Browsing by Author "Krotkov, N."
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Item Detection of volcanic ash clouds from Nimbus 7/total ozone mapping spectrometer(AGU, 1997-07-01) Seftor, C. J.; Hsu, N. C.; Herman, Jay; Bhartia, P. K.; Torres, O.; Rose, W. I.; Schneider, D. J.; Krotkov, N.Measured radiances from the Version 7 reprocessing of the Nimbus 7/total ozone mapping spectrometer (TOMS) 340- and 380-nm channels are used to detect absorbing particulates injected into the atmosphere after the El Chichon eruption on April 4, 1982. It is shown that while the single-channel reflectivity determined from the 380-nm channel is able to detect clouds and haze composed of nonabsorbing aerosols, the spectral contrast between the 340- and 380-nm channels is sensitive to absorbing particulates such as volcanic ash, desert dust, or smoke from biomass burning. In this paper the spectral contrast between these two channels is used to detect the volcanic ash injection into the atmosphere and to track its evolution for several days. The movement of the ash clouds is shown to be consistent with the motions expected from the National Centers for Environmental Prediction (NCEP)-derived balanced wind fields in the troposphere and lower stratosphere. The movement of the volcanic SO₂ cloud detected from TOMS data was also in agreement with the NCEP wind at higher altitudes of up to 100–10 mbar. The vertical wind shear in the neighborhood of the eruption site resulted in a clear separation of the ash and SO₂ clouds. The location and movement of the ash cloud are consistent with information obtained by the advanced very high resolution radiometer (AVHRR) instrument on board the NOAA 7 satellite and to ground reports of ash fall.Item Distribution of UV radiation at the Earth's surface from TOMS-measured UV-backscattered radiances(AGU, 1999-05-01) Herman, Jay; Krotkov, N.; Celarier, E.; Larko, D.; Labow, G.Daily global maps of monthly integrated UV-erythemal irradiance (290–400 nm) at the Earth's surface are estimated using the ozone amount, cloud transmittance, aerosol amounts, and surface reflectivity from the solar UV radiation backscattered from the Earth's atmosphere as measured by the total ozone mapping spectrometer (TOMS) and independently measured values of the extraterrestrial solar irradiance. The daily irradiance values at a given location show that short-term variability (daily to annual) in the amount of UV radiation, 290–400 nm, reaching the Earth's surface is caused by (1) partially reflecting cloud cover, (2) haze and absorbing aerosols (dust and smoke), and (3) ozone. The reductions of UV irradiance estimated from TOMS data can exceed 50 ± 12% underneath the absorbing aerosol plumes in Africa and South America (desert dust and smoke from biomass burning) and exceeded 70 ± 12% during the Indonesian fires in September 1997 and again during March 1998. Recent biomass burning in Mexico and Guatemala have caused large smoke plumes extending into Canada with UV reductions of 50% in Mexico and 20% in Florida, Louisiana, and Texas. Where available, ground-based Sun photometer data show similar UV irradiance reductions caused by absorbing aerosol plumes of dust and smoke. Even though terrain height is a major factor in increasing the amount of UV exposure compared to sea level, the presence of prolonged clear-sky conditions can lead to UV exposures at sea level rivaling those at cloudier higher altitudes. In the equatorial regions, ±20°, the UV exposures during the March equinox are larger than during the September equinox because of increased cloudiness during September. Extended land areas with the largest erythemal exposure are in Australia and South Africa where there is a larger proportion of clear-sky days. The large short-term variations in ozone amount which occur at high latitudes in the range ±65° cause changes in UV irradiance comparable to clouds and aerosols for wavelengths between 280 nm and 300 nm that are strongly absorbed by ozone. The absolute accuracy of the TOMS monthly erythemal exposure estimates over a TOMS field of view is within ±6%, except under UV-absorbing aerosol plumes (dust and smoke) where the accuracy is within ±12%. The error caused by aerosols can be reduced if the height of the aerosol plume is more accurately known. The TOMS estimated irradiances are compared with ground-based Brewer spectroradiometer data obtained at Toronto, Canada. The Brewer irradiances are systematically 20% smaller than TOMS irradiance estimates during the summer months. An accounting of systematic errors brings the Brewer and TOMS irradiances into approximate agreement within the estimated instrumental uncertainties for both instruments.Item Numerical results for polarized light scattering in a spherical atmosphere(Elsevier, 2022-05-02) Korkin, Sergey; Yang, E.-S.; Spurr, R.; Emde, C.; Zhai, Peng-Wang; Krotkov, N.; Vasilkov, A.; Lyapustin, A.We report numerical results for polarized light reflection from the top of a Rayleigh scattering spherical atmosphere with height-dependent single scattering albedo over a dark surface. Michael Mishchenko considered this scenario back in the 1990’s, for a plane-parallel atmosphere of unit optical thickness (OT = 1), for which radiance errors arising from neglecting polarization reaches their highest values. To further extend Mishchenko's results, we consider a value of OT = 0.25, for which the effect of atmospheric curvature is pronounced. New results are generated using three state-of-the art radiative transfer (RT) codes. These are: the MYSTIC and MCSSA models, which simulate light scattering in a true-spherical atmosphere using Monte Carlo methods; and the discrete ordinate code VLIDORT, operating with a new multiple-scatter spherical correction designed to deliver reasonable approximations to spherical-medium scattering. In this work, we report results for both single and multiple scattering; this will help to support the validation of existing and future polarized spherical RT codes, especially those using approximative methods to deal with sphericity.Item Volcanic Cloud and Aerosol Monitor (VOLCAM) for Deep Space Gateway(2018) Krotkov, N.; Bhartia, P. K.; Torres, O.; Li, C.; Sanders, S.; Realmuto, V.; Carn, S.; Herman, JayFrequent (~15 min) imaging of reflected solar ultra-violet (UV) and thermal ifrared (TIR) radiation of the whole Earth from cislunar vantage point offer unique possibilities to answer NASA’s Earth System Science (ESS) questions and further advance volcanic ash (VA) and sulfur dioxide (VSO2) aviation safety applications. We propose complementary ultraviolet (UV) and ther-mal Infrared (TIR) filter cameras for a dual-purpose whole Earth imaging with complementary natural haz-ards applications and Earth System science goals.