The Experts below are selected from a list of 252 Experts worldwide ranked by ideXlab platform
Roger Davies - One of the best experts on this subject based on the ideXlab platform.
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modeling Zenith Angle dependence of outgoing longwave radiation implication for flux measurements
Remote Sensing of Environment, 1997Co-Authors: J Otterman, Roger Davies, D Starr, Thomas W Brakke, H Jacobowitz, A Mehta, F Cheruy, C PrabhakaraAbstract:Abstract As shown in previous studies, outgoing-longwave radiation (OLR) can be in error by ∼7% when evaluated from directional measurements by applying time-averaged angular distribution models (ADMs) to account for the emission anisotropy. In order to develop an insight into the problem of assessing hemispheric emission from directional measurements, we formulate the directional (monochromatic) greenhouse factor g d specified as the ratio of a longwave radiance measured above the atmosphere to that emitted from the surface at the same Zenith Angle θ v . The explicit expressions for g d involve two atmospheric parameters, the optical thickness and the temperature-profile parameter. Our analysis indicates that under clear conditions a narrow-band radiance, if measured at θ v ≈57° in atmospheric windows (low values of optical thickness) and at θ v ≈47° in absorption bands, determines the hemispheric-average radiance to within about 1%. For broad spectral bands, whether under clear or cloudy (solid cover, or “randomly scattered” clouds) conditions, the same finding applies at θ v ≈50°. Thus, the, Zenith Angle of equivalence θ eq varies by about ±5° for different values of the optical thickness; it varies only slightly for different temperature profiles (even though different temperature profiles produce quite different patterns of radiance vs. θ v , that is, different ADMs apply). Measurements at or near θ eq therefore constitute direct assessment of OLR, without resorting to ADMs to adjust for the variations of emission with view Angle (anisotropy). The existing OLR data should be reexamined, accepting measurements only within the range 45–60% of view Zenith Angles.
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Effect of cloud inhomogeneities on the solar Zenith Angle dependence of nadir reflectance
Journal of Geophysical Research: Atmospheres, 1997Co-Authors: Norman G. Loeb, Tamás Várnai, Roger DaviesAbstract:A significant discrepancy has been noted between satellite measurements of shortwave reflectance at nadir and the results of plane-parallel model calculations: For moderate to large solar Zenith Angles, observed nadir reflectances increase with solar Zenith Angle, whereas plane-parallel values decrease. Consequently, cloud optical depths retrieved using one-dimensional (1-D) theory have a bias which increases systematically with solar Zenith Angle. Using Monte Carlo model simulations of photon transport through stochastic, isotropic, scale-invariant cloud fields with variable cloud top heights and volume extinction coefficients, we show that nadir reflectances from three-dimensional cloud fields increase with solar Zenith Angle, consistent with the observations. The difference from the 1-D case is shown to be explainable by cloudside illumination as well as by the presence of structured (i.e., non-flat) cloud tops. Cloud sides enhance the amount of incident solar radiation intercepted by cloud, allowing more radiation to be scattered upward in the nadir direction. Structured cloud tops change the slope of illuminated cloud top surfaces, such that nadir reflectance at low solar elevations increases with the slope of the illuminated surface. For simple cloud geometries the two effects make equivalent contributions to the increase in nadir reflectance with solar Zenith Angle. While this increase is most pronounced for vertically extensive broken cloud fields, it also affects reflectances from overcast cloud fields with inhomogeneous (bumpy) cloud tops. Thus the observed solar Zenith Angle bias in cloud optical depth for the general cloud scene likely also occurs for extensive overcast cloud fields. Internal inhomogeneities due to small-scale liquid water content variations within clouds are shown to cause no changes at low Sun and only slight decreases in nadir reflectance for high solar elevations.
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Observational evidence of plane parallel model biases: Apparent dependence of cloud optical depth on solar Zenith Angle
Journal of Geophysical Research, 1996Co-Authors: Norman G. Loeb, Roger DaviesAbstract:This study directly compares plane parallel model calculations with 1 year of Earth Radiation Budget Satellite shortwave observations at nadir over ocean between 30°S and 30°N. When plane parallel model calculations are matched to the observations on a pixel-by-pixel basis by adjusting cloud fraction and cloud optical depth, the resulting frequency distributions of cloud optical depth show a systematic shift towards larger values with increasing solar Zenith Angle, regardless of the assumptions made in the calculations. This dependence is weak for thin clouds but gets progressively stronger as the clouds become thicker. For the thinnest 50% of the clouds (optical depths ≲6), it occurs only at oblique solar Zenith Angles, whereas it is observed at all solar Zenith Angles for the thickest 10% of clouds (optical depths ≳12). On average, the increase is extremely large for solar Zenith Angles ≳63°. Such behavior is unrealistic since average cloud optical depths from such an extensive data set should be almost independent of solar Zenith Angle. The cause is traced to a fundamental flaw in plane parallel theory applied to real clouds: the solar Zenith Angle dependence of model reflectance is opposite to that of the observations. The one-dimensional nadir reflectance remains within 10% of the observed reflectance for solar Zenith Angles ≲53° when applied to a general ensemble of real clouds, and for solar Zenith Angles ≲63° when applied to the thinnest 50% of such clouds. Uncertainties are found to increase rapidly as the Sun becomes more oblique, easily reaching 30% at the lowest solar elevations. Based on results from theoretical studies, it is concluded that three-dimensional cloud structures not accounted for by plane parallel theory have a statistically important effect on the radiation field. As a minimum requirement, application of one-dimensional theory to the remote sensing of cloud optical thickness from measurements of nadir reflectance should therefore be restricted to thin clouds and small solar Zenith Angles.
Alexei Yu. Smirnov - One of the best experts on this subject based on the ideXlab platform.
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Zenith Angle distributions at Super-Kamiokande and SNO and the solution of the solar neutrino problem
Physical Review D, 2001Co-Authors: Ma. Concepción González García, Carlos Peña Garay, Alexei Yu. SmirnovAbstract:We have performed a detailed study of the Zenith Angle dependence of the regeneration factor and distributions of events at SNO and SK for different solutions of the solar neutrino problem. In particular, we discuss the oscillatory behavior and the synchronization effect in the distribution for the LMA solution, the parametric peak for the LOW solution, etc. A physical interpretation of the effects is given. We suggest a new binning of events which emphasizes the distinctive features of the Zenith Angle distributions for the different solutions. We also find the correlations between the integrated day-night asymmetry and the rates of events in different Zenith Angle bins. The study of these correlations strengthens the identification power of the analysis.
Yushu Zhou - One of the best experts on this subject based on the ideXlab platform.
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Effects of diurnal variation of solar Zenith Angle on a tropical coupling system: A two‐dimensional coupled ocean‐cloud resolving atmosphere modeling study
Geophysical Research Letters, 2008Co-Authors: Shouting Gao, Yushu ZhouAbstract:[1] The effects of diurnal variation of solar Zenith Angle on tropical atmospheric and oceanic variability are investigated with a two-dimensional coupled ocean-cloud resolving atmosphere model. The experiment with a time-invariant solar Zenith Angle is compared to the control experiment with a diurnally-varied solar Zenith Angle. In both experiments, the model, with imposed large-scale vertical velocity and zonal wind derived from Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE), is integrated over a 30-day period. The control simulation shows a good agreement with the observations in terms of atmospheric temperature, specific humidity, and mixed-layer temperature and salinity. The experiment with the time-invariant solar Zenith Angle produces a colder and drier atmosphere and a colder and saltier ocean mixed layer than the experiment with the diurnally-varied solar Zenith Angle does. The atmospheric temperature and precipitable water budgets and oceanic mixed-layer temperature and salinity budgets are analyzed. Compared to the experiment with the diurnally-varied solar Zenith Angle, the experiment with the time-invariant solar Zenith Angle has smaller solar heating, consumes more atmospheric water vapor through more condensation, and generates smaller thermal forcing through deeper mixed layer and more saline entrainment. The results indicate the importance of the inclusion of diurnal variation of solar Zenith Angle in coupled model simulations to avoid atmospheric and oceanic cooling biases and atmospheric drying bias.
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effects of diurnal variation of solar Zenith Angle on a tropical coupling system a two dimensional coupled ocean cloud resolving atmosphere modeling study
Geophysical Research Letters, 2008Co-Authors: Shouting Gao, Yushu ZhouAbstract:[1] The effects of diurnal variation of solar Zenith Angle on tropical atmospheric and oceanic variability are investigated with a two-dimensional coupled ocean-cloud resolving atmosphere model. The experiment with a time-invariant solar Zenith Angle is compared to the control experiment with a diurnally-varied solar Zenith Angle. In both experiments, the model, with imposed large-scale vertical velocity and zonal wind derived from Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE), is integrated over a 30-day period. The control simulation shows a good agreement with the observations in terms of atmospheric temperature, specific humidity, and mixed-layer temperature and salinity. The experiment with the time-invariant solar Zenith Angle produces a colder and drier atmosphere and a colder and saltier ocean mixed layer than the experiment with the diurnally-varied solar Zenith Angle does. The atmospheric temperature and precipitable water budgets and oceanic mixed-layer temperature and salinity budgets are analyzed. Compared to the experiment with the diurnally-varied solar Zenith Angle, the experiment with the time-invariant solar Zenith Angle has smaller solar heating, consumes more atmospheric water vapor through more condensation, and generates smaller thermal forcing through deeper mixed layer and more saline entrainment. The results indicate the importance of the inclusion of diurnal variation of solar Zenith Angle in coupled model simulations to avoid atmospheric and oceanic cooling biases and atmospheric drying bias.
Maurizio Lusignoli - One of the best experts on this subject based on the ideXlab platform.
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Energy and Zenith Angle distribution of upward going muons and neutrino oscillations
Physical Review D, 1998Co-Authors: Paolo Lipari, Maurizio LusignoliAbstract:The energy and Zenith Angle distribution of neutrino induced, upward going muons can give direct information on the presence of $\nu$--oscillations in precisely the range of parameters suggested as a solution of the atmospheric neutrino problem. We discuss here the uncertainties in the theoretical prediction. The shape of the Zenith Angle distribution of the muon flux is quite insensitive to modifications of the theoretical input and is a good probe for the existence of neutrino oscillations. We conclude that the existing data sample on $\nu$--induced muons has the statistical power to confirm or refute the $\nu$--oscillation solution of the atmospheric neutrino problem.
Zhuo Wang - One of the best experts on this subject based on the ideXlab platform.
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dependence of land surface albedo on solar Zenith Angle observations and model parameterization
Journal of Applied Meteorology and Climatology, 2008Co-Authors: Fanglin Yang, Zhuo Wang, Xubin Zeng, Kenneth E Mitchell, Yu Tai Hou, Yongjiu Dai, Xinzhong LiangAbstract:Abstract This study examines the dependence of surface albedo on solar Zenith Angle (SZA) over snow-free land surfaces using the intensive observations of surface shortwave fluxes made by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program and the National Oceanic and Atmospheric Administration Surface Radiation Budget Network (SURFRAD) in 1997–2005. Results are used to evaluate the National Centers for Environmental Prediction (NCEP) Global Forecast Systems (GFS) parameterization and several new parameterizations derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) products. The influence of clouds on surface albedo and the albedo difference between morning and afternoon observations are also investigated. A new approach is taken to partition the observed upward flux so that the direct-beam and diffuse albedos can be separately computed. The study focused first on the ARM Southern Great Plains Central Facility site. It is found that the diffuse albedo prescribe...
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the solar Zenith Angle dependence of desert albedo
Geophysical Research Letters, 2005Co-Authors: Zhuo Wang, Michael Barlage, Xubin Zeng, Robert E Dickinson, Crystal B SchaafAbstract:[1] Most land models assume that the bare soil albedo is a function of soil color and moisture but independent of solar Zenith Angle (SZA). However, analyses of the Moderate Resolution Imaging Spectroradiometer (MODIS) Bidirectional Reflectance Distribution Function (BRDF) and albedo data over thirty desert locations indicate that bare soil albedo does vary with SZA. This is further confirmed using the in situ data. In particular, bare soil albedo normalized by its value at 60° SZA can be adequately represented by a one-parameter formulation (1 + C)/(1 + 2C * cos(SZA)) or a two-parameter formulation (1 + B1 * f1(SZA) + B2 * f2(SZA)). Using the MODIS and in situ data, the empirical parameters C, B1, and B2 are taken as 0.15, 0.346 and 0.063. The SZA dependence of soil albedo is also found to significantly affect the modeling of land surface energy balance over a desert site.
