• Cloud Detection And Trace Gas Retrieval From The Next Generation Satellite Remote Sensing Instruments

      Larsen, North Fredrick; Stamnes, Knut (2005)
      The objective of this thesis is to develop a cloud detection algorithm suitable for the National Polar Orbiting Environmental Satellite System (NPOESS) Visible Infrared Imaging Radiometer Suite (VIIRS) and methods for atmospheric trace gas retrieval for future satellite remote sensing instruments. The development of this VIIRS cloud mask required a flowdown process of different sensor models in which a variety of sensor effects were simulated and evaluated. This included cloud simulations and cloud test development to investigate possible sensor effects, and a comprehensive flowdown analysis of the algorithm was conducted. In addition, a technique for total column water vapor retrieval using shadows was developed with the goal of enhancing water vapor retrievals under hazy atmospheric conditions. This is a new technique that relies on radiance differences between clear and shadowed surfaces, combined with ratios between water vapor absorbing and window regions. A novel method for retrieving methane amounts over water bodies, including lakes, rivers, and oceans, under conditions of sun glint has also been developed. The theoretical basis for the water vapor as well as the methane retrieval techniques is derived and simulated using a radiative transfer model.
    • Extinction of ultraviolet-A, visible and near-infrared wavelength light in snow and antarctic sea ice

      Quakenbush, Timothy Kyle; Wendler, Gerd; Weller, Gunter; Shaw, Glen; Stamnes, Knut; Stringer, William (1994)
      The optical properties of sea ice are important in the understanding of sea ice thermodynamics, growth and decay processes, polar climates, and remote sensing. The optical properties of ice have been fairly well described, but most studies have focused on wavelengths longer than 400 nm, and on Arctic sea ice. With increased interest in the effects of changing ultraviolet light levels from Antarctic ozone depletion, there has been recent work done on the extinction of ultraviolet light in Antarctic sea ice, including the study reported here. A spectrometer was modified, and taken on two trips to Antarctica to measure the extinction of light in sea ice. Extinction in summer and in winter sea ice were measured in the wavelength range 320 to 900 nm with the resolution range of 1 to 6 nm. This wavelength range covers the ultraviolet A band (320-400 nm), photosynthetically active region (PAR, 400-700 nm), and a small part of the near infrared (NIR, 700-1000 nm). The extinction coefficient in the middle of melting sea ice had a minimum of 0.6 m$\sp{-1}$ in the range of 450-500 nm, rising to 1 m$\sp{-1}$ at 350 nm, and 1.8 at 700 nm. Winter sea ice had spectrally flat extinction between 320-600 nm with values ranging from 1.7-5 m$\sp{-1}$ for different ice floes. This is in contrast to an expected minimum around 450 nm from absorption by ice and water. Extinction increased to 10-15 m$\sp{-1}$ at 900 nm in the winter sea ice. Both warm and cold ice had increased extinction at all wavelengths near the bottom of the floes. Algae were identified in several ice floes by the chlorophyll absorption peaks at 330, 430 and 680 nm. The presence of algae in the ice was associated with an increase in the extinction by a factor of up to 5 for wavelengths shorter than 600 nm, with a much smaller effect on longer wavelengths. Absorption by algae was evident at nearly all depths in some of the ice, but was mostly concentrated in the bottom 15% to 35%.
    • Ozone depletion and biologically relevant ultraviolet radiation

      Zeng, Jun; Stamnes, Knut; Benner, Richard; Bowling, Sue Ann; Eslinger, David; Kawasaki, Koji; Watkins, Brenton (1995)
      An atmospheric radiative transfer model is used to calculate surface spectral ultraviolet irradiance under cloud-free conditions, and compared with measurements made at Lauder, New Zealand (45$\sp\circ{S}$, 170$\sp\circ{E})$ before and after the eruption of Mt. Pinatubo, and including a snow-covered surface. The ratios of diffuse to direct irradiance depend critically on solar elevation, surface albedo, and aerosol extinction. Ozone changes have pronounced effects on the global UVB irradiance, but have only a minor effect on these ratios. The comparison suggests that the ultraviolet radiation exposure can be computed with confidence for clear sky conditions, if the appropriate atmospheric pressure and temperature profiles, ozonesonde data, surface albedo, and aerosol optical properties are available. The total ozone abundances are derived by using ground-based UV irradiance measurements and compared with TOMS in Antarctica and the Arctic from 1990 to 1994. The comparisons show that they are generally in good agreement. Possible reasons for the discrepancies between the two methods are discussed. The equivalent cloud optical depths are also inferred from these data. Ozone depletion can also increase the penetration of ultraviolet radiation into the aquatic system. A coupled atmosphere-ocean radiative transfer model is used to investigate the effect of ozone depletion on UV penetration through the atmosphere and into the underlying water column. Comparisons between model computations and in situ measurements of irradiances made in Antarctic water show good agreement in the UV spectral range between 300 and 350 nm. The ratio of UVB (280-320 nm) to total (280-700 nm) irradiance also compared well. For a given ozone reduction the largest relative increase of UVB radiation arriving at the surface and penetrating to various depths in the ocean occurs at large solar zenith angles. At high latitudes the most pronounced increase in UVB exposure due to an ozone depletion occurs in the early spring, when ozone depletion is expected to be the most severe. The sensitivities of irradiance reflectance and diffuse attenuation coefficients to solar zenith angle, sky conditions, and chlorophyll concentration are discussed by using a coupled atmosphere-ocean radiative transfer model. The irradiance reflectance is sensitive to solar zenith angle, cloud cover, and chlorophyll concentration; the diffuse attenuation coefficient is sensitive to solar zenith angle and chlorophyll concentration, but less sensitive to sky conditions.
    • Single and multiple electromagnetic scattering by size -shape distributions of small nonspherical particles

      Schulz, Frank Michael (1998)
      A comprehensive model for light scattering by size-shape distributions of randomly oriented nonspherical particles is developed. The model uses spheroids as model particles. The vector Helmholtz equation is solved with a new separation of variables (SVM) approach that allows one to calculate the ensemble-averaged single scattering optical properties of ensembles of randomly oriented particles analytically. Since the use of the SVM in spheroidal coordinates properly accounts for the geometry of the particles, the method is applicable to a large range of shapes ranging from elongated prolate needles via spheres to flat oblate disks. The relation between geometric symmetries of particles and symmetry relations of the electromagnetic scattering solution is investigated systematically in the general framework of the theory of point groups. The results are exploited in the model for increasing the computational efficiency. A comprehensive vector radiative transfer model is in part developed in this work. This radiative transfer model takes the output of the single scattering model as input and computes the Stokes vector components in a vertically inhomogeneous, plane parallel medium as a function of polar and azimuth angle and as a function of optical depth. The single scattering model is applied to investigate the impact of particle shape on the optical properties of size-shape distributions of randomly oriented particles, such as aerosol layers or ice clouds in the atmosphere. The optical properties are found to be much more sensitive to a variation in the effective aspect ratio than to a variation in the effective variance of a shape-distribution. The results of this study are used as input to the vector radiative transfer model in order to study the shape-sensitivity of the radiation field in a macroscopic medium containing a size-shape distribution of randomly oriented particles. It is found that both the radiance, and the degree of linear polarization, and the degree of circular polarization are strongly shape-sensitive in most viewing directions.