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Levin, Z.

Publications and source records attributed to Levin, Z..

7 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Uncertainties in Remote Sensing of Aerosol Radiative Forcing

The role of aerosol forcing remains one of the largest uncertainties in estimating man's impact on the global climate system. One school of thought suggests that remote sensing by satellite sensors will provide the data necessary to narrow these uncertainties. Much effort has gone into the development of new satellite sensors specifically designed to retrieve aerosol loading and some information about the sizes of the aerosols. These next generation remote sensing instruments (EOS-MODIS, EOS-MISR, POLDER, ATSR) will provide unprecedented accuracy in the retrieval of aerosol loading. Although the new generation of sensors has excellent accuracy compared to the heritage instruments of the past, they still have measurement limitations. In clean, pristine regions the absolute magnitude of the uncertainty in the aerosol retrieval becomes comparable in magnitude to the signal itself. If much of the aerosol forcing is occurring at very low magnitudes of aerosol concentrations, satellite remote sensing will miss it. This study attempts to quantify remote sensing limitations due to the accuracy limits of the retrieval algorithms. We use a combination of numerical aerosol transport models, ground-based AERONET data and ISCCP cloud climatology to determine how much of the forcing occurs in regions too clean to determine from satellite retrievals. This study is not an intercomparison of global transport models. It is not an estimation of global aerosol forcing. This study is an exercise to determine whether satellite remote sensing can live up to our high expectations.

Remer, L.↗
NASA NTRS1999

Towards a Strategy of Remote Sensing of Aerosol Direct and Indirect Radiative Forcing

The role of aerosol forcing remains one of the largest uncertainties in estimating man's impact on the global climate system. One school of thought suggests that remote sensing by satellite sensors will provide the data necessary to narrow these uncertainties. Much effort has gone into the development of new satellite sensors specifically designed to retrieve aerosol loading and some information about the sizes of the aerosols. These next generation remote sensing instruments (EOS-MODIS, EOS-MISR, POLDER, ATSR) will provide unprecedented accuracy in the retrieval of aerosol loading. Although the new generation of sensors has excellent accuracy compared to the heritage instruments of the past, they still have measurement limitations. In clean, pristine regions the absolute magnitude of the uncertainty in the aerosol retrieval becomes comparable in magnitude to the signal itself. If much of the aerosol forcing is occurring at very low magnitudes of aerosol concentrations, satellite remote sensing will miss it. This study attempts to quantify remote sensing limitations due to the accuracy limits of the retrieval algorithms. We use a combination of numerical aerosol transport models, ground-based AERONET data and ISCCP cloud climatology to determine how much of the forcing occurs in regions too clean to determine from satellite retrievals. This study is not an intercomparison of global transport models. It is not an estimation of global aerosol forcing. This study is an exercise to determine whether satellite remote sensing can live up to our high expectations. The results show that while remote sensing will be able to measure a majority of the direct forcing, it will miss roughly half of the indirect forcing.

Remer, L. A.↗
NASA NTRS1987

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

Computational results are given for Titan atmosphere aerosol electrical conductivity and charge at altitudes up to 400 km, together with a consideration of ionization from such sources as galactic cosmic rays and electron precipitation from the Saturnian magnetosphere. Predicted conductivity shows the existence of substantial electron concentrations up to the Titan surface. At altitudes of more than 100 km, and aerosol concentrations greater than 10/cu cm, electron/positive ion-recombination is found to be controlled by the recombination of the aerosols' surfaces rather than by the gas-kinetic recombination rate.

Borucki, W. J.↗
NASA NTRS1983

Lightning generation in planetary atmospheres

The possibilities of lightning generation on other planets are considered, and the basic conditions that exist in terrestrial clouds during lightning discharges and the various theories of charge separation are reviewed. The properties of terrestrial clouds that produce lightning, the properties of lightning itself, and the fairweather field are first reviewed. The general requirements and the different proposed charging mechanisms for electrification of terrestrial clouds are discussed. The mechanisms of electrical breakdown and whistler production are considered, and recent observations of extraterrestrial lightning and of clouds on other planetary bodies are summarized. Circumstances likely to account for lightning activity on Jupiter and Venus and for electrical activity on Mars and in the rings of Saturn are discussed.

Borucki, W. J.↗
NASA NTRS1982

A one-dimensional model of the atmospheric electric field near the Venusian surface

A one-dimensional model is developed for the development of an electric field below the Venus cloud layer, assuming an absence of cloud-to-ground lightning. The ion-bearing medium is considered as a collision dominated, partially ionized gas. Ion production by cosmic rays is accompanied by a sink of recombining positive and negative ions. The net diffusion of ions to the surface was examined, and modeled as resulting from differences between the ion diffusivities and electrical activity within the clouds. Calculations were made of the conduction and diffusion currents profiles, positive and negative ion densities, and the net space charge. Higher positive ion diffusivity was found to enhance the surface positive charge, producing an electric field which depended on the boundary layer mixing. Charge separation in the clouds also produces an electric field. Assuming a fair weather conduction current similar to earth leads to an electric field of 5 kV/sq m at the surface, with continuity to a few kilometers altitude.

Tzur, I.↗
NASA NTRS1982

Predicted electrical conductivity between 0 and 80 km in the Venusian atmosphere

Calculations of the space charge, ion density, and conductivity in the Venus atmosphere were made. The presence of the cloud particles on Venus causes a profund reduction in the calculated values of the ion density and conductivity compared to the values that are obtained without consideration of the cloud particles. When the cloud particles are included in the calculations, the results for the ion density and conductivity are approximately the same as those of the terrestrial atmosphere at the same pressure-altitude. Because the particles span such a large range of sizes and are abundant over a substantial range of pressure, the space charge varies strongly with altitude and particle size. Differential settling of the particles is expected to produce weak electric fields in the clouds.

Borucki, W. J.↗