R. J. Swap

Variability of biomass burning aerosol optical characteristics in southern Africa during the SAFARI 2000 dry season campaign and a comparison of single scattering albedo estimates from radiometric measurements

A priority network utilizing a common bus coupled to a plurality of priority seeking peripheral devices wherein a processor or any number of processors is connected to the common bus. Each successive peripheral device is connected to the common bus in increasing priority order, such that each device will have a unique priority defined. Each peripheral device is provided with an associated peripheral control unit. Each of the peripheral control units is connected in serial fashion on an enabling line with the output of the higher priority control unit providing an enabling input to the next lowest priority peripheral control unit, such that the highest priority device requesting bus access prevents all lower priority devices from gaining access to the common bus until the higher priority device has completed its data transfer.

Horizontal and vertical transport of air over southern Africa

Tropospheric air trajectories that occurred during the Southern African Fire-Atmosphere Research Initiative (SAFARI) in August-October 1992 are described in terms of a circulation classification scheme and the vertical stability of the atmosphere. Three major and frequently occurring stable discontinuities are found to control vertical transport of aerosols in the subtropical atmosphere at the end of the dry season. Of these, the main subsidence-induced feature is a spatially ubiquitous and temporally persistent absolutely stable layer at an altitude of about 5 km (3.5 km above the interior plateau elevation). This effective obstacle to vertical mixing is observed to persist without break for up to 40 days. Below this feature an absolutely stable layer at 3 km (1.5 km above the surface) prevails on and off at the top of the surface mixing layer for up to 7 days at a time, being broken by the passage of regularly occurring westerly wave disturbances. Above the middle-level discontinuity a further absolutely stable layer is frequently discerned at an altitude of about 8 km. It is shown that five basic modes can be used to describe horizontal aerosol transportation fields over southern Africa. Dominating these is the anticyclone mode which results in frequent recirculation at spatial scales varying from hundreds to thousands of kilometers. In exiting the anticyclonic circulation, transport on the northern periphery of the system is to the west over the Atlantic Ocean via a semistationary easterly wave over the western part of the subcontinent. On the southern periphery, wave perturbations in the westerly enhance transports which exit the subcontinent to the east into the Indian Ocean. Independently derived data suggest that during SAFARI only 4% of the total transport of air from three locations south of 18 degrees 8 was into the Atlantic Ocean. Over 90% of the transport was into the Indian Ocean across 35 degrees E. This result reflects circulation fields typical of the extremely dry conditions prevailing in 1992. The integrated effect of the control exerted by atmospheric stability on vertical mixing, on the one hand, and the nature of the horizontal circulation fields, on the other, is to produce a distinctive suite of transport patterns that go a long way to explain the observed high concentrations of tropospheric aerosols and trace gases observed over the subcontinent in winter and spring, as well as over the tropical South Atlantic and southwestern Indian Oceans.

Temporal and spatial characteristics of Saharan dust outbreaks

Temporal and spatial characteristics of moderate to heavy northern African aerosol outbreaks over the North Atlantic Ocean for the period of January 1989 through December 1992 are presented using NOAAs advanced very high resolution radiometer (AVHRR) aerosol optical thickness (AOT) data. The episodic nature of the transport and deposition of northern African aeolian material as well as the intra-annual variability of these aerosol outbreaks is detailed. Saharan dust outbreaks are generally observed within relatively well-defined zonal corridors that are approximately 10° wide latitude. The latitude of these westward outbreaks progresses from south to north during the first 6–7 months of a year. After reaching its northernmost extent (∼20°−25°N) around August, the area of aerosol outbreaks then moves southward. The first six months of a year are the most active in terms of aerosol outbreak frequency and spatial extent. The annual peak in outbreak activity occurs most frequently during the months of February through April. The annual westward mass flux of northern African aeolian material over the North Atlantic during the years detailed in this study range from an annual minimum of 130 Mt for 1990 to an annual maximum of 460 Mt for 1991. These fluxes are not supplied in a steady fashion. Approximately 50% of the annual westward mass flux of northern African aerosols is transported during 20% of a given year. The annual mass fluxes presented in this study are found to be consistent with other independent estimates of the westward mass flux of northern African aerosols. The relationship of these aerosol loadings to sub-Saharan rainfall is consistent as well with earlier observations. Aerosol loadings are also presented and compared to regional sedimentation rates. The impact of the transport and deposition of this aeolian material on marine biogeochemical cycles of the North Atlantic is addressed briefly with the calculation of the annual mass flux of various nutritive species. Lastly, the continued development of the National Oceanographic and Atmospheric Administrations AVHRR AOT observations and analyses may lead to a North African aeolian emission system analog useful in the geologic sedimentary record.

The long-range transport of southern African aerosols to the tropical South Atlantic

Two episodes of long-range aerosol transport (4000 km) from southern Africa into the central tropical South Atlantic are documented. Stable nitrogen isotope analysis, multielemental analysis, and meteorological observations on local and regional scales are used to describe the observed surface aerosol chemistry during these transport episodes. The chemical, kinematic, and thermodynamic analyses suggest that for the central tropical South Atlantic, west Africa between 0° and 10°S is the primary air mass source region (over 50%) during austral spring. Over 70% of all air arriving in the lower and middle troposphere in the central tropical South Atlantic comes from a broad latitudinal band extending from 20°S to 10°N. Air coming from the east subsides and is trapped below the midlevel and trade wind inversion layers. Air from the west originates at higher levels (500 hPa) and contributes less than 30% of the air masses arriving in the central tropical South Atlantic. The source types of aerosols and precursor trace gases extend over a broad range of biomes from desert and savanna to the rain forest. During austral spring, over this broad region, processes include production from vegetation, soils, and biomass burning. The aerosol composition of air masses over and the atmospheric chemistry of the central South Atlantic is a function of the supply of biogenic, biomass burning, and aeolian emissions from tropical Africa. Rainfall is a common controlling factor for all three sources. Rain, in turn, is governed by the large-scale circulations which show pronounced interannual variability. The field measurements were taken in an extremely dry year and reflect the circulation and transport fields typical of these conditions.

Causes of bulk carbon and nitrogen isotopic fractionations in the products of vegetation burns: laboratory studies

Abstract Bulk stable isotope analysis is a means for the characterization of the sources of carbonaceous and nitrogenous material aerosols derived from biogenic sources. In order to use stable isotope techniques for characterizing the products of vegetation burns the isotope effect of combustion must be known. The C3 vegetation Colospherum mopane and Eucalyptus sp. and the C4 vegetation Cenchris cilliarus, Antephora pubesence and Saccharum officinarum, were burned under controlled conditions in the laboratory in order to better understand how the process of combustion affects the isotopic fractionation of the produced material. Carbon isotopes for aerosol particles formed during controlled laboratory burns of C3 vegetation were higher in δ 13 C by 0.5‰ compared to the source vegetation. Aerosol particles captured above the controlled laboratory burns of C4 vegetation were lower in δ 13 C by 3.5‰ compared to the source vegetation. The proposed causes for the different isotope effects shown for C3 and C4 sourced products are differences in the oxidation chemistry of these two plant types. Aerosol particulate material and ashes produced during the controlled laboratory burns of the vegetation are higher in δ 15 N than the source vegetation by 6.6‰ and 2.5‰, respectively. Furthermore, δ 15 N values for the residual material produced when Eucalyptus sp. samples were heated at discrete temperatures, suggest that different pools of nitrogenous compounds are accessed at different temperatures of heating.

AN AIR TRANSPORT CLIMATOLOGY FOR SUBTROPICAL SOUTHERN AFRICA

An air transport climatology is derived for subtropical southern Africa (Africa south of 15°S) by classifying daily synoptic situations into predominant circulation types. The annual variation of these provides the basis for determining month-by-month transport. Percentage zonal transport in easterly and westerly directions, levels of transport, and times of transit are derived from forward trajectory analyses using European Centre for Medium- range Weather Forecasts (ECMWF) data for a 7-year period. It is shown that semi-permanent subtropical continental anticyclones, transient mid-latitude ridging anticyclones and mid-latitude westerly disturbances produce major transport into the south-western Indian Ocean in the Natal plume. Only quasi-stationary tropical easterly waves result in appreciable transport into the tropical South Atlantic Ocean in the Angolan plume. Total transport is a function of circulation type and frequency, as well as plume dimensions. Transport in continental highs follows an annual cycle reaching peak values in excess of 70 per cent in winter. That in easterly waves also exhibits an annual cycle, but one peaking in summer, when up to 55 per cent transport may occur in north-western regions. Transport in ridging highs and westerly perturbations is much less and occurs throughout the year, with a slight tendency to peak in spring. Recirculation of air is shown to be considerable when anticyclonic conditions prevail. Monthly, seasonal, and annual mass fluxes over and out of southern Africa are determined from transport fields, frequency of occurrence of circulation types and from measurements of aerosol concentrations. An annual mass flux of aerosols some 134 Mtons is generated over the subcontinent. About 60 Mtons year−1 are deposited, and approximately 29 Mtons year−1 are exported westward over the Atlantic Ocean and 45 Mtons year−1 eastward over the Indian Ocean. Twenty-six million tons of the 74 Mtons of aerosols exported annually to the adjacent oceans on each coast are a product of recirculation. Deposition within 10° latitude of the coast is nearly 10 times greater on the east than on the west coast.

Large-Scale Recirculation of Air over Southern Africa

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 tran...

The Amazon Boundary-Layer Experiment (ABLE 2B): A Meteorological Perspective

Abstract In July and August 1985, and April and May 1987, two atmospheric chemistry field experiments called the Amazon Boundary-layer Experiments, (ABLE 2A and 2B) were conducted from a base near Manaus, Brazil in the central Amazon basin. The experiments were designed to determine sources, sinks, concentrations, and transports of trace gases and aerosols originating from the tropical rain forest soils, wetlands, and vegetation. We describe in this paper the design of these experiments and some of the preliminary results which have emerged. We wish, in particular, to illustrate the complexities of determining the inter-related roles between meteorological scales of motion and the fluxes, transports, and reactions of chemical species and aerosols embedded in the atmospheric fluid. Illustrative results from ABLE 2A and 2B are presented which represent both meteorological findings largely independent of the chemistry and results which stem from the chemical nature of the experiment and might not have been f...

Regional fuel load for two climatically contrasting years in southern Africa

[1] Available fuel loads for burning in savanna ecosystems in the southern African region have been estimated using a new Fuel load-Net Primary Production model based on ecophysiological processes such as respiration and Potential Evapotranspiration. The model outputs 15-day standing available fuel load layers for an entire year (a total of 24 layers). Published data from the Southern African Fire-Atmosphere Research Initiative (SAFARI-92) project and from the Southern African Regional Science Initiative (SAFARI 2000) field campaigns were generally in agreement with the estimations. Consistently with previous studies, precipitation was recognized to be the major climatic driver for fuel production. As a consequence, even though there was a regional increase in precipitation in 1999–2000 as compared to the 1991–1992 periods, the temporal and spatial variability in precipitation at fine scales (site level) was important enough to restrict generalities over the entire region for fuel load production. Four areas of interest, Etosha National Park (Namibia), Mongu (Zambia), Kasama (Zambia), and Kruger National Park (South Africa), were selected to reconstruct an aridity gradient and analyze their fuel load variability over the two years. These areas presented contrasting fuel load distributions for the two very different studied periods with arid areas producing heavier fuel loads in 1999–2000, and the more humid areas producing heavier fuel loads in 1991–1992. The consequences of such fuel load variability and the use of such results are discussed. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 1640 Global Change: Remote sensing; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 9305 Information Related to Geographic Region: Africa; KEYWORDS: climate, precipitation, net primary production, Fire, NOVI, tree cover

Eolian-transported freshwater diatoms and phytoliths across the equatorial Atlantic record: Temporal changes in Saharan dust transport patterns

The deposition of freshwater diatoms (FD) and phytotliths (P) was determined from sediment traps for a wide region (20°N–7°S) in the tropical and equatorial Atlantic, along a N-S transect in the eastern equatorial Atlantic around 10°W and in the western equatorial Atlantic around 25°W. These siliceous organisms are derived from the Sahara and Sahel regions of Africa, and eolian transport with direct settling over the open ocean is assumed to be the transport agent. Depositional rates of FD and P revealed strong coupling with seasonal changes in Saharan dust transport that are associated with seasonal precipitation patterns, major wind systems, and the geographical extension of the dust plume across the Atlantic. Mean daily fluxes were highest south of Cape Verde (FD = 9 × 104 valves m−2 d−1; P = 2 × 104 bodies m−2 d−1), moderately high off Cape Blanc and in the Guinea Basin north of the equator (of the order of 3 × 104 valves m−2 d−1 for FD, and 0.7 × 104 bodies m−2 d−1 for P), and consistently low south of the equator and in the western equatorial Atlantic. In traps north of the equator, seasonal changes were marked. Aulacoseira granulata and A. islandica were the most abundant FD in the traps, regardless of trap location and season. However, the number of FD species was higher in the Cape Blanc and Cape Verde areas. The morphological diversity (shape and size) of the P assemblage decreased with increasing distance from the African continent. Patterns of FD and P accumulation rates in surface sediments coincided with those in the traps. Robust freshwater diatom and phytolith records associated with seasonal eolian transport from Saharan and Sahelian regions into the Atlantic furnish clues that can help in our present understanding of the processes linking transport between land, atmosphere, and ocean.