Showing papers on "Zenith published in 1982"

Optical Properties of Snow

Abstract: Measurements of the dependence of snow albedo on wavelength, zenith angle, grain size, impurity content, and cloud cover can be interpreted in terms of single-scattering and multiple-scattering radiative transfer theory. Ice is very weakly absorptive in the visible (minimum absorption at λ = 0.46 µm) but has strong absorption bands in the near infrared (near IR). Snow albedo is therefore much lower in the near IR. The near-IR solar irradiance thus plays an important role in snowmelt and in the energy balance at a snow surface. The near-IR albedo is very sensitive to snow grain size and moderately sensitive to solar zenith angle. The visible albedo (for pure snow) is not sensitive to these parameters but is instead affected by snowpack thickness and parts-per-million amounts (or less) of impurities. Grain size normally increases as the snow ages, causing a reduction in albedo. If the grain size increases as a function of depth, the albedo may suffer more reduction in the visible or in the near IR, depending on the rate of grain size increase. The presence of liquid water has little effect per se on snow optical properties in the solar spectrum, in contrast to its enormous effect on microwave emissivity. Snow albedo is increased at all wavelengths as the solar zenith angle increases but is most sensitive around λ =1 µm. Many apparently conflicting measurements of the zenith angle dependence of albedo are difficult to interpret because of modeling error, instrument error, and inadequate documentation of grain size, surface roughness, and incident radiation spectrum. Cloud cover affects snow albedo both by converting direct radiation into diffuse radiation and also by altering the spectral distribution of the radiation. Cloud cover normally causes an increase in spectrally integrated snow albedo. Some measurements of spectral flux extinction in snow are difficult to reconcile with the spectral albedo measurements. The bidirectional reflectance distribution function which apportions the reflected solar radiation among the various reflection angles must be known in order to interpret individual satellite measurements. It has been measured at the snow surface and at the top of the atmosphere, but its dependence on wavelength, snow grain size, and surface roughness is still unknown. Thermal infrared emissivity of snow is close to 100% but is a few percent lower at large viewing angles than for overhead viewing. It is very insensitive to grain size, impurities, snow depth, liquid water content, or density. Solar reflectance and microwave emissivity are both sensitive to various of these snowpack parameters. However, none of these parameters can be uniquely determined by satellite measurements at a single wavelength; a multichannel method is thus necessary if they are to be determined by remote sensing.

Determination of surface albedos and aerosol extinction characteristics from satellite imagery

Abstract: A procedure is developed for calculating atmospheric extinction characteristics (optical thickness, scattering height, single scattering albedo) and surface albedo from radiometric images made at satellite altitudes. The procedure - a fast, computerized method - is suitable for the high-volume processing of satellite imagery data and thus can be used to map temporal and spatial distributions of aerosol parameters. Based on an analytical approximate solution to the equation of radiative transfer in a plane parallel atmosphere, the procedure is primarily applicable to the 0.4-micron to 0.8-micron wavelength range for solar zenith angles from 10 deg to 60 deg, surface albedos between 0.03 and about 0.5, and atmospheric optical thicknesses from 0.2 to 2. With a step-like change in the surface albedo (for example, a seashore or river bank or other similar change of terrain), both surface reflectivity and atmospheric optical thickness can be derived from radiance measurements. In this case, the resultant optical thickness is not based on a known surface reflectivity and is in essence independent of the radiometer calibration.

ANALYTICAL CHARACTERIZATION OF SPECTRAL ACTINIC FLUX and SPECTRAL IRRADIANCE IN THE MIDDLE ULTRAVIOLET

Abstract: — We present an analytic characterization of upward and downward diffuse spectral irradiance for the wavelength range 280–380 nm, solar zenith angle range from 0 to 86, altitude range from 0 to 5 km and for non-zero surface albedo. This work is a modification and extension of the previous work of Green, Cross and Smith based upon the radiative transfer calculations of Braslau, Dave and Halpern. Guided by these irradiance systematics we develop an analytic characterization of diffuse spectral scalar irradiance or actinic flux also broken down into upward and downward components for the above wavelengths, solar zenith angles and altitudes, for non-zero surface albedo utilizing the actinic flux calculations of Peterson.

Infrared radiative transfer through a regular array of cuboidal clouds

Abstract: Infrared radiative transfer through a regular array of cuboidal clouds is studied and the interaction of the sides of the clouds with each other and the ground is considered. The theory is developed for black clouds and is extended to scattering clouds using a variable azimuth two-stream approximation. It is shown that geometrical considerations often dominate over the microphysical aspects of radiative transfer through the clouds. For example, the difference in simulated 10 micron brightness temperature between black isothermal cubic clouds and cubic clouds of optical depth 10, is less than 2 deg for zenith angles less than 50 deg for all cloud fractions when viewed parallel to the array. The results show that serious errors are made in flux and cooling rate computations if broken clouds are modeled as planiform. Radiances computed by the usual practice of area-weighting cloudy and clear sky radiances are in error by 2 to 8 K in brightness temperature for cubic clouds over a wide range of cloud fractions and zenith angles. It is also shown that the lapse rate does not markedly affect the exiting radiances for cuboidal clouds of unit aspect ratio and optical depth 10.

Soybean canopy reflectance as a function of view and illumination geometry

Abstract: The results of an experiment designed to characterize a soybean field by its reflectance at various view and illumination angles and by its physical and agronomic attributes are presented. Reflectances were calculated from measurements at four wavelength bands through eight view azimuth and seven view zenith directions for various solar zenith and azimuth angles during portions of three days. An ancillary data set consisting of the agronomic and physical characteristics of the soybean field is described. The results indicate that the distribution of reflectance from a soybean field is a function of the solar illumination and viewing geometry, wavelength and row direction, as well as the state of development of the canopy. Shadows between rows greatly affected the reflectance in the visible wavelength bands and to a lesser extent in the near infrared wavelengths. A model is proposed that describes the reflectance variation as a function of projected solar and projected viewing angles. The model appears to approximate the reflectance variations in the visible wavelength bands from a canopy with well defined row structure.

Vertical motions of the neutral thermosphere at midlatitude

Abstract: Determination of vertical motions in the upper thermosphere at midlatitudes from high time resolution Doppler shift measurements of the [OI]³P - ¹D transition emission at night show that there exist oscillations with ∼ 40 min periodicity and large magnitude (ca. 50 m/s). The behavior of these oscillations is ascribed to the passage of gravity waves through that region of the atmosphere. The results also show that individual (high time resolution) zenith measurements of the neutral thermosphere emissions can not be used as (zero) reference winds with any degree of certainty.

Measurements of atmospheric water vapor with microwave radiometry

Abstract: A dual-channel, ground-based microwave radiometer, working at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer, and radiosondes have been used for comparative measurements of the integrated amount of precipitable water vapor in the atmosphere over a period with zenith water vapor contents varying between 6 and 26 mm. The microwave radiometer was found to give comparable or better formal accuracy than the radiosondes, the absolute accuracy of which is believed to be about 1 mm. The rms difference of the integrated amount of water vapor in the zenith direction measured with the microwave radiometer and with radiosondes was 1.2 mm for all data, and 0.8 mm for a selected group of good weather data. These are lower formal errors than previously reported. It is shown that the simplified relation between the radiometer antenna termperatures and the integrated amount of water vapor in this case contributes with a formal error of about 0.3 mm. It is suggested that mean ground weather data can be used to adapt this relation to other sites and seasons.

Measurement of the fluctuations in the depth of maximum of showers produced by primary particles of energy greater than 1.5×1017 eV

Abstract: Measurements of the RMS fluctuation of the depth of shower maximum sigma (Xm) as a function of energy are reported from 1.5*1017 to 1019 eV. The results are based on a study of the risetimes of pulses recorded from four 34 m2 water-Cerenkov detectors in 2691 showers having zenith angle less than 40 degrees . The energy dependence of sigma (Xm) is determined and found to be in reasonable agreement with what is expected for a constant mass composition and an energy-dependent nucleon-air cross section.

Tropospheric wet path-delay measurements

Abstract: A dual-channel microwave radiometer measuring the sky brightness temperature at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer (IRSH) measuring the ratio of the radiation from the sun at the wavelengths 931 and 880 nm, and radiosondes have been used simultaneously to determine the excess path length due to water vapor (wet path delay) of radio waves propagating through the troposphere. By a least squares fit of the measured parameters from the microwave radiometer and the infrared spectral hygrometer, respectively, to the wet path delay calculated from the radiosonde profiles, the following root mean square (rms) differences of the wet path delay in the zenith direction were obtained: infrared spectral hygrometer-radiosondes, 1.1 cm; microwave radiometer-radiosondes, 0.7 cm; and 0.5 cm for a selected group of "good weather" data. The wet path delay was also calculated from surface meteorological measurements alone and the rms difference compared with corresponding radiosonde data was 2.0 cm in the zenith direction.

Estimation of diffuse irradiance on sloping, obstructed surfaces: An error analysis

Abstract: In several branches of climatology, and in related disciplines, estimates of shortwave and/or longwave diffuse irradiance are required. In this study, these irradiances are obtained by numerical integration of the fundamental radiance and source-receptor geometry equations for a variety of receptor slopes, receptor orientations, skyline patterns and sky radiance distributions. The angular increment used in the numerical integration over zenith and azimuth angle is varied to determine the optimal value in the trade-off between computing expense and accuracy. It is recommended that an angle ofπ/36 radians (5°) or smaller is required to evaluate the diffuse irradiance to 5 percent; for 10 percent error, the angle should not exceedπ/18 radians (10°).

Middle Ultraviolet Irradiance at the Ocean Surface: Measurements and Models

Abstract: Measurements of the downward spectral middle ultraviolet (MUV, 280–380 nm) irradiance E(O+,λθ) have been made just above the ocean surface in the central equatorial Pacific. These data were obtained for sun zenith angles in the range 12° to 70°. Analytic models have been fit to these data. The data and the models, their use and their limitations, will be summarized here.Most of this work including the data (Smith and Baker, 1980; hereafter referred to as SB) and the model(Baker et al. 1980; hereafter referred to as BSG) has been explored in detail in previously published works.

Observation of horizontal air showers with energies of 1 to 100 TeV

Abstract: An observation of horizontal air showers of sizes from 103 to 105 particles (1 to 100 TeV in energy) has been performed. For showers of sizes up to ∼2.104, the size spectrum and zenith angle distributions are observed to be consistent with expectation. Some deviation appears in both the size spectrum and angle distributions for showers above ∼2.104, which suggests that they may be due to a short-lived muon parent particle.

Influence of sky radiance distribution on the ratio technique for estimating bidirectional reflectance

Abstract: The technique of ratioing scene radiance to the radiance obtained from standard Lambertian reference panels in order to estimate bidirectional reflectance factors may depend on the angular distribution of the diffuse irradiance field as well as the direct solar irradiance. A simulation study was performed to estimate the magnitude of this effect for differing clear sky irradiance distributions for a variety of vegetated surfaces. For the seven surfaces and wavelengths analyzed, the error induced in the estimation of bidirectional reflectance factors using the standard ratio technique was less than 5 percent for zenith view and sun angles less than 55 degrees.

Computer simulations of atmospheric Čerenkov radiation in large cosmic-ray showers

Abstract: The computations of Protheroe and Turver (1) have been extended to include the Cerenkov light in showers incident at zenith angles up to 60°. We use results from a wide range of scaling-based models to demonstrate the existence of a unique model-independent relationship between a range of observable Cerenkov parameters and the depth of the electron cascade maximum.

The Zenith, the Mountain, the Center, and the Sea

Abstract: OST BASIC TO Chamula spatial organization is the belief that they M live at the center . . . 'the navel of the earth,' \" Gary Gossen explains in his description of modern Tzotzil Maya cosmology.' He goes on to describe their belief that the vertical axis of the universe is associated with the north-south axis on the surface of the earth: north is equivalent to up and the sky, and south is the equivalent of down and the underworld.' Independently, Gordon Brotherston and Dawn Ades came to the same conclusion about the cosmology of the 16th century Yucatecan Maya2 and I have recently postulated that this same belief can be demonstrated for the Classic period Maya as well.3 In this paper, I will suggest that this central cosmic axis governed the orientation and the iconographic program of pre-Classic Izapa, and that such a preoccupation with the zenith may have led to the creation of the 260-day ritual calendar at that site, as Malmstrom4 suggested in 1973. Izapa is in a region that was always noted for its rich volcanic soils and abundant agricultural production, especially of cacao; it is also located on a heavily traveled route that links nothern Mesoamerica with the south (FIGURE 1). There are more carved stelae at Izapa than at any other pre-Classic site (with the possible exception of Kaminaljuyu which displayed no formal or thematic unity in its sculpture) and it has more carved monuments than most Classic Maya sites. There is no question that Izapa was an important center and that its role was qualitatively different from that of most contemporary sites. Izapa is located on a latitude close to 15\" N, which crosses the Pacific Ocean to the west and includes the post-Classic site of Mixco Viejo and

Application of the Adjoint Method in Atmospheric Radiative Transfer Calculations

Abstract: The transfer of solar radiation through a standard mid-latitude summer atmosphere including different amounts of aerosols (from clear to hazy) has been computed. The discrete-ordinates (S/sub N/) method, which has been developed to a high degree of computational efficiency and accuracy primarily for nuclear radiation shielding applications, is employed in a forward as well as adjoint mode. In the adjoint mode the result of a transfer calculation is an importance function (adjoint intensity) which allows the calculation of transmitted fluxes, or other radiative responses, for any arbitrary source distribution. The theory of the adjoint method is outlined in detail and physical interpretations are developed for the adjoint intensity. If, for example, the downward directed solar flux at ground level, F/sub lambda/ (z = 0), is desired for N different solar zenith angles, a regular (forward) radiative transfer calculation must be repeated for each solar zenith angle. In contrast, only 1 adjoint transfer calculation gives F/sub lambda/ (z = 0) for all solar zenith angles in a hazy aerosol atmosphere, for 1 wavelength interval, in 2.3 seconds on a CDC-7600 computer. A total of 155 altitude zones were employed between 0 and 70 km, and the convergence criterion for the ratiomore » of fluxes from successive iterations was set at 2 x 10/sup -3/. Our results demonstrate not only the applicability of the highly efficient modern S/sub N/ codes, but indicate also conceptual and computational advantages when the adjoint formulation of the radiative transfer equation is used.« less

Energy radiation reflector

Abstract: The "ENERGY RADIATION REFLECTOR" is a reflective surface for the concentration of light and heat from the sun's radiation to a particular area. Its unique shape will accept incident rays from a large variety of solar zenith angles and reflect them into a focusing pattern of one long narrow straight line which falls mostly within the confines of the shape of the reflective surface and the encompassing enclosure into which it is mounted onto a heat and light receptor structure containing a circulating heat transfer medium, either liquid or gaseous. This reflective surface is three dimensional and bent in its long horizontal plane in accordance with the following mathematical equation, in which "x" is the vertical axis and "y" is the horizontal axis, then, expressed by the following 8TH degree polinomial, with minor deviations: y ? -(6.4550 x 10-13)x8 + (2.7799 x 10-10)x7 -(4.9542 x 10-8) x6 + (4.7343 x 10-6) x5 -(2.6403 x 10-4) x4 + (8.8896 x 10-3) x3 -(1.9194 x 10-1) x2 + (2.7232)x (2.1227 x 10-1).

Snow‐sky and water‐sky luminance at an arctic station

Abstract: Luminance measurements of water-sky and snow-sky were carried out at Ny-Alesund, Svalbard. The luminance of a well-developed water-sky is found to be about 40% of that of the adjacent snow-sky. Once the‘luminance level’of a comparatively pure snow-sky is reached, there is only a very slight further luminance increase towards zenith. This is in good agreement with FRITZ’theory as well as with the light conditions experienced during‘white-out’situations. By measuring the angular elevation of the border between water-sky and snow-sky and the height of the cloud base, good estimates are obtained of the distance to the corresponding border on the ground between snow surface and open water.