Michael Garstang

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.

International geosphere-biosphere programme/international global atmospheric chemistry SAFARI-92 field experiment: Background and overview

The International Geosphere-Biosphere Programme/International Global Atmospheric Chemistry (IGBP/IGAC) Southern Africa Fire-Atmosphere Research Initiative (SAFARI-92) field experiment was conducted in the 1992 dry season in southern Africa. The objective of the experiment was a comprehensive investigation of the role of vegetation fires, particularly savanna fires, in atmospheric chemistry, climate, and ecology. During SAFARI-92 experimental fires were conducted in Kruger National Park, South Africa, and at some sites in Zambia, in order to study fire behavior and trace gas and aerosol emissions. Regional studies on atmospheric chemistry and meteorology showed that vegetation fires account for a substantial amount of photochemical oxidants and haze over the subcontinent, and that the export of smoke-laden air masses contributed strongly to the ozone burden of the remote atmosphere in the southern tropical Atlantic region. The relationships between fire, soil moisture status, and soil trace gas emissions were investigated for several climatically and chemically important gases. Remote sensing studies showed that advanced very high resolution radiometer/local area coverage (AVHRR/LAC) imagery was valuable for fire monitoring in the region and in combination with biomass models could be used for the estimation of pyrogenic emissions.

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.

Amazon Coastal Squall Lines. Part I: Structure and Kinematics

Abstract Mesoscale to synoptic-scale squall lines that form along the northeastern coast of South America as sea-breeze-induced instability lines and propagate through the Amazon Basin are investigated using data collected during the April–May 1987 Amazon Boundary Layer Experiment (ABLE 2B). These systems, termed “Amazon coastal squall lines” (ACSL), have been noted by others, but details of the structure and evolution of the ACSL are limited. The present paper uses Geostationary Operational Environmental Satellite, radar, upper-air rawinsonde, and surface Portable Automated Mesonet data to describe the structure, dynamics, and life cycle of the ACSL. Twelve ACSL were sampled during ABLE 2B, and three cases are discussed in detail. The ACSL are discontinuous lines of organized mesoscale cloud clusters that propagate across the central Amazon Basin at speeds of 50–60 km h−1. The ACSL undergo six possible life cycle stages: coastal genesis, intensification, maturity, weakening, reintensification, and dissip...

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...

Cloud modulation of surface solar irradiance at a pasture site in southern Brazil

Broken cloud fields create mosaic radiative landscapes with interchanging cloud-shaded and sunlit areas. While clouds attenuate solar radiation incident on cloud-shaded areas, sunlit ground surfaces may actually receive more irradiance than under a clear sky due to light scattering and reflection from neighboring clouds. In this paper, we studied these two opposite but closely related aspects of cloud modulation of surface solar irradiance at a pasture site in southern Brazil. We analyzed a high-resolution time series of surface measurements obtained during the 1999 wet season. Surface solar irradiance frequently (more than 20% of the time) exceeded clear-sky levels and occasionally surpassed the extraterrestrial radiation. Clouds created a bimodal frequency distribution of surface solar irradiance, producing an average of approximately 50 and 14% for attenuation and enhancement, respectively, as compared to corresponding clear-sky level irradiance. The average duration of enhancement periods was about 1/3 of the average duration of attenuation periods. On the daily basis, cloud-induced enhancement contributed an average of 4% of the daily solar input to the surface and compensated for more than 10% of the attenuation due to the presence of clouds. Through spectral analysis, two temporal regimes were shown to modulate the surface irradiance by clouds. One was a convective/mesoscale of tens of minutes to hours and the other was a turbulent scale of several minutes corresponding to the classical Kolmogorov f−5/3 power law.

The Role of Surface Divergence and Vorticity in the Life Cycle of Convective Rainfall. Part I: Observation and Analysis

Abstract The role of the surface velocity fields in the formation, maintenance and decay of convective storms is examined using approximately 90 days of measurements in a densely instrumented network (660 km2) in south Florida. The results show statistically strong cause and effect relationships between surface convergence and onset of rain, storm intensity and duration. Short-term prediction of the onset of rain and the amount of rain produced proves possible. The surface fields of divergence provide an estimate of storm mass and moisture transports. The size of the surface area of convergence, by governing the supply of moisture, plays a controlling role in storm intensity. Large storms are efficient (72%), in terms of moisture supplied to rain produced, compared to smaller storms (37%). Within the confines of the experiment network, weak storms are in near mass balance, while inflow greatly exceeds outflow in the intense storm. The near mass balance of the weak storm suggests cloud-to-subcloud layer in...