For remote sensing over snow-covered surfaces, the bidirectional reflectance distribution function (BRDF) of snow plays an important role that should be considered in inverse algorithms for the retrieval of snow properties. However, to simplify retrievals, many researchers assume that snow is a Lambertian reflector. This “forward model” error affects the accuracy of retrieved snow parameters (such as albedo, snow grain size, and impurity concentration). To quantify this error and to compensate for it, we provide a simple yet accurate semi-empirical correction formula. It allows for easy conversion of top-of-the-atmosphere (TOA) reflectance arising from an anisotropically reflecting snow surface to an equivalent TOA reflectance for a Lambertian surface with the same albedo. Conversely, this correction can be used to translate TOA radiance computed with the Lambertian assumption into a more realistic value based on a BRDF treatment. The coefficients in this correction formula are stored in a look-up table (LUT), and a simple LUT interpolation program is provided to allow the user to extract TOA reflectances for any sun-satellite geometry by quick interpolation in the LUTs. For the first 8 channels of the VIIRS spectrometer, the R-square regression coefficient for fitting this correction formula is better than 0.95 for a wide range of sun-satellite geometries.
Recent technological advances have made measurements of UV doses and ozone column amounts with multichannel filter instruments not only possible, but also an attractive alternative to other more labor-intensive and weather-dependent methods. Filter instruments can operate unattended for long periods of time, and it is possible to obtain accurate ozone column amounts even on cloudy days. We present results from extensive comparisons of the performance of several Norwegian Institute for Air Research UV (NILU-UV) and ground-based (GUV) filter instruments against Dobson and Brewer instruments and the earth probe–total ozone mapping spectrometer (EP-TOMS) instrument. The data used in the comparisons are from four different sites where we have had the opportunity to operate more than one type of UV instrument for extended periods of time. The sites include the University of Oslo, Norway; Ny-Ålesund, Spitzbergen, Norway; the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center facilities at Wallops Island, Virginia, and Greenbelt, Maryland; and the University of Alaska, Fairbanks. Our results show that ozone column amounts obtained with current filter-type instruments have an accuracy similar to those obtained with the Dobson instrument. The mean difference between NILU-UV and Dobson direct sun measurements were 0.4±1.9% (1) in Oslo for the period 2000 to 2003. The difference between a GUV and the same Dobson was 1.7±1.4% for the same time period. The mean difference between GUV and TOMS in Ny-Ålesund 79 deg N and Oslo 60 deg N in the period 1996 to 1999 was <0.5±3% for days with noon solar zenith angles (SZAs)<80 deg.
Successful retrieval of surface properties from space is hampered by the presence of atmospheric aerosol particles that contribute significantly to the measured signal. Our ability to obtain reliable information about surface properties depends to a large extent on how well we can account for the influence of aerosols. The problem is complicated by the fact that aerosols often consist of a multicomponent mixture of particles with different chemical compositions and different affinities to water. For example, to predict how the optical properties of such particles change with increasing humidity, we must make assumptions about how the particles grow, change their refractive indices, and mix as a function of humidity. We discuss possible strategies for reliable atmospheric correction over dark as well as bright surfaces. Also, we discuss the role of realistic simulations of the radiative transfer process in the coupled atmosphere-ocean system in the solution of the inverse problem required to retrieve surface properties is also discussed.
Recent work has shown the need for accurate treatment of radiative transfer in ocean color retrieval. The plane-parallel coupled atmosphere-ocean discrete ordinate model CAO-DISORT has been used to investigate the validity of current approximative inverse methods and to study new techniques for improved ocean color retrieval. In this paper we show that CAO-DISORT is fully differentiable with respect to its input optical properties, so that we can define analytic Jacobians with respect to any profile element in the atmosphere and ocean. A single call to the linearized model will produce radiances and Jacobians at arbitrary optical depth and viewing geometry in either medium. The model also has a pseudo-spherical treatment for solar beam attenuation in a curved atmosphere. The linearized model can be used directly in iterative least-squares retrievals requiring forward model simulations of backscatter measurements and their parameter derivatives; there is no need for approximations involving an atmospheric correction. We demonstrate the model's new capability by performing closed-loop least squares fitting to simultaneously retrieve the aerosol optical thickness and marine chlorophyll concentration from a set of 6 synthetic measurements at SeaWifs wavelengths.
Retrieval of surface properties of highly reflecting targets such as snow and ice is a challenging problem due to the influence of aerosols
which varies considerably in space and time. Also, accounting for the bidirectional properties of a bright surface such as snow is very important for reliable retrievals. The main purpose of the work described in this paper is to explore the opportunities and possibilities offered by multi- and hyperspectral data such as those available provided by MODIS, GLI, the Advanced Land Imager (ALI), and Hyperion to retrieve reliable aerosol and surface properties. Over snow and ice surfaces these include aerosol optical depth and single scattering albedo, the mean size of snow grains and ice "particles" (inclusions), and the spectral and broadband snow/ice albedo. In particular the following question will be addressed: To what extent can multi- and hyperspectral data help improve our knowledge of snow and ice parameters that are important for understanding global climate change?
A new method for simultaneous retrieval of aerosol properties and marine constituents in turbid waters is described. This method is an extension to turbid waters of an approach developed previously for simultaneous retrieval of aerosol properties and chlorophyll concentrations in clear waters. This extension is accomplished by employing near-infrared (NIR) channels not available on the SeaWiFS and MERIS instruments to help retrieve aerosol parameters over turbid waters. Optimal estimation theory is used to retrieve in-water parameters from multi- and hyperspectral information. Both forward and inverse modeling strategies will be discussed, as well as the uniqueness of the solutions, the information content available in multi- and hyperspectral data, and the error analysis approach. Our results indicate that it is important to use forward models that accurately treat the radiative transfer in the coupled (combined) atmosphere-ocean system, and to carefully select the most suitable bio-optical models for the in-water inherent optical properties (IOPs). Synthetic data, as well as multi- and hyperspectral images of data obtained over clear as well as turbid waters, are used to test the validity of the new retrieval approach.
A number of remote sensing instruments with multi-spectral imaging capabilities (SeaWiFS, MODIS, GLI on ADEOS-II, and others) have recently been launched on earth-orbiting satellites or will soon be launched into space. Many of these sensors offer unique opportunities for studies of sea ice and ocean properties at high latitudes. There are a number of challenges associated with the inversion of data received from satellite such instruments in order to retrieve meaningful information. Here we discuss some of these challenges with emphasis on the derivation of sea ice and marine parameters from satellite data.
Successful retrieval of surface properties from space is hampered by the presence of atmospheric aerosol particles that contribute significantly to the measured signal. Our ability to obtain reliable information about surface properties depends to a large extent on how well we can account for the influence of aerosols. The problem is complicated by the fact that these aerosols often consist of a multi-component mixture of particles with different chemical compositions and different affinities to water. For example, in order to predict how the optical properties of such particles change with increasing humidity, we need to make assumptions about how the particles grow, change their refractive indices, and mix as a function of humidity. The purpose of this paper is to discuss possible strategies for reliable atmospheric correction over dark as well as bright surfaces. The role of realistic simulations of the radiative transfer process in the coupled atmosphere-surface system in order to solve the inverse problem required to retrieve surface properties will also be discussed.
At Stevens Institute of Technology, Hoboken, NJ we have operated a site with NILU-UV instruments for nearly two years. For most of this time only one instrument has been in operation, but we also have
data for extended periods of time when up to three instruments have been working in parallel. The site is in close proximity to New York City and it is equipped with basic radiation sensors in addition to the NILU-UV sensors. In a companion paper we present results from intercomparisons between filter-based instruments, such as the NILU-UV, and the Dobson and Brewer instruments. Here we describe our experience operating filter-based radiation instruments. In particular, we discuss data quality issues and describe how one can detect and correct for drift in filter-based instruments. We also investigate the effect of elevated detector temperatures due to over-heating of the instrument by solar radiation on very warm days. Our experience with the newer versions of the filter instruments is that most of them have only minor problems with filter drift over time, and that this drift (if any) is easily detectable and can be corrected for. A potential problem is that varying detector temperature can degrade the instrument performance. Since filter UV instruments are normally set to operate with detector temperatures much higher than ambient temperatures this is a minor issue for most locations, and one that can easily be prevented.
Recent technology advances have made measurements of UV doses and ozone column amounts with multi-channel filter instruments not only possible, but also an attractive alternative to other more labor-intensive and weather dependent methods. Filter instruments can operate unattended for long periods of time, and it is possible to obtain accurate ozone column amounts even on cloudy days. We present results from extensive comparisons of the performance of several NILU-UV and GUV filter instruments against Dobson and Brewer instruments and the EP-TOMS instrument. The data used in the comparisons are from four different sites where we have had the opportunity to operate more than one type of UV instruments for extended periods of time. The sites include the University of Oslo, Norway, Ny-Alesund, Spitzbergen, Norway, the NASA Goddard Space Flight Center facilities at Wallops Island, VA, and Greenbelt, MD and the University of Alaska, Fairbanks. Our results show that ozone column amounts obtained with current filter-type instruments are just as good as those obtained with the Dobson instrument. The mean difference between NILU-UV and Dobson direct sun measurements were 0.4% ± 1.9% (1σ) in Oslo 2000-2003. The difference between a GUV and the same Dobson was 1.7% ± 1.4% for the same time period. The mean difference between GUV and TOMS in Ny-Alesund 79°N and Oslo 60°N in the period 1996-1999 was < 0.5% ± 3% for days with noon SZA < 80°.
Applying scalar diffraction theory in the near field of weak phase objects, a simple relationship with the diffracted field behind an aperture of identical form in a black screen is observed which allows a formulation of a modified Babinet principle for complementary weak phase objects. Its validity is demonstrated in the near field behind complementary dielectric objects using a modified Mach-Zehnder interferometer for 3 cm microwaves.
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