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Outward flux and intensity of scattered radiation for top of Rayleigh atmosphere lying above smooth water surface that reflects radiation according to Fresnel law
The contribution of inelastic molecular scattering (Rayleigh-Brillouin and rotational Raman scattering) to the filling-in of Fraunhofer lines in the light of the blue sky is studied. Aerosol fluorescence is shown to be negligible, and aerosol scattering is ignored. The angular and polarization dependences of the filling-in detail for single scattering are discussed. An approximate treatment of multiple scattering, using a backward Monte Carlo technique, makes it possible to investigate the effects of the ground albedo. As the molecular scatterings alone produce more line-filling than is observed, it seems likely that aerosols dilute the effect by contributing unaltered sunlight to the observed spectra.
Atmospheric radiation polarization characteristics, determining concentration and distribution of atmospheric scatterers
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A modification of the classical Rayleigh technique utilizing the fast Fourier transform, developed into the so-called Rayleigh-FFT method for electromagnetic scattering, is shown to obtain valid scattering results from arbitrary irregular periodic structures composed of a rough layer over a rough half-space. The considerable usefulness of the Rayleigh approach is thus established, and Beckmann's (1968) contention (that the convergence of the method is reasonably rapid only if the roughness is very slight) is refuted.
The Raman shifted return of a lidar, or optical radar, system has been utilized to make atmospheric temperature measurements. These measurements were made along a horizontal path at temperatures between -20 C and +30 C and at ranges of about 100 meters. The temperature data were acquired by recording the intensity ratio of two portions of the Raman spectrum which were simultaneously sampled from a preset range. The lidar unit employed in this testing consisted of a 4 joule-10ppm laser operating at 694.3 nm, a 10-inch Schmidt-Cassegrain telescope, and a system of time-gated detection and signal processing electronics. The detection system processed three return signal wavelength intervals - two intervals along the rotational Raman scattered spectrum and one interval centered at the Rayleigh-Mie scattered wavelength. The wavelength intervals were resolved by using a pellicle beam splitter and three optical interference filters. Raman return samples were taken from one discrete range segment during each test shot and the signal intensities were displayed in digital format. The Rayleigh-Mie techniques. The test site utilized to evaluate this measurement technique encompassed a total path length of 200 meters. Major components of the test site included a trailer-van housing the lidar unit, a controlled environment test zone, and a beam terminator. The control zone which was located about 100 meters from the trailer was 12 meters in length, 2.4 meters in diameter, and was equipped with hinged doors at each end. The temperature of the air inside the zone could be either raised or lowered with respect to ambient air through the use of infrared heaters or a liquid-nitrogen cooling system. Conditions inside the zone were continuously monitored with a thermocouple rake assembly. The test path length was terminated by a 1.2 meter square array of energy absorbing cones and a flat black screen. Tests were initially conducted at strictly ambient conditions utilizing the normal outside air temperatures as a test parameter. These tests provided a calibration of the Raman intensity ratio as a function of' temperature for the particular optical-filter arrangement used in this system while also providing a test of' the theoretical prediction formulated in the design of the system. Later tests utilized zone temperatures above and below ambient to provide temperature gradient data. These tests indicate that ten shots, or one minute of' data acquisition, from a 100 meter range can provide absolute temperature measurements with an accuracy of + 30 C and a range resolution of about 5 meters. Because this measurement accuracy compares well with that predicted for this particular unit, it is suggested that a field-application system could be built with signif'icant improvements in both absolute accuracy and range.
NASA Goddard Space Flight Center's Airborne Raman Ozone Temperature Lidar (AROTEL) made temperature retrievals within Polar Stratospheric Clouds (PSCs) on several flights during the SAGE III Ozone Loss and Validation Experiment (SOLVE) campaign. The location of the PSCs was confirmed using simultaneously acquired data from the NASA Langley Aerosol lidar. Retrievals were made on flight dates 991207, 991210 and 000127 from just above the aircraft to 25 kilometers geometric altitude. Raman temperature retrievals are, to first order, insensitive to Mie interference because the Raman signals are red shifted by 2331 cm(exp -1) from the initial laser wavelength. Backscattering from clouds and aerosols is consequently not observed by the detector; however, extinction does impact the measurement and limits retrievals to optically thin clouds. Comparisons between retrievals employing Rayleigh and Raman scattering show the Raman temperatures to be significantly warmer than those employing Rayleigh scattering. Uncertainties are a function of altitude: at 25 km they were approximately 3 K. Temperature profiles could not be retrieved for optically thick clouds.
Ultraviolet spectra of planetary atmospheres and the near infrared carbon dioxide bands of venus
Phonon-drag thermopower variation in dilute copper alloys assuming Rayleigh law for phonon scattering
Aerosols affect the polarization of radiation in scattering, hence measured polarization can be used to infer the nature of the particles. Size distribution, particle shape, real and absorption parts of the complex refractive index affect the scattering. From Lorenz-Mie calculations of the 4-Stokes parameters as a function of scattering angle for various wavelengths the following polarization parameters were plotted: total intensity, intensity of polarization in plane of observation, intensity perpendicular to the plane of observation, polarization ratio, polarization (using all 4-Stokes parameters), plane of the polarization ellipse and its ellipticity. A six-component log-Gaussian size distribution model was used to study the effects of the nature of the polarization due to variations in the size distribution and complex refractive index. Though a rigorous inversion from measurements of scattering to detailed specification of aerosol characteristics is not possible, considerable information about the nature of the aerosols can be obtained. Only single scattering from aerosols was used in this paper. Also, the background due to Rayleigh gas scattering, the reduction of effects as a result of multiple scattering and polarization effects of possible ground background (airborne platforms) were not included.
Theoretical evidence for the existence and behavior of polarization in astronomical spectra is provided. The theory for the study of spectral multiple scattering of arbitrarily polarized light is first developed, and the detailed and integrated spectropolarimetry of a planetary atmosphere is then studied for cases in which the spectra are formed in the presence of either very small nonspherical particles (Rayleigh-Cabannes scattering) or large polydisperse spherical particles (Mie scattering). It is shown in both cases that polarization is indeed present; it increases with the line strength but decreases afterwards as the line becomes very strong and tends to saturation. A polarization reversal is also predicted during latitudinal (pole-to-equator) scan and possibly also during longitudinal (terminator-to-limb) scan of the planet. The reversal happened at all phase angles considered. Our companion article (Forbes and Fymat) will provide observational substantiation to these theoretical predictions.
The ellipticity and the direction of polarization are calculated for radiation that has undergone multiple scattering from plane parallel layers. Both the radiation emerging from the top of the layer and that transmitted through the bottom are considered. Two different phase functions are used for the scattering layer: Rayleigh and haze L. The direction of polarization of the reflected radiation shows little variation as the optical depth of the layer increases, while there is a much larger variation for the transmitted radiation. When the optical thickness is small, the direction of polarization for haze L varies rapidly with zenith angle near those angles at which the single scattered polarization is zero. The ellipticity of the radiation from haze L layers increases at first in direct proportion to the optical thickness of the layer. In general the ellipticity of the transmitted radiation is considerably greater than that of the reflected because of the greater average number of photon collisions in the former case.
We consider scattering by arbitrarily shaped particles that satisfy two conditions: (1) that the polarizability of the particle relative to the ambient medium be small compared to 1 and (2) that the phase shift introduced by the particle be less than 2. We solve the integro-differential equation proposed by Shifrin by using the method of successive iterations and then applying a Fourier transform. For the second iteration, results are presented that accurately describe scattering by a broad class of particles. The phase function and other elements of the scattering matrix are shown to be in excellent agreement with Mie theory for spherical scatterers.
The majority of lightning-related casualties typically occur during thunderstorm initiation (e.g., first flash) or dissipation (e.g., last flash). The physics of electrification and lightning production during thunderstorm initiation is fairly well understood. As such, the literature includes a number of studies presenting various radar techniques (using reflectivity and, if available, other dual-polarimetric parameters) for the anticipation of initial electrification and first lightning flash. These radar techniques have shown considerable skill at forecasting first flash. On the other hand, electrical processes and lightning production during thunderstorm dissipation are not nearly as well understood and few, if any, successful techniques have been developed to anticipate the last flash and subsequent cessation of lightning. One promising approach involves the use of dual-polarimetric radar variables to infer the presence of oriented ice crystals in lightning producing storms. In the absence of strong vertical electric fields, ice crystals fall with their largest (semi-major) axis in the horizontal associated with gravitational and aerodynamic forces. In thunderstorms, strong vertical electric fields (100-200 kV m(sup -1)) have been shown to orient small (less than 2 mm) ice crystals such that their semi-major axis is vertical (or nearly vertical). After a lightning flash, the electric field is typically relaxed and prior radar research suggests that ice crystals rapidly resume their preferred horizontal orientation. In active thunderstorms, the vertical electric field quickly recovers and the ice crystals repeat this cycle of orientation for each nearby flash. This change in ice crystal orientation from primarily horizontal to vertical during the development of strong vertical electric fields prior to a lightning flash forms the physical basis for anticipating lightning initiation and, potentially, cessation. Research has shown that radar reflectivity (Z) and other co-polar back-scattering radar measurements like differential reflectivity (Z(sub dr)) typically measured by operational dual-polarimetric radars are not sensitive to these changes in ice crystal orientation. However, prior research has demonstrated that oriented ice crystals cause significant propagation effects that can be routinely measured by most dual-polarimetric radars from X-band (3 cm) to S-band (10 cm) wavelengths using the differential propagation phase shift (often just called differential phase, phi(sub dp)) or its range derivative, the specific differential phase (K(sub dp)). Advantages of the differential phase include independence from absolute or relative power calibration, attenuation, differential attenuation and relative insensitivity to ground clutter and partial beam occultation effects (as long as the signal remains above noise). In research mode, these sorts of techniques have been used to anticipate initial cloud electrification, lightning initiation, and cessation. In this study, we develop a simplified model of ice crystal size, shape, orientation, dielectric, and associated radar scattering and propagation effects in order to simulate various idealized scenarios of ice crystals responding to a hypothetical electric field and their dual-polarimetric radar signatures leading up to lightning initiation and particularly cessation. The sensitivity of the K(sub dp) ice orientation signature to various ice properties and radar wavelength will be explored. Since K(sub dp) is proportional to frequency in the Rayleigh- Gans scattering regime, the ice orientation signatures should be more obvious at higher (lower) frequencies (wavelengths). As a result, simulations at radar wavelengths from 10 cm down to 1 cm (Ka-band) will be conducted. Resonance effects will be considered using the T-matrix method. Since most K(sub dp) Vbased observations have been shown at S-band, we will present ice orientation signatures from C-band (UAH/NASA ARMOR) and X-bd (UAH MAX) dual-polarimetric radars located in Northern Alabama. Issues related to optimal radar scanning for the detection of oriented ice will be discussed. Preliminary suggestions on how these differential phase signatures of oriented ice could contribute to lightning initiation and cessation algorithms will be presented.
The complete radiation field is calculated for scattering layers of various optical thicknesses. Results obtained for Rayleigh and haze scattering are compared. Calculated radiances show differences as large as 23% compared to the approximate scalar theory of radiative transfer, while the same differences are approximately 0.1% for a continental haze phase function. The polarization of reflected and transmitted radiation is given for various optical thicknesses, solar zenith angles, and surface albedos. Two types of neutral points occur for aerosol phase functions. Rayleigh-like neutral points arise from zero polarization that occurs at scattering angles of 0 deg and 180 deg. For Rayleigh phase functions, the position of these points varies with the optical thickness of the scattering layer. Non-Rayleigh neutral points are associated with the zeros of polarization which occur between the end points of the single scattering curve, and are found over a wide range of azimuthal angles.
Presentation of OAO-2 broad-band ultraviolet photometry data for Jupiter, Uranus, and Neptune. These data have been combined with observations of 23 late-type stars to derive planetary albedos over the region 2000-4300 A. The new broad-band albedo for Jupiter agrees well with a previous measurement based on OAO-2 spectrometer data. The albedos for Uranus and Neptune are in good agreement with ground measurements in the region of overlap. The results for Uranus and Neptune have been compared with theoretical calculations for semiinfinite and finite pure H2 Rayleigh-Raman scattering atmospheres. Except for the shortest wavelengths, the measurements lie below the semiinfinite theoretical calculations. The calculations for the finite atmospheres suggest that an additional absorbing constituent will be needed to explain the observed albedos.
Observations on Uranus, Neptune, and four G-type stars which were obtained with the five channel ultraviolet spectrometer aboard the Astronomical Netherlands Satellite are reported. The paper uses these data to derive geometric albedos between 1800 and 3300 A for Uranus and between 2200 and 3300 A for Neptune. It is shown that the albedos are in good agreement with ground measurements near 3300 A and with OAO 2 measurements near 2500 A. In addition, it is noted that the albedos are smaller than theoretical predictions for deep pure H2 Rayleigh-Raman scattering atmospheres, which confirms earlier predictions that additional sources of ultraviolet absorption are required in the upper atmosphere of these planets. Consideration is given to the possibility that the Uranian atmosphere contains an extra absorber, noting that the identification of this absorber is difficult from broad-band spectrophotometric measurements.
The ultraviolet spectra of Uranus, Neptune, and Titan do not reveal absorption features at 7-A resolution from 2100 to 3200 A. Upper limits of from 1 to 3 A are set for the equivalent widths of narrow absorptions, which corresponds to a CO/H2 mixing ratio less than 2 x 10 to the -4th in the case of Uranus. The slopes of the continuum reflectivities of Uranus and Neptune are consistent with the semi-infinite Rayleigh-Raman scattering model of Cochran, while the absolute levels are matched only if solar photometry is modified within acceptable limits. An alternative, but less satisfactory explanation of the new data is that the UV reflectivities of both Uranus and Neptune are depressed uniformly by a continuum absorber. The suggestion by Savage et al (1980) that the albedo of Uranus decreases by approximately 20% from 2200 to 1800 A is not confirmed. For Titan, the albedo decreases monotonically toward shorter wavelengths down to 2300 A. New limits are set for the pressure level in the atmosphere of Titan up to which the real, but presently unidentified, UV absorber there must extend, and for the fraction of Titan that must be covered by this absorber