173 research outputs found
Electron transport and energy degradation : on the numerical solution to the two-stream equations
No full textUAG R-286; Scientific Report.A new numerical solution to the two-stream equations for electron transport and thermalization is presented and discussed. The numerical code is verified by comparing computed results with the analytic results of Stamnes (1981) and with the Nagy-Banks code (Nagy and Banks, 1970). Further verification is provided by carefully testing for energy conservation. The computational speed depends primarily on the number of grid points in altitude utilized. It is demonstrated that useful results can be obtained with as little as 20-40 altitude grid points. The present numerical code represents an extension of the Nagy-Banks code to high electron energies appropriate for auroral applications. This extension is accomplished by using the energy degradation method of Swartz (1977) and the backscatter ration of Stamnes (1981).This work was supported by the National Science Foundation through grant ATM 79-23266 to the University of Alaska.Abstract - 1. Introduction -- 2. The electron transport equation in the two-stream approximation -- 3. Numerical procedures -- 3.1. Computational aspects -- 4. Verification of numerical results -- 4.1. Comparison with analytic results for monoenergetic electrons -- 4.2. Comparison with the analytic multi-energy results of Stamnes (1981) -- 4.2.1. Energy conservation -- 4.3. Results for an imbedded source -- 4.4. Comparison with the Nagy-Banks two-stream code -- 5. Conclusion -- 6. Appendix: analytic proof of energy conservation -- Acknowledgements -- References
On the discrete ordinate method for radiative transfer calculations in anisotropically scattering atmospheres : intensity computations
No full textUAG R-281The recently developed matrix method to solve the discrete ordinate approximation to the radiative transfer equation in plane parallel geometry (Stamnes, 1980) is extended to compute the full azimuthal dependence of the intensity. Comparing computed intensities with those obtained by other established methods, we find that for phase functions typical of atmospheric haze 32 streams are sufficient for better than 1% agreement, while 16 streams yield an accuracy of about 1-5% except for angles close to the forward and backward directions for which the error is about 10-15%. The results of the intensity computations are summarized by presenting three-dimensional “stack plots” of the intensity as a function of polar and azimuthal angles. We also show that for flux calculations 4 streams suffice to obtain 1% accuracy, while 8 streams yield an accuracy better than 0.1%.This research was supported by the National Science Foundation through grants ATM 79-23266 and ATM 79-26406 (Acknowledgements).Abstract – 1. Introduction – 2. Basic equations – 3. Matrix solution – 4. Source function and angular distributions – 5. Boundary conditions – 6. Results and comparisons – a. Comparison with results provided in “standard procedures” – b. Comparison with Fricke (1979) – 7. Summary and conclusions – Acknowledgements – References
AccuRT: A versatile tool for radiative transfer simulations in the coupled atmosphere-ocean system
No full textCloud and surface properties and the solar radiation budget derived from satellite data over the Arctic Ocean: Comparisons with surface measurements and in situ aircraft data
No full textDissertation (Ph.D.) University of Alaska Fairbanks, 2000Use of satellite data to study the surface and cloud properties and the solar radiation budget (SRB) is very important for improving our understanding of cloud and sea-ice albedo feedback in the Arctic. Based on an accurate and comprehensive Radiative Transfer Model (RTM), algorithms were developed for using the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) data for the discrimination of cloud from snow/ice surfaces, retrieval of snow surface properties and surface albedo, and retrieval of cloud optical depth (tau) and effective droplet size ( re). Through the improved estimation of solar reflectance in AVHRR channel 3 (3.75 mum) and atmospheric anisotropic correction, a threshold function was found and used for developing an automatic cloud discrimination algorithm over snow/ice surfaces. Thin cirrus was discriminated using the brightness temperature difference between AVHRR channels 4 and 5 and brightness temperature in channel 4. Retrieval of snow grain size and mass-fraction of soot from AVHRR is difficult because of the effects of aerosol in channel 1 and the strong water vapor absorption in channel 2. Retrieval of surface albedo is more promising, but, with the melt of snow/ice, different narrow-to-broadband conversion relations should be used to derive broadband albedo. AVHRR channels 2, 3 and 4 are used to retrieve tau, r e and cloud top temperature simultaneously. Validation of these algorithms with in-situ aircraft measurements by the NCAR C-130 and the NASA ER-2 and with surface measurements obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment indicates that the retrieved re is close to the "true" value of re, but the retrieved tau tends to be overestimated. Uncertainties of cloud retrievals with regard to cloud cover fraction, vertical inhomogeneity, multi-layer stratification and cloud phase were examined. Inter comparison of different satellite data demonstrates that NOAA-14 AVHRR data for SHEBA is overestimated by 10--20% using the calibration by Rao and Chen (1996). Finally, seasonal variation of surface albedo, cloud properties and SRB over SHEBA was derived based on 1 or 2 AVHRR overpasses per day from April to August, 1998
Cloud Detection And Trace Gas Retrieval From The Next Generation Satellite Remote Sensing Instruments
No full textDissertation (Ph.D.) University of Alaska Fairbanks, 2005The 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
Experimental And Theoretical Investigation Of Stratospheric Ozone Depletion In The Northern Hemisphere Caused By Heterogeneous Chemistry
No full textDissertation (Ph.D.) University of Alaska Fairbanks, 2001Stratospheric ozone is of crucial importance for life on Earth. This thin layer protects us from the ultraviolet solar radiation and also works as a greenhouse gas that helps maintaining our climate. Large changes in thickness and vertical distribution of the ozone abundance may have detrimental effects on life on Earth. But even small changes could have considerable impact on UV irradiance, bio-production and cancer rates. During the last decade record low spring time vertical column amounts of stratospheric ozone have been observed over Northern Europe. However, this decrease is not as severe as the depletion observed over Antarctica and at mid-latitudes in the Southern Hemisphere. The discovery of the spring time stratospheric ozone depletion first in Antarctica and later in the Arctic has triggered international research efforts on stratospheric ozone chemistry and the possible effects of human activities on the ozone layer. Ground-based differential optical absorption spectroscopy measurements of NO2 and ozone have been performed over Fairbanks (65�N) and Ny-Alesund (79�N) during the 1994--95 season. In this work we present improvements to ground based differential optical spectroscopy measurements by improving dark current corrections and spectral fitting of spectrographic photo diode array detector measurements. We have also improved the retrieval of vertical column amounts from diffuse light measurements by improving the corrections for seasonal changes in absorber air mass. This is particularly important at high latitudes. We used these data together with local weather and ozone sounding data, and with trace gas and aerosol data measured by other ground based instruments and by instruments deployed on satellites. This comprehensive dataset was used to investigate the performance of two current state of the art chemical transport models with and without the presence of heterogeneous chemistry. These are the University of Cambridge SLIMCAT model and the University of Oslo SCTM-1 model. They were selected because the SLIMCAT is designed for process studies and comparison with measured data while the SCTM-1 is designed for prognostic and sensitivity studies aimed at predicting future development of the stratospheric ozone layer. We have used the models to study the sensitivity of the heterogeneous chemistry to stratospheric meteorological conditions and the effect of sulfuric acid aerosols and polar stratospheric clouds on the stratospheric ozone abundance and ozone chemistry at high- and mid-latitudes in the Northern Hemisphere
Electromagnetic scattering by spheroidal particles: Applications to the atmosphere
No full textDissertation (Ph.D.) University of Alaska Fairbanks, 2000An efficient and reliable method is presented for computing the expansion coefficients in the eigenfunction series representing the prolate and oblate spheroidal functions. While the traditional method is based on recurrence relations, infinite continued fractions, and a variational procedure, the new method is based on reformulating the computational task as an eigenvalue problem. In contrast with the traditional method, the new method requires no initial estimates of the eigenvalues, and the computations can be performed using readily available computer library routines. The new method is shown to produce accurate expansion coefficients for the spheroidal functions required to study scattering by particles with a wide range of shapes, sizes, and complex refractive indices [1]. In a previous study in which the scattering characteristics of Polar Stratospheric Clouds (PSCs) were calculated using randomly oriented monodispersions of prolate spheroids [2], the scattering signature of the main types of PSC particles was related to particles of a certain shape and size range. Here these results are used as a reference for testing a new method for calculating the scattering characteristics of PSC like clouds. The method is based on finding the single scattering solution for spheroids using the rigorous Separation of Variables Method (SVM), and then from it obtain the so called T-matrix The following question is addressed: Can the backscatter depolarization returns be used together with other remote sensing techniques to determine either basic shape (degree of needle- or disc-like asphericity) or size information of ice cloud particles? To this end the SVM is again utilized to obtain the T-matrix for a variety of size, shape, and size-shape distributions of ensembles of randomly oriented spheroidal particles. The results indicate that single-wavelength depolarization lidar returns are insufficient to uniquely determine both the size and the shape of the particles in an ice cloud. However, a combination of an NIR depolarization lidar and additional information obtained by complementing instruments---from which either size or shape can be estimated---has the potential for determining the mean size and shape of particles in an ice cloud
Longwave Radiative Transfer In The Atmosphere: Model Development And Applications
No full textDissertation (Ph.D.) University of Alaska Fairbanks, 2003A FLexible Radiative Transfer Tool (FLRTT) has been developed to facilitate the construction of longwave, correlated k-distribution, radiative transfer models. The correlated k-distribution method is a technique which accelerates calculations of radiances, fluxes, and cooling rates in inhomogeneous atmospheres; therefore, correlated k-distribution models are appropriate for simulations of satellite radiances and inclusion into general circulation models. FLRTT was used to build two new rapid radiative transfer models, RRTM_HIRS and RRTM_v3.0, which maintain accuracy comparable to the line-by-line radiative transfer model LBLRTM. Iacono et al. [2003] evaluated upper tropospheric water vapor (UTWV) simulated by the National Center for Atmospheric Research Community Climate Model, CCM3, by comparing modeled, clear-sky brightness-temperatures to those observed from space by the High-resolution Radiation Sounder (HIRS). CCM3 was modified to utilize the rapid radiative transfer model RRTM and the separate satellite-radiance module, RRTM_HIRS, which calculates brightness temperatures in two HIRS channels. By incorporating these accurate radiative transfer models into CCM3, the longwave radiative transfer calculations have been removed as a significant source of error in the simulations. An important result of this study is that CCM3 exhibits moist and dry discrepancies in UTWV of 50% in particular climatic regions, which may be attributed to errors in the CCM3 dynamical schemes. RRTM_v3.0, an update of RRTM, is a rapid longwave radiative transfer appropriate for use in general circulation models. Fluxes calculated by RRTM_v3.0 agree with those computed by the LBLRTM to within 1.0 W/m2 at all levels, and the computed cooling rates agree to within 0.1 K/day and 0.3 K/day in the troposphere and stratosphere, respectively. This thesis also assessed and improved the modeling of clear-sky, longwave radiative fluxes at the Atmospheric Radiation Measurement Program North Slope of Alaska site by simultaneously addressing the specification of the atmosphere, radiometric measurements, and radiative transfer modeling. Consistent with findings from other field sites, the specification of the atmospheric water vapor is found to be a large source of uncertainty in modeled radiances and fluxes. Improvements in the specification of carbon dioxide optical depths within LBLRTM resulted, in part, from this analysis
The influence of inclusions on light scattering by large hexagonal and spherical ice crystals
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