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At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The estimation of convective rainfall by area integrals. I - The theoretical and empirical basis. II - The height-area rainfall threshold (HART) method

A theory is developed which establishes the basis for the use of rainfall areas within present thresholds as a measure of either the instantaneous areawide rain rate of convective storms or the total volume of rain from an individual storm over its lifetime. The method is based upon the existence of a well-behaved pdf of rain rate either from the many storms at one instant or from a single storm during its life. The generality of the instantaneous areawide method was examined by applying it to quantitative radar data sets from the GARP Tropical Atlantic Experiment for South Africa, Texas, and Darwin (Australia). It is shown that the pdf's developed for each of these areas are consistent with the theory.

Rosenfeld, Daniel↗
NASA NTRS1990

New mineralogical and chemical data on the Machinga (L6) chondrite, Malawi

The Machinga, southern Malawi, Africa, L6 chondrite (observed fall, January 22, 1981) contains accessory phases of metal, troilite, chromite, and native Cu (which is associated with limonite and found in zones of aqueous alteration). Rare accessory phases are apatite and pentlandite, which are uncommon in L6 chondrites. Major mineral constituents (olivine, orthopyroxene, and plagioclase) indicate shock effects at a level of about 15-20 GPa shock pressure. The meteorite is thus classified to be of L6d type. Melt pockets of widely variable composition are abundant.

Koeberl, Christian↗
NASA NTRS1990

Distribution of tropospheric ozone determined from satellite data

An analysis of more than 22,000 ozone profiles from Stratospheric Aerosol and Gas Experiment I (SAGE I) (1979-1981) and SAGE II (1984-1987) between 50 deg N and 50 deg S is used in conjunction with 9 years (1979-1987) of daily global depictions of total ozone from the TOMS instrument aboard Nimbus 7 to investigate the spatial distribution and seasonal cycle of the integrated amount of ozone in the troposphere. In the tropics, highest concentrations are found in the eastern Atlantic Ocean downwind (west) of Africa and maximize during the time when biomass burning is most prevalent, between July and October. A different seasonal cycle in the tropics is also observed over Indonesia, where a relative maximum is present in the March-April time frame, likewise consistent with when biomass burning is most prevalent. At mid-latitudes, highest concentrations are found downwind of Asia and maximize in the summer. Relatively higher amounts of tropospheric ozone are similarly observed downwind of North America and Europe. At mid-latitudes, the ratio between the amount of tropospheric ozone in the Northern Hemisphere and the amount in the Southern Hemisphere is 1.4, in good agreement with in situ measurements.

Fishman, Jack↗
NASA NTRS1990

Investigation of biogeophysical feedback on the African climate using a two-dimensional model

A numerical scheme is specifically designed to develop a time-dependent climate model to ensure the conservation of mass, momentum, energy, and water vapor, in order to study the biogeophysical feedback for the climate of Africa. A vegetation layer is incorporated in the present two-dimensional climate model. Using the coupled climate-vegetation model, two tests were performed involving the removal and expansion of the Sahara Desert. Results show that variations in the surface conditions produce a significant feedback to the climate system. It is noted that the simulation responses to the temperature and zonal wind in the case of an expanded desert agree with the climatological data for African dry years. Perturbed simulations have also been performed by changing the albedo only, without allowing the variation in the vegetation layer. It is shown that the variation in latent heat release is significant and is related to changes in the vegetation cover. As a result, precipitation and cloud cover are reduced.

Xue, Yongkang↗
NASA NTRS1990

Earth observations during Space Shuttle mission STS-30

The earth observations that were conducted during the STS-30 mission in May, 1989 are examined. An overview of the STS-30 mission is given, noting the launch of the Magellan spacecraft. The major positions of STS-30 photographs are illustrated and the sun elevation angles of the photographs and the types of films used during the mission are listed. Photographs of various regions are described, focusing on North Africa and the Middle East, South and Southeast Asia, Central America, the Caribbean, and the Southern U.S. The use of the photographs for various research purposes is discussed, including oceanography, meteorology, and polarization experiments. Also, consideration is given to the use of video/camcorders for earth observations.

Helfert, Michael R.↗
NASA NTRS1990

The distribution of middle tropospheric carbon monoxide during early October 1984

The measurement of Air Pollution from Satellite experiment measured the distribution of middle-tropospheric CO from the Space Shuttle during October 1984. The data represent average mixing ratios between 57 deg N and 57 deg S. Approximately 75,000 individual CO measurements were obtained during the 9-day mission. The data are presented in maps that show the CO mixing ratios averaged over 5 x 5-deg areas for 6 days of the mission. Comparisons with concurrent direct measurements taken aboard aircraft show that the inferred concentrations are systematically low by 20-40 percent, depending upon which direct measurement calibration standard is used. The data show that there are very large CO sources resulting from biomass burning over South America and southern Africa. Measured mixing ratios were high over northeast Asia and were highly variable over Europe.

Reichle, Henry G., Jr.↗
NASA NTRS1990

Global-scale, intraseasonal fluctuations of diabatic forcing of the atmosphere

Fields of diabatic heating rate estimates (H) for 5-day periods were calculated from the European Center for Medium Weather Forecasts (ECMWF) analyses since 1985 as the residual of the dry thermodynamic equation. Included in these fields are the horizontal and vertical divergences of heat for both mean and eddy statistics. Previous work dealt with 4-day periods, however, with the emphasis in the Global Precipitation Climatology Project (GPCP) for 5-day period totals the change was made to accommodate the GPCP product. H has long been associated with cold tropical cloud-top temperatures as measured by polar orbiting outgoing longwave radiation (OLR) sensors. Correlations between H and OLR fields on three time scales indicate a moderate amount of agreement. For periods less than 90 days, significant negative correlations are found between H and OLR for (1) tropical and NH midlatitude oceanic areas, and (2) for zonal and hemispheric mean values. Positive correlations are seen in NH mean and continental areas of N. Africa, N. America, N. Asia and Antarctica. These latter results reflect seasonal heating and cooling. Comparisons have been made between H as H as calculated from the ECMWF analyses and output from the CCM1 T42 simulations. The CCM1 tends to have a more cellular structure with more heating (precipitation) over land versus that observed (ECMWF) over oceans.

Christy, John R.↗
NASA NTRS1990

Geochemistry of Precambrian carbonates - 3-shelf seas and non-marine environments of the Archean

Samples from the Pangola and Ventersdorp Supergroups (Kaapvaal Craton, South Africa) and from the Fortescue and Hamersley Groups (Pilbara Block, Australia) were analyzed, using XRF, AAS, and isotope-analysis techniques to investigate the mineralogical, chemical, and isotopic features of these representatives of contemporary shelf carbonates (Pangola and Hamersley samples) and nonmarine carbonates (the Ventersdorp and Fortescue samples). Results show that, mineralogically, the shelf carbonates are almost exclusively dolostones, while the lacustrine facies are predominantly limestones. Geological, trace-element, and oxygen-isotope results of the shelf carbonates suggest that their original mineralogy may have been aragonite, and that the Pangola dolostones may represent a direct dolomitization product of this precursor. By contrast, the stabilization of the Hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone.

Veizer, Jan↗
NASA NTRS1990

Monitoring arid lands using AVHRR-observed visible reflectance and SMMR37-GHz polarization difference

Visible reflectance along a transect through the Sahel and Sudan zones of Africa has been derived from observations by the AVHRR on the NOAA-7 and NOAA-9 satellites and compared with concurrent observations of the 37-GHz polarization difference by the SMMR on the Nimbus-7 satellite. The study period was January 1982 to December 1986, which included an unprecedented drought during 1984 over the Sahel zone. While spatial and temporal patterns of these two data sets are found to be highly correlated, there are also quantitative differences which need to be understood.

Choudhury, B. J.↗
NASA NTRS1990

Interannual variability of the global net radiation balance and its consequence on global energy transport

Global cloudiness and radiation budget data from Nimbus 6 and 7 are used to investigate the role of cloud and surface radiative forcing and elements of the earth's general circulation. Although globally integrated cloud forcing is nearly zero, there are large regional imbalances and well regulated processes in the shortwave and longwave spectrum that control the meridional gradient structure of the net radiation balance and the factors modulating the east-west oriented North Africa-western Pacific energy transport dipole. The analysis demonstrates that clouds play a dual role in both the shortwave and longwave spectra in terms of tropical and midlatitude east-west gradients. The key result is that cloud forcing, although not always the principle regulator of interannual variability of the global climate, serves to reinforce the basic three-cell meridional circulation.

Smith, Eric A.↗
NASA NTRS1989

Modelling Rift Valley fever (RVF) disease vector habitats using active and passive remote sensing systems

The NASA Ames Ecosystem Science and Technology Branch and the U.S. Army Medical Research Institute of Infectious Diseases are conducting research to detect Rift Valley fever (RVF) vector habitats in eastern Africa using active and passive remote-sensing. The normalized difference vegetation index (NDVI) calculated from Landsat TM and SPOT data is used to characterize the vegetation common to the Aedes mosquito. Relationships have been found between the highest NDVI and the 'dambo' habitat areas near Riuru, Kenya on both wet and dry data. High NDVI values, when combined with the vegetation classifications, are clearly related to the areas of vector habitats. SAR data have been proposed for use during the rainy season when optical systems are of minimal use and the short frequency and duration of the optimum RVF mosquito habitat conditions necessitate rapid evaluation of the vegetation/moisture conditions; only then can disease potential be stemmed and eradication efforts initiated.

Ambrosia, Vincent G.↗
NASA NTRS1990

On the interannual variation of the earth radiation balance

The interannual variability of the top of the atmosphere net radiation budget as measured from the Nimbus-7 earth radiation budget instruments was calculated for an eight-year period 1979-1986. The largest fluctuations are shown to occur in three tropical areas: the Atlantic off the west coast of Africa, the eastern Pacific near South America, and the western Pacific northeast of Indonesia. The variability in the Atlantic was 20 percent greater than in the eastern Pacific and 35 percent greater than in the Indonesian area. The maximum anomalies in these two Pacific regions occurred during the El Nino year 1982-1983, while the maximum Atlantic anomalies, south of the Gulf of Guinea, were during 1984. An independent data set of derived cloud type and amount from the Temperature Humidity Infrared Radiometer and the Total Ozone Mapping Spectrometer instruments shows interannual changes in multilevel convective cloud systems have a minimal effect of the net balance. However, changes in middle and low clouds drastically affect the balance, and are the most likely cause of the maximum radiation balance variability in the Gulf of Guinea region. This observed interannual variation of the top of the atmosphere net balance denotes the most variable 'cloud radiative forcing' situation observed to date.

Randel, David L.↗
NASA NTRS1991

South Atlantic interbasin exchange

The exchange of mass and heat between the South Atlantic and the neighboring ocean basins was estimated using hydrographic data and inverse methods, in order to gain information on the links between the deep-water formation processes occurring within the Atlantic and the global thermohaline circulation. Results demonstrate that the global thermohaline cell associated with the formation and export of North Atlantic deep water (NADW) is closed primarily by a 'cold water path' in which deep water leaving the Atlantic ultimately returns as intermediate water entering the basin through Drake Passage. This conclusion conflicts with the suggestion by Gordon (1986) that the global thermohaline circulation associated with the formation of NADW is closed primarily by a 'warm water path', in which the export of NADW is compensated by an inflow of warm Indian Ocean thermocline water south of Africa.

Rintoul, Stephen Rich↗
NASA NTRS1991

Diabatic heating rate estimates from European Centre for Medium-Range Weather Forecasts analyses

Vertically integrated diabatic heating rate estimates (H) calculated from 32 months of European Center for Medium-Range Weather Forecasts daily analyses (May 1985-December 1987) are determined as residuals of the thermodynamic equation in pressure coordinates. Values for global, hemispheric, zonal, and grid point H are given as they vary over the time period examined. The distribution of H is compared with previous results and with outgoing longwave radiation (OLR) measurements. The most significant negative correlations between H and OLR occur for (1) tropical and Northern-Hemisphere mid-latitude oceanic areas and (2) zonal and hemispheric mean values for periods less than 90 days. Largest positive correlations are seen in periods greater than 90 days for the Northern Hemispheric mean and continental areas of North Africa, North America, northern Asia, and Antarctica. The physical basis for these relationships is discussed. An interyear comparison between 1986 and 1987 reveals the ENSO signal.

Christy, John R.↗
NASA NTRS1991

Representation of vegetation by continental data sets derived from NOAA-AVHRR data

Images of the normalized difference vegetation index (NDVI) are examined with specific attention given to the effect of spatial scales on the understanding of surface phenomena. A scale variance analysis is conducted on NDVI annual and seasonal images of Africa taken from 1987 NOAA-AVHRR data at spatial scales ranging from 8-512 km. The scales at which spatial variation takes place are determined and the relative magnitude of the variations are considered. Substantial differences are demonstrated, notably an increase in spatial variation with coarsening spatial resolution. Different responses in scale variance as a function of spatial resolution are noted in an analysis of maximum value composites for February and September; the difference is most marked in areas with very seasonal vegetation. The spatial variation at different scales is attributed to different factors, and methods involving the averaging of areas of transition and surface heterogeneity can oversimplify surface conditions. The spatial characteristics and the temporal variability of areas should be considered to accurately apply satellite data to global models.

Justice, C. O.↗
NASA NTRS1991

Mean and inter-year variation of growing-season normalized difference vegetation index for the Sahel 1981-1989

Images are presented that show the mean and coefficient of variation of nine years (1981-1989) of NOAA AVHRR normalized difference vegetation index (NDVI) data for the growing season (July-October) in Africa, north of the equator. The variation in the growing season NDVI is represented by the coefficient of variation image that shows the large variation in the Sahelian growing season between years. It is concluded that these images illustrate some aspects of the perspective being brought to regional and continental scale processes by coarse resolution satellite sensors and the potential of these sensors to provide consistent, long-term datasets.

Tucker, C. J.↗
NASA NTRS1991

Atmospheric effects on NOAA AVHRR data over Sahelian regions

Variable atmospheric conditions have a significant impact on satellite monitoring of vegetation in the Sahelian zone of Africa. The effects of atmospheric O3, H2O, and aerosols on spectral, directional reflectances and derived measures, such as the normalized difference vegetation index are discussed using a sensitivity study. An atmospheric correction scheme is presented based on measurements of optical thicknesses from the ground and is applied to NOAA AVHRR data. The use of the reflectance of the ocean surface close to the terrestrial target to estimate atmospheric effects on terrestrial reflectance is also discussed.

Soufflet, V.↗
NASA NTRS1991

Melt droplet formation in energetic impacts

Impacts between rocky bodies at velocities exceeding about 15 km/sec are capable of melting or vaporizing both the impacting object and a portion of the target. Geological materials initially shocked to high pressure approach the liquid-vapor phase boundary from the liquid side as they decompress, breaking up into an expanding spray of liquid droplets. A simple theory is presented for estimating the sizes of these droplets as a function of impactor size and velocity. It is shown that these sizes are consistent with observations of microtektites and spherules found in the Cretaceous-Tertiary boundary layer, the Acraman impact structure, Archean beds in South Africa and lunar regolith. The model may also apply to the formation of chondrules.

Vickery, A. M.↗