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At least 649 records · Page 36

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NASA NTRS1986

Tropical-extratropical geopotential height teleconnections during the Northern Hemisphere winter

Simultaneous and lagged correlation patterns between height anomalies in the Northern Hemisphere (NH) or in the Southern Hemisphere (SH) and height anomalies at tropical stations in two major tropical precipitation zones (the Indochina maritime continent and Africa) were examined. The height anomalies among the tropical stations in these zones and its nearby Pacific Ocean, South America, and Africa are strongly correlated; they are also correlated with stations in Australia and the North PLacific Ocean. The correlations between height anomalies at any of these stations and NH height anomalies show a well-defined global pattern with major features concentrated in the United States and adjacent oceans. Depending upon the location of the stations, the pattern is either a Pacific North American (PNA), a Tropical NH (TNH), or a mixed pattern having both elements. During the El Nino/Southern Oscillation (ENSO) years both TNH and PNA patterns appear in simultaneous and lagged maps, but in non-ENSO years the TNH is weak in simultaneous charts.

Mo, Kingtse C.↗

NASA NTRS1987

Enhancement of surface-atmosphere fluxes by desert-fringe vegetation through reduction of surface albedo and of soil heat flux

Under the arid conditions prevailing at the end of the dry season in the western Negev/northern Sinai region, vegetation causes a sharp increase relative to bare soil in the daytime sensible heat flux from the surface to the atmosphere. Two mechanisms are involved: the increase in the surface absorptivity and a decrease in the surface heat flux. By increasing the sensible heat flux to the atmosphere through the albedo and the soil heat flux reductions, the desert-fringe vegetation increases the daytime convection and the growth of the planetary boundary layer. Removal of vegetation by overgrazing, by reducing the sensible heat flux, tends to reduce daytime convective precipitation, producing higher probabilities of drought conditions. This assessment of overgrazing is based on observations in the Sinai/Negev, where the soil albedo is high and where overgrazing produces an essential bare soil. Even if the assessment for the Sinai/Negev does not quantitatively apply throughout Africa, the current practice in many African countries of maintaining a large population of grazing animals, can contribute through the mesoscale mechanisms described to reduce daytime convective precipitation, perpetuating higher probabilities of drought. Time-of-day analysis of precipitation in Africa appears worthwhile, to better assess the role of the surface conditions in contributing to drought.

Otterman, J.↗

NASA NTRS1988

Summary of along-track data from the Earth radiation budget satellite for several major desert regions

For several days in January and August 1985, the Earth Radiation Budget Satellite, a component of the Earth Radiation Budget Experiment (ERBE), was operated in an along-track scanning mode. A survey of radiance measurements is given for four desert areas in Africa, the Arabian Peninsula, Australia, and the Sahel region of Africa. Each overflight provides radiance information for four scene categories: clear, partly cloudy, mostly cloudy, and overcast. The data presented include the variation of radiance in each scene classification as a function of viewing zenith angle during each overflight of the five target areas. Several features of interest in the development of anisotropic models are evident, including day-night differences in longwave limb darkening and the azimuthal dependence of short wave radiance. There is some evidence that surface features may introduce thermal or visible shadowing that is not incorporated in the usual descriptions of the anisotropic behavior of radiance as viewed from space. The data also demonstrate that the ERBE scene classification algorithms give results that, at least for desert surfaces, are a function of viewing geometry.

Brooks, David R.↗

NASA NTRS1988

Modeling and estimation of a low degree geopotential model from terrestrial gravity data

The development of appropriate modeling and adjustment procedures for the estimation of harmonic coefficients of the geopotential, from surface gravity data was studied, in order to provide an optimum way of utilizing the terrestrial gravity information in combination solutions currently developed at NASA/Goddard Space Flight Center, for use in the TOPEX/POSEIDON mission. The mathematical modeling was based on the fundamental boundary condition of the linearized Molodensky boundary value problem. Atmospheric and ellipsoidal corrections were applied to the surface anomalies. Terrestrial gravity solutions were found to be in good agreement with the satellite ones over areas which are well surveyed (gravimetrically), such as North America or Australia. However, systematic differences between the terrestrial only models and GEMT1, over extended regions in Africa, the Soviet Union, and China were found. In Africa, gravity anomaly differences on the order of 20 mgals and undulation differences on the order of 15 meters, over regions extending 2000 km in diameter, occur. Comparisons of the GEMT1 implied undulations with 32 well distributed Doppler derived undulations gave an RMS difference of 2.6 m, while corresponding comparison with undulations implied by the terrestrial solution gave RMS difference on the order of 15 m, which implies that the terrestrial data in that region are substantially in error.

Pavlis, Nikolaos K.↗

NASA NTRS1988

Exotic minerals in 3,500 million year old rocks: Evidence for large meteorite impacts

A relatively small area of mountainous terrain in southern Africa provides scientists from all over the world a look at what the surface of the earth was like three and a half billion years ago. The Barberton Mountains lie astride the borders of the Republic of South Africa, Mozambique, and the Kingdom of Swaziland. The discovery of several widely distributed deposits that were likely formed by major terrestrial impacts of large extraterrestrial bodies during this early period of earth's history is reported. The Barberton impact deposits are being studied by electron microscopy. The impact deposits were examined for minerals that show the effects of shock metamorphism or compositions unusual in terrestrial rocks.

Byerly, G. R.↗

NASA NTRS1987

Climatic consequences of anthropogenic vegetation changes from 1880-1980

The changes in the surface albedo associated with land-use alterations from 1880-1980 and the climatic implications of these changes are discussed. Changes in atmospheric CO2, the hydrologic cycle, atmospheric particulates and aerosols are reviewed. A global survey of the growth of agriculture and forestry data is presented. The anthopogenic vegetation changes in western Africa are examined in detail. Maps and tables of anthopogenic vegetation and albedo changes are presented for western Africa, South America, and worldwide. It is found that the mean regional albedo increased from 17.4 in 1880 to 17.8 in 1980. No apparent secular decrease in precipitation linked to vegetation clearing is observed. It is suggested that disruption of the hydrologic cycle as a result of devegetation may have led to reductions in soil moisture.

Gornitz, Vivien↗

NASA NTRS1988

Invertible canopy reflectance modeling of vegetation structure in semiarid woodland

The Li-Strahler canopy reflectance model, driven by Landsat Thematic Mapper (TM) data, provided regional estimates of tree size and density in two bioclimatic zones in West Africa. This model exploits tree geometry in an inversion technique to predict average tree size and density from reflectance data using a few simple parameters measured in the field (spatial pattern, shape, and size distribution of trees) and in the imagery (spectral signatures of scene components). The model was tested in sparse woodland and wooded grassland in the Sahelian and Sudanian bioclimatic zones in West Africa.

Franklin, Janet↗

NASA NTRS1988

A GCM simulation study of the influence of Saharan evapotranspiration and surface-albedo anomalies on July circulation and rainfall

The influence of surface albedo and evapotranspiration anomalies that could result from the hypothetical semiarid vegetation over North Africa on its July circulation and rainfall is examined using the Goddard Laboratory for Atmospheres GCM. It is shown that increased soil moisture and its dependent evapotranspiration produces a cooler and moister PBL over North Africa that is able to support enhanced moist convection and rainfall in Sahel and southern Sahara. It is found that lower surface albedo yields even higher moist static energy in the PBL and enhances the local moist convection and rainfall. Modifying the rain-evaporation parameterization in the model produces changes in the hydrological cycle and rainfall anomalies in distant regions. The effects of different falling rain parameterizations are discussed.

Sud, Y. C.↗

NASA NTRS1989

Earth observations during Space Shuttle flight STS-27: A high latitude observations opportunity - 2-6 December 1988

The earth observations from the STS-27 mission on December 2-6, 1988 are reported. The film and generic scene characteristics chosen for the mission are given. Results are given from geological observations of the Ruwenzori Mountains between Uganda and Zaire, four rift valley systems in Africa and Asia, and several volcanoes and impact craters. Environmental observations of Africa, the Middle East, South Asia, North America and the Soviet Union, are presented. Also, meteorological and oceanographic observations are discussed. The uniqueness of the high-inclination winter launch of the STS-27 mission for obtaining observations of specific features is noted.

Whitehead, Victor S.↗

NASA NTRS1990

Transport of continental air to the subantarctic Indian Ocean

The occurrence of high Rn-222 episodes (radonic storms) observed at three islands (Crozet, Kerguelen, and Amsterdam) in the subantarctic Indian Ocean is simulated using a three-dimensional chemical tracer model. The chemical tracer model is described and the simulated time series of Rn-222 concentrations at the three islands are compared to observations. The origin, seasonal frequencies, and periodicities of the storms are examined. It is found that the storms are due to fast boundary layer advection of air from South Africa, made possible by the conjunction of a subtropical high SE of Madagascar and a midlatitudes low off the southern tip of Africa. The implications of the results for the transport of continental air to the subantarctic Indian Ocean are discussed.

Balkanski, Yves J.↗

NASA NTRS1990

The significance of biomass burning as a source of carbon monoxide and ozone in the Southern Hemisphere tropics - A satellite analysis

Carbon monoxide mixing ratios obtained by the October 1984 Measurement of Air Pollution from Satellites (MAPS) experiment are compared with the distribution of October 1984 Total Ozone Mapping Spectrometer (TOMS) ozone concentrations. The TOMS and MAPS data show coincident high values of ozone and carbon monoxide over central South America and southeastern Africa. The 1984 MAPS data are also compared with tropospheric ozone concentrations derived from 6 years of TOMS and Stratospheric Aerosol and Gas Experiment (SAGE) I and II measurements. Examination of the October climatological distribution of tropospheric ozone also reveals high concentrations over central South America and southeastern Africa. These coincident high values of CO and ozone in the rural southern tropics are most likely due to biomass burning and the subsequent transport of CO and ozone by large-scale weather patterns. It appears that both CO and ozone are being transported thousands of kilometers from their source regions by the prevailing winds.

Watson, Catherine E.↗

NASA NTRS1990

Orientation of late Precambrian sutures in the Arabian-Nubian shield

Recent tectonic models have resulted in conflicting descriptions of how the late Precambrian sutures of the Arabian-Nubian shield extend into northeast Africa. The Hamisana shear zone in northeastern Sudan is critical to this discussion because it truncates and disrupts two sutures, the Allaqi-Heiani and the Onib-Sol Hamed. Analysis of field structural data, Thematic Mapper imagery, and Rb-Sr and U-Pb geochronology suggests that the Allaqi-Heiani suture is the western extension of the Onib-Sol Hamed suture and that both make up the exposed parts of a far-traveled, polydeformed ophiolitic nappe complex. Subsequent deformation localized in the Hamisana shear zone disrupted this nappe and displaced the suture between 660 and 550 Ma during regional deformation associated with the Najd fault system. These results indicate that at least one suture extends westward into the interior of northern Africa.

Stern, Robert J.↗

NASA NTRS1990

Earth observations during Space Shuttle Mission STS-33 - November 23-27, 1989

An overview is presented of photographs, astronaut impressions, and photographic equipment and techniques coincident with the Discovery STS-33 mission of November 23-27, 1989. Particularly significant geographical photographs of southern Africa, Madagascar, northern Africa, Southeast Asia islands, Pacific islands, and Australia are shown and reviewed. Meteorological and oceanographic observations are summarized along with film anomalies that occurred during this mission.

Amsbury, David L.↗

NASA NTRS1991

Global biomass burning - Atmospheric, climatic, and biospheric implications

The present volume discusses the biomass burning (BMB) studies of the International Global Atmospheric Chemistry project, GEO satellite estimation of Amazonian BMB, remote sensing of BMB in West Africa with NOAA-AVHRR, an orbital view of the great Chinese fire of 1987, BMB's role in tropical rainforest reduction, CO and O3 measurements of BMB in the Amazon, effects of vegetation burning on the atmospheric chemistry of the Venezuelan savanna, an assessment of annually-burned biomass in Africa, and light hydrocarbon emissions from African savanna burnings. Also discussed are BMB in India, trace gas and particulate emissions from BMB in temperate ecosystems, ammonia and nitric acid emissions from wetlands and boreal forest fires, combustion emissions and satellite imagery of BMB, BMB in the perspective of the global carbon cycle, modeling trace-gas emissions from BMB, NO(x) emissions from BMB, and cloud-condensation nuclei from BMB.

Levine, Joel S.↗

NASA NTRS1993

The meteorological environment of the tropospheric ozone maximum over the tropical South Atlantic Ocean

Atmospheric flow patterns are examined over the South Atlantic Ocean where a maximum of tropospheric ozone has been observed just west of southern Africa. We investigate the flow climatology during October and perform a case study for six days during October 1989. Horizontal and vertical motions are examined and used to prepare 3D backward trajectories from the region of greatest ozone. An initially zonally symmetric distribution of ozone is treated as a passive tracer and advected by 3D flows forecast by the global model. Results from the passive tracer simulation indicate that 3D advection alone can produce a maximum of tropospheric ozone in the observed location. In addition, the trajectories suggest that by-products of biomass burning could be transported to the area of maximum ozone. Low-level flow from commonly observed regions of burning in Africa streams westward to the area of interest. Over Brazil, if the burning by-products are carried into the upper troposphere by convective process, they then could be transported eastward to the ozone feature in approximately five days. There is considerable subsidence over the tropical southern Atlantic, such that stratospheric influences also are a factor in producing the ozone maximum. Both planetary-scale and transient synoptic-scale circulation features play major roles in the various transport processes that influence the region. In summary, the observed tropospheric ozone maximum appears to be caused by a complex set of horizontal and vertical advections, transport from regions of biomass burning, and stratospheric influences.

Krishnamurti, T. N.↗

NASA NTRS1993

An investigation of Ekman upwelling in the North Atlantic

The spatial and temporal variability of the North Atlantic Ekman upwelling fields on seasonal and interannual time scales is investigated on the basis of surface winds from the Fleet Numerical Oceanography Center for 1979-1986. A pronounced minimum in the basin-wide monthly mean vertical Ekman velocities during 1981-1982 is found. It is shown that the primary source of the interannual signal was the region off NW Africa in the vicinity of the Guinea Dome. Other sectors of the basin experienced no significant interannual trends. Hydrographic data and SST data from the NW Africa sector for 1981-1986 indicate a cooling trend beginning in late 1982, consistent with increased upwelling. The fall and winter seasons' mixed layers at the center of the Guinea Dome were deeper in 1984 and 1985 than in previous years. The potential impact of large interannual variations in Ekman upwelling on basin-wide primary productivity is discussed.

Mcclain, Charles R.↗

NASA NTRS1993

The AGRHYMET data communications project

The U.S. Geological Survey (USGS) and the U.S. Agency for International Development (USAID) are providing technical assistance to the AGRHYMET program in West Africa. AGRHYMET staff use remote sensing technology to produce satellite image maps of the Sahel region of West Africa. These image maps may show vegetation greenness, sea surface temperatures, or processed weather satellite imagery. The image maps must be distributed from the AGRHYMET Regional Center in Niger to national AGRHYMET centers in the member countries of Burkina Faso, Cape Verde, Chad, Gambia, Guinea-Bissau, Mali, Mauritania, Niger, and Senegal. After consideration of a number of land- and space-based solutions for image map distribution, the best solution was determined to be use of International Maritime Satellite Organization (INMARSAT) land-based terminals. In April 1992, a field test and proof-of-concept demonstration using land-mobile terminals produced favorable results. The USGS and USAID are setting up a wide area network using INMARSAT terminals to link the AGRHYMET sites for image data transfer. The system is in the procurement and installation phase and initial opening capability may be operational for the 1993 growing season, starting in May 1993.

Mah, G. R.↗

NASA NTRS1994

Impacts, extinctions, volcanism, glaciations, and tectonics: Matches and mismatches

The debate concerning possible reactions between impacts, extinction events, and volcanism has recently taken a new turn. Diamictites and associated sedimentary deposits long regarded by geologists as glaciogenic, have been reinterpreted as impact-related. Going further, the Permo-Carboniferous diamictites that are widespread in the southern continents and India are not put forward as evidence that fragmentation of the Gondwana supercontinent in the Mesozoic was a direct result of meteorite impact. In an abstract at a meeting of the American Geophysical Union, and in an article in the popular press, one member of the earth science community has made a specific claim to identify the site of the supercontinent-destroying bolide on the Falkland/Malvinas Plateau. It is claimed by this scientist that the Cape Fold belt in Africa represents a 'breaking wave' of deformation resulting from this impact, and that fractures in the clasts of the Dwyka diamictite in southern Africa represent impact-induced cataclasis of the target rock. These hypotheses fly in the face of the well-established tectonic history of the Gondwana supercontinent in several respects.

Dalziel, I. W. D.↗