G. S. Kent

NASA, Washington, West Virginia, United States

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Publications (63)139.14 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The latest in a series of solar occultation satellite instruments, Stratospheric Aerosol and Gas Experiment (SAGE) III, was placed into orbit in December 2001, and data were obtained until March 2006. Measurements were made of the extinction attributable to aerosols and cloud at a number of wavelengths between 290 and 1550 nm. The analysis of data obtained by its predecessor, SAGE II, has shown that an intercomparison of such data at two or more wavelengths may be used to separate the effects of cloud and aerosol. This analysis has been done on a routine basis for many years using SAGE II data at 525 and 1020 nm and applied extensively to global studies of tropospheric cloud and aerosol. Here we describe the aerosol-cloud separation algorithm developed for use with the SAGE III data, which uses the extinction at 525, 1020, and 1550 nm. This algorithm is now being used to produce vertical profiles of cloud presence as a standard SAGE III data product. These profiles have a vertical resolution of 0.5 km and cover the altitude range from 6.0 to 30.0 km, and data are presently available from March 2002 onward. An outline is given of the development of this algorithm, the nature of the SAGE III data, and the algorithm performance. To maintain continuity with SAGE II cloud data, the relative performances of the SAGE II and SAGE III algorithms are also examined. An example of the application of the algorithm to SAGE III tropospheric data is shown and discussed.
    Applied Optics 04/2007; 46(8):1261-78. · 1.69 Impact Factor
  • K. H. Sage, G. S. Kent
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    ABSTRACT: The Stratospheric Aerosol and Gas Experiment (SAGE) III is the latest in a series of solar occultation satellite instruments designed for the measurement of aerosol and gases. SAGE III extinction data obtained at three wavelengths (525, 1020 and 1550 nm) is used to determine whether cloud is present along the optical path from the sun to the satellite instrument. The algorithm used differs from that previously used to detect cloud using the SAGE II instrument, where data was not available at 1550 nm. Due to the long optical path through the atmosphere, both instruments are extremely sensitive to low values of extinction. In the troposphere, cloud data is divided into two classes: non-opaque, which is mainly subvisual, and opaque. SAGE III is also able to detect the presence of polar stratospheric cloud. Unlike SAGE II where cloud presence was a research product, cloud presence is a standard data product for SAGE III. SAGE III cloud data from May, 2001 onwards, at altitudes between 6 and 30 km, is currently being made available for general use. The theoretical background to the SAGE III algorithm is described and contrasted with that used with data from the SAGE II instrument. Examples showing how the algorithm is applied to the data are presented for a cloud-free atmosphere, for non-opaque stratospheric and tropospheric clouds, and for opaque clouds. Under some circumstances the signature of thin cloud in the data set can be confused with that of dense aerosol, produced for example as a result of volcanic activity or by lofting of dust from the surface of the earth. This potential confusion necessitates a quality control procedure for the data; this procedure is explained, together with the changes that this operation produces on the output data format and timing. In the interest of the long-term continuity of the SAGE II/ SAGE III cloud data set some of the SAGE III data has been processed using both the current SAGE III algorithm and the older SAGE II algorithm. Preliminary results of this intercomparison are briefly described.
    AGU Fall Meeting Abstracts. 12/2003;
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    ABSTRACT: Stratospheric Aerosol and Gas Experiment II aerosol data obtained at two wavelengths, 525 nm and 1020 nm, have been used for some time to identify the presence of cloud along the optical path from the Sun to the satellite instrument. Examination of data obtained over desert regions in the Northern Hemisphere, particularly the Taklimakan Desert, indicates that this separation method does not always operate correctly. In regions where there is expected to be a large amount of lofted dust, unexpectedly low values of mean extinction are found, combined with higher than expected amounts of cloud. These anomalous data have been analyzed in detail, and the discrepancy is plausibly shown to be due to faulty identification of lofted dust as cloud. Six Northern Hemisphere desert regions, together with three comparison regions, have been identified for study and the anomalies used to develop a description of the seasonal and altitudinal characteristics of the lofted aerosol over these regions.
    Journal of Geophysical Research 01/2003; 108. · 3.17 Impact Factor
  • Geoffrey S. Kent, Charles R. Trepte
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    ABSTRACT: Three methods of analyzing Stratospheric Aerosol and Gas Experiment (SAGE) II tropospheric aerosol extinction data are described and intercompared in terms of global maps and vertical contour plots of the extinction coefficient, or its equivalent. The first method, which has been in use for several years, is found to be biased toward smaller aerosols (effective radius 0.25 mum). The third method which, unlike the first two methods, is capable of producing an altitude resolved aerosol climatology down to about 1 km above the earth's surface, requires an assumption about the amount of cloud contamination in the data set. Given the correctness of this assumption, the method is able to derive the total extinction due to both large and small aerosols. Aerosol climatologies produced by all three methods are shown and intercompared, with particular emphasis on the lofting of dust from Asian and other Northern Hemisphere deserts and its subsequent advection over the western Pacific Ocean.
    Proc SPIE 01/2003;
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    ABSTRACT: Information on vertical cloud distribution is important to atmospheric radiative calculation, general circulation modeling, and climate study. The method used for estimating the vertical structure of opaque cloud occurrence from the solar occultation observations obtained by the Stratospheric Aerosol and Gas Experiment (SAGE) II has been reviewed for further understanding of the nature of the derived cloud statistics. Most importantly, based on the SAGE II tropical observations (1985-1998), the present study illustrates that the derived opaque cloud occurrence at a given altitude is generally independent of the cloud occurrence at other altitudes, except for some anticorrelation between high-level (12.5 km) and low-level (1-3 km) clouds. This feature of the layer cloud frequency independence is also evident when regional data over the Pacific warm pool and the eastern Pacific are examined. The independ- ent information of the layer cloud frequency is significant and makes it possible to use the derived vertical distribution of cloud occurrence to estimate the probability of multilayer clouds. The limitation is that it is difficult to determine how frequently the multilayer clouds are actually overlapping or how frequently thick cloud (> 1 km) really occurs based on the SAGE II observations alone. A discussion of the SAGE II tropical opaque cloud occurrence in relation to the cloud climatology based on visual observations from surface stations and ships, the International Satellite Cloud Climatology Project data, and the cloud statistics using rawin- sonde records is also provided.
    Journal of Geophysical Research 01/2001; 106:12603-12613. · 3.17 Impact Factor
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    ABSTRACT: The present study investigates the 1.0-mum extinction coefficient measurements obtained in the Antarctic region in 1979 from the Stratospheric Aerosol Measurement (SAM) II, with particular focus on the background aerosol properties. Correlative meteorological information from the National Centers for Environmental Prediction is incorporated in this investigation. The results indicate that the data frequency distribution of the background aerosol extinction coefficient in the local summer and fall can be adequately modeled by using a single-mode normal distribution, and that a binormal distribution is needed for modeling the distribution in the local winter and spring because of the different characteristics of the aerosols inside and outside the polar vortex. In general, the vertical distribution of the aerosol mean extinction coefficient exhibits two regions of different seasonal variation. Above 16 km the extinction coefficient is the highest during the local summer, and the lowest during the local spring inside the polar vortex. Below 16 km the aerosol seasonal variation is more complex, but the winter enhancement of the aerosol extinction coefficient inside the Antarctic polar vortex is clearly evident. As the season changes from winter to spring, the results inside the Antarctic polar vortex also indicate a reduction in aerosol optical depth in the stratosphere, but no significant changes in the upper troposphere. The present study further indicates that the bottom of the winter polar vortex in Antarctica is located at an altitude as low as 8 to 9 km, which is about 4 to 5 km lower than the bottom of the Arctic polar vortex. This difference may be attributable to the different strengths of the winter polar vortex and the planetary wave activities between the two hemispheres. In summary, the properties of the Antarctic background aerosol are very consistent with the effect of polar stratospheric clouds on the aerosol vertical distribution through their formation, sedimention, and evaporation, and with the seasonal evolution of the polar vortex. Finally, the result of the present study provides valuable opportunities for fully utilizing the multiyear SAM II tropospheric and stratospheric measurements to investigate the aerosol climatology and long-term variations in the Arctic and Antarctic regions.
    Journal of Geophysical Research 01/2000; 105:9407-9420. · 3.17 Impact Factor
  • G S Kent, G M Hansen
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    ABSTRACT: A small scanning three-wavelength lidar system at NASA Langley Research Center in Hampton, Virginia, has been used since 1992 to make atmospheric measurements on stratospheric and upper tropospheric aerosols and on the evolution of aircraft exhaust plumes. Many of these measurements have been made away from the zenith, and, to reduce the hazard to air traffic produced by the laser beam, a radar safety device has been installed. The radar application is original in that the radar beam is made collinear with the laser beam by use of a dichroic mirror that transmits the laser radiation and reflects the microwaves. This mirror is inserted into the outgoing optical path prior to the radiation from both the radar and the laser passing through the independent scanning unit. Tests of the complete system show that the lidar and radar beams remain collocated as they are scanned and that the radar can be used to inhibit the laser prior to an aircraft passing through the beam.
    Applied Optics 11/1999; 38(30):6383-7. · 1.69 Impact Factor
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    ABSTRACT: A model is proposed for identifying the aerosol mode of the second Stratospheric Aerosol and Gas Experiment (SAGE II) 1.02-mum extinction coefficient measurements at altitudes below 6.5 km, which also contain cloud samples. This development allows one to extend the SAGE II satellite data analysis from the lower limit at 6.5 km of the SAGE II two-wavelength method into the lower troposphere. Thus the proposed model provides opportunities for fully utilizing the SAGE II tropospheric measurements important to the understanding of the global behavior of tropospheric aerosols, clouds, and ozone and related transports. The effectiveness of this model is examined by using the SAGE II two-wavelength technique at 6.5 km. Sample applications of the proposed model reveal encouraging results. To assess the quality of the aerosol 1.02-mum data, it is recommended that a comprehensive data comparison analysis be conducted by using tropospheric measurements from different instruments.
    Journal of Geophysical Research 01/1999; 104:9609-9616. · 3.17 Impact Factor
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    ABSTRACT: Results of large-eddy simulations of an aircraft wake are compared with results from ground-based lidar measurements made at NASA Langley Research Center during the Subsonic Assessment Near-Field Interaction Flight Experiment field tests. Brief reviews of the design of the field test for obtaining the evolution of wake dispersion behind a Boeing 737 and of the model developed for simulating such wakes are given. Both the measurements and the simulations concentrate on the period from a few seconds to a few minutes after the wake is generated, during which the essentially two-dimensional vortex pair is broken up into a variety of three-dimensional eddies. The model and experiment show similar distinctive breakup eddies induced by the mutual interactions of the vortices, after perturbation by the atmospheric motions.
    AIAA Journal 09/1998; · 1.08 Impact Factor
  • G S Kent, G M Hansen
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    ABSTRACT: A small three-wavelength (355-, 532-, and 1064-nm) lidar system at NASA Langley Research Center in Hampton, Virginia, has been used since 1992 to make measurements on stratospheric aerosols. The data have been processed to study the decay rate of the stratospheric aerosol layer formed after the eruption of Mount Pinatubo in 1991 and its modulation, the aerosol effective radius, and the column mass loading. The stratospheric aerosol decay curves show annual and biennial cycles as well as short-term changes. At 532 nm, the decay time constant was 302 days for the period from February 1992 to August 1994 and had increased to 645 days for the period from September 1994 to December 1997. By 1996 the integrated stratospheric aerosol backscatter had fallen to levels (7.7 x 10(-5) sr(-1) at 532 nm) close to those seen in 1979 and 1989-1991. This decreasing trend was still continuing in 1997, showing no evidence for any anthropogenic contribution to the stratospheric aerosol.
    Applied Optics 07/1998; 37(18):3861-72. · 1.69 Impact Factor
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    ABSTRACT: The tropical cloud data obtained by the satellite instrument of the Stratospheric Aerosol and Gas Experiment (SAGE) II from October 1984 to May 1991 have been used to study cloud vertical distribution, including thickness and multilayer structure, and to estimate cloud optical depth. The results indicate that the SAGE-II-observed clouds are generally optically thin clouds, corresponding to a range of optical depth between approximately 8×10−4 and 3×10−1 with a mean of about 0.035. Two-thirds are classified as subvisual cirrus and one-third thin cirrus. Clouds between 2- to 3-km thick occur most frequently. Approximately 30% of the SAGE II cloud measurements are isolated single-layer clouds, while 65% are high clouds contiguous with an underlying opaque cloud that terminates the SAGE II profile. Thin clouds above detached opaque clouds at altitudes greater than 6.5 km occur less often. Only about 3% of the SAGE II single-layer clouds are located above the tropopause, while 58% of the cloud layers never reach the tropopause. More than one-third of the clouds appear at the tropopause. This study also shows that clouds occur more frequently and extend higher above the tropopause over the western Pacific than over the eastern Pacific, especially during northern winter. The uncertainty of the derived results due to the SAGE II sampling constraints, data processing, and cloud characteristics is discussed.
    Atmospheric Research. 02/1998;
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    ABSTRACT: To provide observational evidence on the extratropical cross-tropopause transport between the stratosphere and the troposphere via quasi-isentropic processes in the middleworld (the part of the atmosphere in which the isentropic surfaces intersect the tropopause), this report presents an analysis of the seasonal variations of the ozone latitudinal distribution in the isentropic layer between 330 K and 380 K based on the measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) II. The results from SAGE II data analysis are consistent with (1) the buildup of ozone-rich air in the extratropical middleworld through the large-scale descending mass circulation during winter, (2) the spread of ozone-rich air in the isentropic layer from midlatitudes to subtropics via quasi-isentropic transport during spring, (3) significant photochemical ozone removal and the absence of an ozone-rich supply of air to the layer during summer, and (4) air mass exchange between the subtropics and the extratropics during the summer monsoon period. Thus the SAGE II observed ozone seasonal variations in the middle- world are consistent with the existing model calculated annual cycle of the diabatic circulation as well as the conceptual role of the eddy quasi-adiabatic transport in the stratosphere-troposphere exchange reported in the literature.
    Journal of Geophysical Research 01/1998; 103:28647-28659. · 3.17 Impact Factor
  • G. S. Kent, C. R. Trepte, P. L. Lucker
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    ABSTRACT: A detailed analysis has been made of Stratospheric Aerosol and Gas Experiment I and II aerosol extinction data for the upper troposphere (6-km altitude to the seasonally averaged tropopause) taken between 1979 and 1998. An improved method of separation of the volcanic and surface-derived components of the aerosol optical depth has been used. The mean extinction, at a wavelength of 1.02 mum, of the nonvolcanic component of the upper tropospheric aerosol is found to increase from approximately 1×10-4km-1 at 70°S to about 7 times that value at 70°N. Maximum downward transfer of volcanic material into the upper troposphere is observed to take place in local spring in each hemisphere, occurring at a latitude of 70°S or greater in the southern hemisphere and at about 50°N in the northern hemisphere. The almost 20-year data sequence (1979-1981, 1984-1991, 1994-1998) has been examined for evidence of any long-term trends in the aerosol optical depth of the upper troposphere. It is unlikely that any change in the upper tropospheric 1-mum aerosol optical depth greater than 1% per year has taken place when averaged over either hemisphere.
    Journal of Geophysical Research 01/1998; 1032:28863-28874. · 3.17 Impact Factor
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    ABSTRACT: Two global satellite data sets have been used to characterize the behavior of aerosols in the upper troposphere of the southern hemisphere during the spring season. The first data set was obtained by the Lidar-In-Space Technology Experiment (LITE) during 10 days in September 1994 and provides high-resolution information about aerosol layering and optical characteristics. The second data set was obtained by the Stratospheric Aerosol and Gas Experiment (SAGE) II over the time period 1984-1996 and provides information on the aerosol distribution and long-term climatology. During September, elevated aerosol layers are found to occur within a latitude band between 20°S and 40°S that extends to almost all longitudes. The latitude and altitude distribution and the optical characteristics of the aerosol suggest that a major source is smoke from biomass burning within the southern hemisphere. This conclusion is supported by the results of back-trajectory analyses that show airmasses originating in the region of southern Africa and traveling longitudinally across the Indian Ocean and Australia into the western Pacific Ocean. The dominant source of the smoke is uncertain, but quite possibly some of it may have originated from Brazil, with additions from southern Africa. The aerosol distribution shows strong similarities to published distributions for ozone and carbon monoxide, also believed to have originated from biomass burning.
    Journal of Geophysical Research 01/1998; 1031:19111-19128. · 3.17 Impact Factor
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    Journal of Geophysical Research 01/1998; 103:25461-25461. · 3.17 Impact Factor
  • Mary T. Osborn, Geoffrey S. Kent, Charles R. Trepte
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    ABSTRACT: The Lidar in Space Technology Experiment (LITE) is a three-wavelength backscatter lidar developed by NASA Langley Research Center to demonstrate and explore the capabilities of space lidar. LITE was flown on space shuttle Discovery in September 1994. Among the primary experimental objectives of LITE was the measurement of stratospheric aerosols. High-quality stratospheric aerosol measurements at 532 nm and 355 nm were obtained during nighttime, high-gain operation. These LITE data provide a detailed global view of the vertical structure and optical properties of the stratospheric aerosols. The data are also used to study the transport processes influencing the aerosol spatial distribution. LITE data compare well with measurements made by the Stratospheric Aerosol and Gas Experiment (SAGE) II. Individual profile comparisons and comparisons of more global features reinforce and extend the validation of the LITE stratospheric data. LITE demonstrates that a spaceborne lidar, with its high vertical resolution and global coverage, is a powerful tool for tracing atmospheric transport.
    Journal of Geophysical Research 01/1998; 103:11447-11453. · 3.17 Impact Factor
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    ABSTRACT: This study investigates the tropospheric mean meridional circulation important to the development of opaque clouds and the measurement opportunity of the 1.02-mum channel of the Stratospheric Aerosol and Gas Experiment (SAGE) II in the troposphere. A simple empirical model is formulated to derive the mean meridional circulation from the 6-year (1985-1990) statistics of the SAGE II tropospheric measurement frequency. The vertical circulation of the model is assumed to be related to the departure field of the zonally averaged SAGE II measurement frequency from the corresponding global mean in a linear fashion. The proportional constant is calibrated with the observed upwelling circulation statistics in the tropics. The obtained model vertical circulation is then used to determine the distribution of meridional velocity according to the continuity equation. The derived model mean circulation features the influence from both the diabatic circulation and the eddy quasi-isentropic transport, with a distinct pattern of material advection into the upper troposphere from both the lower troposphere and the stratosphere. Most significantly, the model circulation is shown to be highly consistent with the observed free tropospheric aerosol and ozone distributions, particularly with their seasonal variations given the aerosol and ozone source regions. This high degree of consistency illustrates the intimate relationship between the large-scale circulation, cloudiness, and the SAGE II tropospheric measurement frequency, and the robust nature of the empirical model despite the model's simplicity. The discussion in relating the model circulation to the conventional Eulerian circulation and the Lagrangian transport, based on isentropic consideration, is also provided.
    Journal of Geophysical Research 01/1998; 1031:13801-13818. · 3.17 Impact Factor
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    G S Kent, P H Wang, K M Skeens
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    ABSTRACT: The Stratospheric Aerosol and Gas Experiment (SAGE) III, scheduled for a first launch in mid-1998, will be making measurements of the extinction that is due to aerosols and gases at many wavelengths between 385 and 1550 nm. In the troposphere and wintertime polar stratosphere, extinction will also occur because of the presence of cloud along the optical path from the Sun to the satellite instrument. We describe a method for separating the effects of aerosol and cloud using the extinction at 525, 1020, and 1550 nm and present the results of simulation studies. These studies show that the new method will work well under background nonvolcanic aerosol conditions in the upper troposphere and lower stratosphere. Under conditions of severe volcanic contamination, the error rate for the separation of aerosol and cloud may rise as high as 30%.
    Applied Optics 12/1997; 36(33):8639-49. · 1.69 Impact Factor
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    ABSTRACT: Recently NASA Langley Research Center's (LaRC) Aerosol Research Branch conducted an aircraft exhaust particle experiment involving tow ground based lidar systems and NASA's B737-100, T39 and OV10 aircraft. The experiment took place at LaRC in February and March of 1996. During flight, exhaust particles exiting the two wing-mounted engines of the B737 become quickly entrained into the aircraft's wingtip vortices. The LaRC lidar systems were used to measure the distribution and optical properties of these exhaust particles as the B737 overflew the lidar facility. Two lidar systems, located in a common facility, were utilized for this experiment. One system was a fixed zenith- viewing lidar with a 48-inch receiver and a 2J transmitter, and the second was a scanning lidar with a 14-inch receiver and a 600 mJ transmitter. Two measurement geometries were employed for the experiment. In the first geometry, the B737 flew upwind of the lidar facility and perpendicular to the ambient wind. The second design had the aircraft fly directly over the facility, and parallel to the ambient wind.Under either scenario data were acquired at 20 and 30 Hertz, by the fixed zenith and scanning system respectively, as the ambient wind carried the vortex pair across the field of view of the lidars. The two supporting aircraft were used to collect in-situ particle data and to measure atmospheric turbulence, respectively. In this paper all aspects of the experiment will be discussed including the lidar systems, the geometry of the experiment, and the aircraft used. Also, selected data obtained during the experiment will be presented. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints
    Proc SPIE 08/1997;
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    ABSTRACT: The Stratospheric Aerosol and Gas Experiment (SAGE) II satellite instrument measures the attenuation due to stratospheric aerosols, gases, and clouds along a tangential path through the atmosphere from the Sun to the satellite instrument. Data from SAGE II and its predecessor, SAGE I, have been used to study and develop climatologies for high altitude cloud. Because these instruments measure the total attenuation on a long optical path and the data are inverted under the assumption of spherical homogeneity in the region of the measurement, interpretation in terms of individual cloud characteristics is difficult. Airborne lidar data, taken on approximately 8000 km of flight path over the tropical Pacific Ocean, have been used to simulate high altitude SAGE II cloud measurements and their inversion. The lidar data set, not necessarily typical for the region flown over, showed the general presence of patchy cloud with a mean horizontal cloud dimension of 20-25 km and a vertical thickness of 0.6-0.8 km. Use of the lidar data to simulate SAGE II measurements produces cloud extinction values similar in magnitude and distribution to those obtained from SAGE II. These simulations also show the existence of three possible error conditions that may occur as a result of cloud inhomogeneities along the viewing path. In the first error condition, the true altitude of a cloud may be higher than that found as a result of the SAGE II inversion, errors of 1 km or greater occurring in just under 40% of the simulations, mainly at the lower cloud altitudes. In the second error condition, the inverted cloud extinction may differ from the volume averaged extinction along the horizontal ray path, the former often being biased slightly low. In the third error condition, the presence of nonuniform or isolated cloud patches can result in an apparent negative inverted extinction value just below the cloud. Such values were observed in about one third of the simulations. The first two of these errors have been statistically quantified for application to SAGE II cloud data.
    Journal of Geophysical Research 01/1997; 102:21795-21807. · 3.17 Impact Factor

Publication Stats

1k Citations
139.14 Total Impact Points

Institutions

  • 1999–2007
    • NASA
      Washington, West Virginia, United States
  • 1998
    • Hampton University
      Hampton, Virginia, United States
  • 1982
    • Remote Sensing Systems
      Santa Rosa, California, United States