Comment on “radiative properties of snow for clear sky solar radiation”

Choudhury's (1981) addition of a special “surface reflection” term to a theoretical model for snow albedo is both unnecessary and incorrectly formulated.     

Twilight and daytime colors of the clear sky

Digital image analysis of the cloudless sky's daytime and twilight chromaticities challenges some existing ideas about sky colors. First, although the observed colors of the clear daytime sky do lie near the blackbody locus, their meridional chromaticity curves may resemble it very little. Second, analyses of twilight colors show that their meridional chromaticity curves vary greatly, with some surprising consequences for their calorimetric gamuts.     

Color and luminance asymmetries in the clear sky

A long-standing assumption about the clear sky is that its colors and luminances are distributed symmetrically about the principal plane. As useful as this approximation is, our digital-image analyses show that clear-sky color and luminance routinely depart perceptibly from exact symmetry. These analyses reconfirm our earlier measurements with narrow field-of-view spectroradiometers [J. Opt. Soc. Am. A 18, 1325 (2001)], and they do so with much higher temporal and angular resolution across the entire sky dome.     

Visible and near-ultraviolet absorption spectrum of ice from transmission of solar radiation into snow

Snow is a scattering-dominated medium whose scattering is independent of wavelength at 350-600 nm. The attenuation of solar radiation in snow can be used to infer the spectral absorption coefficient of pure ice, by reference to a known value at 600 nm. The method is applied to clean Antarctic snow; the absorption minimum is at 390 nm, and the inferred absorption coefficient is lower than even the lowest values of the Antarctic Muon and Neutrino Detector Array (AMANDA) experiment on glacier ice: The absorption length is at least 700 m, by comparison with 240 m for AMANDA and 10 m from laboratory attenuation measurements.     

Modeling of the influence of snow reflectance on ultraviolet irradiance for cloudless sky

The amplification of UV irradiance at the Earth's surface that is due to successive reflections between the snow-covered ground and the scattering atmosphere is analyzed by a method based on decoupling the atmosphere and the surface functions. For a uniform Lambertian surface the amplification factor for the global irradiance depends only on the product of the surface reflectance and the atmospheric backscatter. It varies with wavelength, reaching a maximum near 320 nm; this maximum is close to 50% for clean snow. In UV-B the amplification depends strongly on tropospheric ozone. For non-Lambertian, nonuniform surfaces it is possible, by the same method, to define effective or average reflectances.     

Charge-coupled device spectrograph for direct solar irradiance and sky radiance measurements

The characterization of a charged-coupled device (CCD) spectrograph developed at the Laboratory of Atmospheric Physics, Thessaloniki is presented. The absolute sensitivity of the instrument for direct irradiance and sky radiance measurements was determined, respectively, with an uncertainty of 4.4% and 6.6% in the UV-B, and 3% and 6% in the UV-A, visible and near-infrared (NIR) wavelength ranges. The overall uncertainty associated with the direct irradiance and the sky radiance measurements is, respectively, of the order of 5% and 7% in the UV-B, increasing to 10% for low signals [e.g., at solar zenith angles (SZAs) larger than 70 degrees ], and 4% and 6% in the UV-A, visible, and NIR. Direct solar spectral irradiance measurements from an independently calibrated spectroradiometer (Bentham DTM 300) were compared with the corresponding CCD measurements. Their agreement in the wavelength range of 310-500nm is within 0.5% +/- 1.1% (for SZA between 20 degrees and 70 degrees ). Aerosol optical depth (AOD) derived by the two instruments using direct Sun spectra and by a collocated Cimel sunphotometer [Aerosol Robotic network (AERONET)] agree to within 0.02 +/- 0.02 in the range of 315-870 nm. Significant correlation coefficients with a maximum of 0.99 in the range of 340-360 nm and a minimum of 0.90 at 870 nm were found between synchronous AOD measurements with the Bentham and the Cimel instruments.     

The solar reflectance of a snow field

The radiative transfer equation has been solved using a modified Schuster-Schwartzschild approximation to obtain an expression for the solar reflectance of a snow field. The parameters in the reflectance formula are the single scattering albedo and the fraction of energy scattered in the backward direction. The single scattering albedo is calculated from the crystal size using a geometrical optics formula and the fraction of energy scattered in the backward direction is calculated from the Mie scattering theory. Numerical results for reflectance are obtained for visible and near infrared radiation for different snow conditions. Good agreement has been found with the whole spectral range. The calculation also shows the observed effect of aging on the snow reflectance.     

Comparison between measurements and modeling of UV-B irradiance for clear sky: a case study

We compare the TJV-B global and direct irradiances computed with a radiative transfer model (discrete ordinate method) and measured during a European intercomparison campaign in Greece in July 1991, with clear sky. The agreement between the model and the measurements is within 6%. The sensitivity of the model to the accuracy of the input parameters as well as the potential of modeling for instrument calibration is discussed.     

Radiative properties of dense nanofluids

The radiative properties of dense nanofluids are investigated. For nanofluids, scattering and absorbing of electromagnetic waves by nanoparticles, as well as light absorption by the matrix/fluid in which the nanoparticles are suspended, should be considered. We compare five models for predicting apparent radiative properties of nanoparticulate media and evaluate their applicability. Using spectral absorption and scattering coefficients predicted by different models, we compute the apparent transmittance of a nanofluid layer, including multiple reflecting interfaces bounding the layer, and compare the model predictions with experimental results from the literature. Finally, we propose a new method to calculate the spectral radiative properties of dense nanofluids that shows quantitatively good agreement with the experimental results.     

太阳辐照对芳纶纤维及其复合材料性能的影响

选择K49和F-Ⅲ两种芳纶纤维及其5224A环氧树脂基复合材料为研究对象,以单丝拉伸强度、FTIR、DSC、SEM、复合材料力学性能和介电性能作为表征手段,研究太阳辐照对芳纶纤维的结构性能以及芳纶/环氧复合材料的结构透波性能的影响作用。结果表明:K49在辐照过程中表现出显著的自我屏蔽效应,400h的强度保持率为82%,F-Ⅲ未观察到明显的自我屏蔽效应,强度保持率为50%。辐照前后的FTIR,XPS,DSC,SEM结果证明纤维表层出现了降解,氢键破坏与结晶度下降是引起纤维性能下降的主要原因。太阳辐照前后复合材料的性能测试结果证明辐照对材料的性能无明显影响,辐照后复合材料的弯曲、层间剪切和介电性能基本无变化,拉伸强度略有升高,压缩强度轻微下降,K49/5224A的压缩强度下降了11.8%,F-Ⅲ/5224A的压缩强度下降了6.6%,这主要是由于太阳辐照老化仅引起复合材料表面树脂基体轻微降解所造成的。     

Albedo and flux extinction coefficients of impure snow for diffuse short-wave radiation

Impurities enter a snowpack as a result of fallout or scavenging by falling snow crystals. Albedos and flux extinction coefficients of soot-contaminated snowcovers are studied using a two-stream approximation of the radiative transfer equation. The effect of soot is calculated by two methods: independent scattering by ice grains and impurities, and the average refractive index for ice grains. Both methods predict a qualitatively similar effect of soot; the albedo is decreased and the extinction coefficient is increased relative to that for pure snow in the visible region, while the infrared properties are largely unaffected. Quantitatively, however, the effect of soot is more pronounced in the average refractive index method. We find that soot contamination provides qualitative explanation for several snow observations.     

Double-cavity radiometer for high-flux density solar radiation measurements

A radiometric method has been developed, suitable for both total power and flux density profile measurement of concentrated solar radiation. The high-flux density radiation is collected by a first optical cavity, integrated, and driven to a second optical cavity, where, attenuated, it is measured by a conventional radiometer operating under a stationary irradiation regime. The attenuation factor is regulated by properly selecting the aperture areas in the two cavities. The radiometer has been calibrated by a pulsed solar simulator at concentration levels of hundreds of suns. An optical model and a ray-tracing study have also been developed and validated, by which the potentialities of the radiometer have been largely explored.     

Characterisation of SiC photo-detectors for solar UV radiation monitoring

Silicon carbide has a potential for solar UV radiation monitoring: extremely resistant to UV radiation damage, nearly-blind to visible and infrared radiation and less sensitive to temperature variations than standard radiometric systems. A radiometer composed by three SiC photodiodes has been designed, manufactured and tested under solar radiation. Two photodiodes are equipped with filters in the UVB (280-315 nm) and UVA (315-400 nm) ranges while a third is filtered to match the erythemal action spectrum. UVA, UVB components of the solar radiation as well as UV index (UVI) at the earth's surface have been determined in two site positions in Tuscany, Italy. Data as a function of day-light allowed us to evaluate total optical thickness for UVA and UVB: τ_UVA=0.46 and τ_UVB=1.8. UVI values measured during the year well compares with computed ones used for weather forecast procedures.     

Review Mechanical properties of ice and snow

The mechanical properties of ice and snow are reviewed. The tensile strength of ice varies from 0.7–3.1 MPa and the compressive strength varies from 5–25 MPa over the temperature range –10°C to –20°C. The ice compressive strength increases with decreasing temperature and increasing strain rate, but ice tensile strength is relatively insensitive to these variables. The tensile strength of ice decreases with increasing ice grain size. The strength of ice decreases with increasing volume, and the estimated Weibull modulus is 5. The fracture toughness of ice is in the range of 50–150 kPa m^1/2 and the fracture-initiating flaw size is similar to the grain size. Ice-soil composite mixtures are both stronger and tougher than ice alone. Snow is a open cellular form of ice. Both the strength and fracture toughness of snow are substantially lower than those of ice. Fracture-initiating flaw sizes in snow appear to correlate to the snow cell size.     

Radiative properties of a gas — Sulfer dioxide

An algorithm is initial data are presented for calculating thermal radiation in six bands and the complete spectrum of a gas.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 3, pp. 387–391, September, 1989.     

Security of solar radiation management geoengineering

Solar Radiation Management (SRM) geoengineering is a proposed response to anthropogenic global warming (AGW) (National Academy of Sciences, 2015). There may be profound – even violent – disagreement on preferred temperature. SRM disruption risks dangerous temperature rise (termination shock). Concentrating on aircraft-delivered Stratospheric Aerosol Injection (SAI), we appraise threats to SRM and defense methodologies. Civil protest and minor cyberattacks are almost inevitable but are manageable (unless state-sponsored). Overt military attacks are more disruptive, but unlikely – although superpowers’ symbolic overt attacks may deter SRM. Unattributable attacks are likely, and mandate use of widely-available weapons. Risks from unsophisticated weapons are therefore higher. An extended supply chain is more vulnerable than a secure airbase – necessitating supply-chain hardening. Recommendations to improve SRM resilience include heterogeneous operations from diverse, secure, well-stocked bases (possibly ocean islands or aircraft carriers); and avoidance of single-point-of-failure risks (e.g. balloons). A distributed, civilian-operated system offers an alternative strategy. A multilateral, consensual SRM approach reduces likely attack triggers.     

Identification of thermoresistive solar radiation sensors

An identification procedure to estimate the parameters of a thermoresistive solar radiation sensor is presented. The proposed technique employs only electrical excitation for the sensor. The estimation algorithm is recursive and is applied to the sensor model derived from the thermodynamic equilibrium differential equations. The simulation and the experimental results demonstrate the validity of the proposed approach     

A note on the solar elevation dependence of clear-sky snow albedo

Recent attempts to match the short-wave albedo of snow for clear skies using approximate spectral solar fluxes and solutions of the radiative transfer equation for snow were unsuccessful until a separate surface reflection term was introduced. A separate consideration of specular reflection from surface snow grains has been objected to as being ad hoc. Results based on a new parametrization of short-wave radiation are discussed. Compared with previous radiation models, the new model gives higher diffuse insolation and predicts higher albedos. The difference between observed and predicted albedos is substantially reduced without invoking surface reflection.