fortaleza in Brazil

A comparative study is carried out of the equatorial ionospheric irregularity occurrences, as observed by range spread F events over Fortaleza (4°S, 38°W, dip latitude 1.8°) and satellite VHF signal scintillation event over Natal (5.8°S, 33.23°W, dip latitude 4.8°S) for a one‐year period 1978. Close association is observed in the occurrences and the durations of the irregularities at the two stations. Marked differences in the seasonal behavior of the occurrences of the irregularity events over these two stations are noted with respect to that over Huancayo, most important of them being that while the latter station presents equinoctial maxima in the occurrence rates (as over some African stations), the Brazilian stations show an additional and pronounced peak in December. This result is explained as possible evidence of magnetic declination control of the spread F irregularity generation and its seasonal dependence. The present results have important implications also on the coexistence of different irregularity scale sizes during equatorial plasma bubble irregularity events.

Source depth determination using multi-modal Rayleigh spectral ratios and linear discriminant analysis: A study of the reservoir-induced seismic sequence at Lake Kariba, Africa (September 1963 to August 1974)

abstract The source depths of 12 reservoir-induced seismic events at Lake Kariba were determined using the short- to long-period spectral ratios of multi-modal Rayleigh data recorded at two African stations. Source depths of less than 10 km were confirmed through the analysis of short-period pP-P time differentials. Two separate source-to-receiver crustal path models were employed in the analysis. An African shield model, with low-velocity surficial layers and a low-shear velocity zone between 7 and 18 km, was utilized to reproduce the observed multi-modal Rayleigh waves acquired from the Kariba to Nairobi path. The model is consistent with published evidence that the East African Rift System extends south to Lake Kariba. A conventional South African shield model was utilized for the Kariba to Pretoria path. Synthetic multi-modal Rayleigh waveforms were computed for a range of source depths for the two crustal models. The spectral ratios obtained from these waveforms provided the synthetic data base to which the observed Rayleigh wave spectral ratios were compared using linear discriminant analysis. Due to the limitations in the available data, the source depths could not be determined at a statistical level of confidence. However, the demonstration of the feasibility of the procedure should stimulate its application to many other problems associated with source parameter inversion.

West African water vapor variations between recent contrasting Subsaharan rainy seasons

Investigation is made of the interannual and intraseasonal variability of the southwest monsoon layer's thickness and moisture content over West Africa, and its water vapor supply to this region from the tropical Atlantic, for the Subsaharan (10°–20° N) rainy season of July–September. Results for the very deficient 1968. 1971. and 1972 Subsaharan rainy seasons. whose departures from the 1941–74 mean ranged between −0.78σ and − 1.39σ. are compared with those for the near-average rainy seasons of 1967, 1969, and 1975 (–0.27σ to + 0.06σ). The observational base consists of monthly mean rawinsonde data from West African stations and seasonal estimates of the tropical Atlantic near-surface specific humidity field. Subsaharan drought does not appear to be associated with the northward supply of unusually dry surface air to West Africa from the tropical Atlantic. Over West Africa, the magnitude of the vertically integrated monsoon layer advective water vapor flux tends to be proportional to the layer thickness. The extremely poor 1972 Subsaharan rainy season (–1.39σ) coincided with particularly shallow southwesterly flow across the Gulf of Guinea coast (–5°N); thicker monsoon layers are found characteristic of the less severe 1968 (–0.78°) and 1971 (–0.88°) droughts. However, the monsoon layer depth here during the nondrought study years does not always exceed that for drought months. The direction of the low-level water vapor flux above Dakar, a sensitive Subsaharan location (−15°N) on the West African coast, shows a stronger tendency to be from north of west during unproductive Subsaharan rainy seasons (− 0.78σ to − 1.39σ) than in near-average ones (–0.27σ to + 0.06σ). This suggests the southwest monsoon does not extend as far north along the West African coast during Subsaharan droughts as in more abundant rainy seasons. In contrast to the foregoing Gulf of Guinea results, the monsoon layer over the interior of the Subsaharan zone at ˜ 13°N exhibits minimal variability for the drought study years. The better developed nondrought southwesterly flows here tended to be thicker than those characteristic of most drought months. DOI: 10.1111/j.1600-0870.1983.tb00197.x

Body wave amplitude patterns and upper mantle attenuation variations across North America

Summary. A broad band amplitude study of P- and SH-waves from deep South American events recorded at WWSSN stations in the United States provides constraints on upper mantle variations in attenuation. The events were selected to minimize contamination due to source complexity, radiation pattern, and source structure. The prevalent feature in the short- and longperiod bands is that the SH amplitudes show the same regional variations as the P amplitudes, with the regional variations of short periods being similar, but enhanced relative to the variations at long periods. The amplitudes show some systematic regional difference between the East Coast and Rocky Mountain provinces, and both short and long periods are enhanced at the Gulf Coast and midwestern stations, probably due to thick sedimentary bed receiver structure. Since detailed receiver structures for individual stations are poorly known, we used the working hypothesis that the regional amplitude patterns are caused by lateral variations in upper mantle attenuation. Both a constant Q and a frequency-dependent attenuation operator were considered, with the relative amplitudes and waveforms in short- and long-period bands used to estimate the acceptable range of At* in the constant Q model, or the acceptable range of the parameter r, , which describes the high-frequency roll-off of the absorption band in the frequency-dependent model. Time domain modelling indicates that the short-period P and SH amplitudes and waveforms in the 1-10s period range do not unambiguously demand frequency-dependent attenuation, allowing for the uncertainty in source frequency content and long-period absorption band amplitude. The short-period P amplitudes can be fit with a range in At; = 0.5 s or a range in r, from 0.001 to 0.25s, with either single parameter variation consistent with the general short-period SH amplitude behaviour. A technique for determining the absolute value of t* and possible r, values appropriate for the 5-20s band was applied to selected data. A value of t* = 0.8 s and small 7, were found for travel paths from the deep events to North America, and lower t* or large r,, values were found for paths to southern African stations. Longperiod amplitude variations cannot be explained by frequency-dependent

Magnetotelluric measurements at Eusebio, an equatorial station

Magnetotelluric measurements in the period range 100–86400 s were conducted at a coastal station under the equatorial electrojet (Eusebio, 3.87°S, 321.58°E). The magnetotelluric data were hand-scaled and analysed to obtain a scalar apparent resistivity profile at Eusebio. The depth of the intermediate conducting layer was found to be in the range 45–75 km and the final conducting layer seems to begin at a depth of about 350 km. Possible effects of the source field equatorial electrojet are discussed and our results are compared with those of an African station under the equatorial electrojet.

Evaporation values in East Africa

Potential evaporation (Ep) and actual evapotranspiration (EA) values for 84 East African stations were estimated and mapped using Morton [12, 13, 14] model. The data used in the study were long term averages up to and including 1974. TheEp andEA distributions for the months of January, April, and July and October; and annual are discussed. HighEp values exist in the semi-arid/arid areas of northern and northeastern Kenya and central Tanzania. LowEp values dominate on the highland areas. HighEA values are observed over the East African rainfall favoured and thickly vegetated areas whereas the lowestEA values are predominantly observed during all months in semi-arid/arid areas. BothEp andEp values show a good agreement with the north/south movement of the rain belts across East Africa.

P-wave travel-time residuals and the crust and upper mantle lateral inhomogeneities in Africa

Teleseismic P-wave travel-time residuals of shallow earthquakes and nuclear explosions were analyzed for 27 African stations. The distribution of positive and negative residuals was determined for Northeast Africa, northern and southern parts of the East African Rift Valleys and South Africa. The existence of lateral inhomogeneities in the upper mantle was found in these regions. African rifts are characterized by lower velocity structure of the uppermost mantle than that of the surrounding areas.

Meridional distribution of ozone in the troposphere and its seasonal variations

Data from 5 years of continuous surface ozone measurements made at 16 European and African stations, located on a meridional chain between 69.5°N and 34°S, are presented. The analysis of monthly averages exhibits an annual sinusoidal variation with mean, amplitude, and phase characteristic for each latitude. Thirty-four meridional cross-sections of ozone in the high troposphere were measured aboard commercial jetliners. Both ground-based and aircraft measurements indicate enhanced stratospheric-tropospheric ozone injection at about 30°, 45°, and 65°N. The average ozone injection fluxes vary between 3×1010 mol/cm2 s at the equator and 10×1010 mol/cm2 s at the main injection latitude.

Black Yellow Fever Immunities, Innate and Acquired, as Revealed in the American South

West Africa’s disease environment of the eighteenth and nineteenth centuries was decidedly hazardous to the health of Europeans who ventured there. Comtemporary observers reported die-offs of white troops reaching the 80 percent mark annually, while the loss of one half of a ship’s company on the coast was not all that unusual. Philip Curtin has calculated that on the average England’s loss of white troops ranged between 300-700 per 1,000 mean strength per annum with his most recent word on the subject placing the overall white death toll at about half of the white soldiers, government officials, and civilian personnel who reached West African shores. K. G. Davies, on the other hand, would have the “risks of the African station” even higher with an individual facing “three chances in five of being dead within a year.”West African natives by contrast positively thrived amidst European death. Again referring to Curtin’s data, the biggest killer of whites by far was “fever.” The fevers of Sierra Leone, for example, dispatched white troops at the rate of 410.2 per thousand per annum during the years 1819-1836, yet caused the death of only 2.5 African troops per thousand per annum. A similar differential experience with fevers occurred throughout West Africa—an experience which Professor Curtin has suggested constituted a crucial reason for the Atlantic slave traded Europeans would have preferred to locate plantations in tropical Africa close to a seemingly inexhaustible source of cheap labor, but they were persuaded by the lethal nature of West African fevers to locate those plantations instead in the more salubrious New World. Put plainly, they found the expense of transporting African workers across the Atlantic eminently preferable to challenging the odds against their own survival in Africa.

The Transport Of Water Vapour In The Atmosphere Over Southern Africa

Abstract The total and eddy transport of water vapour are derived from 1967 upper-air data at South African aerological stations. Broad features of the transport fields are given as well as a brief comparative analysis of water vapour studies. It is shown that the transport of vapour over Southern Africa corresponds closely with results obtained in similar studies elsewhere.

A lunar effect in the occurrence of conjugate echoes on topside sounder ionograms

Radio waves, transmitted by the Alouette 2 and Isis 1 satellites, can become trapped in magnetic field aligned ionization ducts, which can extend between the northern and southern hemispheres of the earth. In a set of data containing 1.8 × 105 ionograms recorded at five low-latitude telemetry stations from December 1965 to November 1971, conjugate echoes were observed on 4.5% of the ionograms. The frequency of occurrence of these echoes has a lunar semimonthly oscillation about the 4.5% average with an amplitude of (0.63 ± 0.13)% and with maximums at lunar ages of 4.9 and 16.9 hours (6 and 21 days after new moon). This oscillation occurs in independent blocks of data at very nearly the same lunar ages. The effect appears to depend on longitude, since it is found to be strongest in the data recorded at the American telemetry stations, intermediate at the African and Hawaiian stations, and weakest at the Asian and Australian stations. This effect is strongest for data recorded at the Quito telemetry station, where for data recorded between December 1965 and December 1966 a lunar semimonthly oscillation of amplitude (3.0 ± 0.2)% was found in the percentage occurrence of conjugate echoes. This semimonthly variation probably indicates that lunar tidal induced electric fields in the dynamo region have some influence in the production or duration of conjugate ducts; it is surprising, however, that no lunar diurnal or semidiurnal variation in occurrence frequency of conjugate ducts was found.

Note on Pressure Oscillations Over South Africa

Abstract Surface and 500-mb pressure series based on 12- and 24-hourly observations for 1958, 1966, 1965–66, and 1965–69 are analyzed for several South African stations. Preferential pressure fluctuations with periods of about 6 days are best-developed over southern and eastern coastal regions and are shown to advance from Cape Town to areas northwest of Durban at rates in good agreement with synoptic experience.

SPATIAL VARIATION OF RAINFALL SPECTRA IN SOUTH AFRICA

ABSTRACT Fourier and spectral analysis of rainfall records at 163 South African stations over the period 1910–1969, and at thirty-nine stations from 1880–1969, shows that cyclical fluctuations with periods of about 18–20, 13–15, 10, 4.5, 3.5, 2.8, and 2.3 years account for much of the observed rainfall variance over the country. These fluctuations exhibit distinctive regional distributions that appear to have changed little over the last century.

Comparative biology and ecology ofApanteles flavipes(Cam.) andA. sesamiaeCam. as parasites of graminaceous borers

A panteles flavipes(Cam.) andA. sesamiaeCam. are important parasites of graminaceous borers in the Oriental and Ethiopian Regions, respectively. Both have been utilised for biological control.A. sesamiaehas become the commonest parasite ofSesamia calamistisHmps. in Mauritius (Anon., 1954), whileA. flavipeshas become remarkably successful againstDiatraea saccharalis(F.) in Barbados (Simmonds, 1969a). In view of the interest engendered in these parasites, particularly the latter, by these successes, a general account is presented here of their biology, based on published data and on the author's observations at the East African Station (Kawanda, Uganda) of the Commonwealth Institute of Biological Control.

The Meridional Flux Of Atmospheric Water Vapour Across 30° s Over South Africa

Abstract The content of atmospheric water vapour and the meridional component of total and eddy transport of vapour across 300 S are derived f,rom 1967 upper-air data at three South African aerological stations. It is shown that the meridional eddy transport of vapour contributes substantially to the total, transport.