Retrieval of the ultraviolet effective snow albedo during 1998 winter campaign in the French Alps

A measurement campaign was carried out in February 1998 at Brian?on Station, French Alps (44.9 degrees N, 6.65 degrees E, 1,310 m above sea level) in order to determine the UV effective snow albedo that was retrieved for both erythemal and UV-A irradiances from measurements and modeling enhancement factors. The results are presented for 15 cloudless days with very variable snow cover and a small snowfall in the middle of the campaign. Erythemal irradiance enhancement due to the surface albedo was found to decrease from approximately +15% to +5% with a jump to +22% after the snowfall, whereas UV-A irradiance enhancement decreased from 7% to 5% and increased to 15% after the snowfall. Thesevalues fit to effective surface albedos of 0.4, 0.1, and 0.5 for erythemal, and to effective albedos of 0.25, 0.1, and 0.4 for UV-A irradiances, respectively. An unexpected difference between the effective albedos retrieved in the two wavelength regions can be explained by the difference of the environment contribution.     

Spectral albedos of an alpine snowpack

Spectral albedos (α_λ) from 380 to 2500 nm are reported for a snowpack in the Cascade Mountains of Washington. Data were obtained from just after an 0.4 m snowfall on 13 March 1980 until the pack had metamorphosed to melting coarse grains about 1 mm in diameter mixed with dust. Measurements were made under cloudy conditions to obtain a diffuse incident radiation field. Structural parameters of the snow were measured concurrently for all cases, and on three occasions, estimates of absorbing impurity content were obtained. The dependence of the spectral albedos of the snowpack on grain size and impurity content is illustrated. Comparison of wavelength-integrated albedos (α_obs) obtained using Kipp and Zonen radiometers with corresponding albedos derived from α_λ data show good agreement, and suggests a correlation between α_obs and the amount of incident radiation transmitted by the cloud layer. Comparison with theoretical models confirms that impurities in the snow depress α_λ at visible wavelengths but have little effect beyond 900 nm in the infrared; however, quantitative agreement with theory is uncertain at present.     

Spectral albedos of mid-latitude snowpacks

Spectral albedos of impure-nonhomogeneous snowpacks, typical for mid-latitudes, at wavelengths from 400 to 2200 nm are modeled through a numerical solution of the radiative transfer equation in the two-stream approximation. Discrete depth-dependent values of density, grain size and impurity concentration are used to characterize the snowpacks. The model is for diffuse incident radiation, and the numerical method is based on doubling and invariant imbedding. The effect of soot impurities on snowpack albedos is illustrated: when a snowpack is semi-infinite (thickness of ten centimeters or more), soot reduces the albedos at visible wavelengths; however, for smaller snowpack thicknesses soot may increase the albedos at visible wavelengths. By adjusting soot content and snow grain size, good quantitative agreement with some observations at the Cascade Mountains (Washington) and at Point Barrow (Alaska) are obtained; however, the model grain sizes are found to be fifty to four hundred percent larger than the measured values. For satellite snowcover observations, a model for effective albedo of partially snowcovered areas is developed and compared with some NOAA-2 observations of the southeastern United States.     

Monte Carlo simulations of spectral albedo for artificial snowpacks composed of spherical and nonspherical particles

The optical properties of snowpacks composed of spherical and nonspherical particles artificially prepared in a cold laboratory are investigated by measuring spectral albedos. The measured spectral albedo in the spectral region lambda=0.35-2.5 microm is compared with the theoretically calculated albedo, for which a Monte Carlo radiative transfer model is employed for multiple scattering combined with the Mie theory and the ray-tracing technique for single scattering by snow particles. Since the spherical particles are a little aggregate, the effects of a cluster of the spheres on snow albedo are examined using a generalized multiparticle Mie-solution model [Appl. Opt. 34, 4573 (1995); J. Quant. Spectrosc. Radiat. Transf. 79-80, 1121 (2003)]. The snow albedo of a cluster of the spheres can be represented with that of the singe sphere slightly larger than its component of the cluster in case of small grains. The observed albedos for the spherical snow particles agree with the theoretically calculated ones for the snow grain size measured in the snow pit work. The snow albedos for the nonspherical particles, which were dendrites, are influenced by the branch width and the branch length, based on a comparison of the theoretically calculated albedo by using circular cylindrical snow particles and the observed albedo. The snow albedo in the near-infrared region depends on the branch width only when the branch length is sufficiently greater than the branch width. The comparison between the spherical and nonspherical snow particles indicates that the spectral albedo of the nonspherical particles can be represented by using an equal volume-area ratio sphere.     

Determining the sign of a polar surface of lithium niobate crystal by UV reflectance spectroscopy

We propose to reveal the +Z and–Z surfaces of a polar cut lithium niobate crystal by measuring its UV reflection spectrum. By the example of a congruent lithium niobate, it is shown that the intensities of light reflection from polar crystal surfaces of different signs in the region of 190—260 nm differ by up to several percent. The depth of short-wave radiation penetration into surface layers of the crystal in the spectral range of intrinsic absorption is estimated. It is shown that the proposed method can be used for determining the surface signs of polar crystal layers with thicknesses from several dozen to several hundred microns.     

Radiative properties of snow for clear sky solar radiation

Spectral and integrated radiative properties (reflection, transmission, and the rate of heating) of finegrained wind-packed snow typical of subpolar regions are studied through a model taking into account surface reflection and volumetric multiple scattering. The surface reflection is modeled by a bidirectional reflectance distribution function applicable to powdered dielectric material. For the volumetric multiple scattering, the radiative transfer equation designed for strongly asymmetric scattering is solved. All multiple scattering parameters (single scattering albedo, various moments of the scattering phase function, and optical depth) are related to measurable physical characteristics (density, grain size, and the absorption coefficient of pure ice).Parameterized atmospheric spectral transmission coefficients for scattering and absorption by aerosols and gases are used to obtain the direct and diffuse components of solar flux, incident on the snow-cover. Calculated values of spectral and integrated visible and near infrared reflection and flux attenuation coefficients of snow are compared with observations. The rate of radiative heating at different depths within the snowcover is calculated from the net flux divergence. It is shown that the conventional method of calculating this rate using measured bulk extinction coefficients grossly underestimates the amount of heating within the top few millimeters. This study provides a better overall understanding of the radiative properties of snow under clear sky conditions in terms of the physical characteristics of the snowcover.     

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.     

Empirical relationship between dust content and Arctic snow albedo

To provide empirical equations that quantify the effects of dust upon snow albedo and the radiation balance in a High Arctic environment, research was carried out at two sites, one within and the other outside the settlement of Resolute. At the settlement, the amount of dust on the snow was several orders of magnitude larger than outside Resolute. The main sources were the unpaved runway and roads. The dust reduced snow surface albedo, resulting in considerable net radiation input to the snowpack. Empirical relationships can be easily obtained to predict the effects of dust on snow albedo and hence the net radiation. It is suggested that during the snowmelt period, radiation data from the government station may not be representative because it is affected by the dusty conditions of the town.     

一种新型红外多谱段高温计的研究

基于辐射学原理的辐射高温计是一种非常实用的非接触式表面温度测量仪器。但是,在许多的实际情况下,辐射测温的精度受到一定的影响,这主要是由于被测试件表面发射率的不解定性、较强的环境辐射等。本在研究了复杂环境中的辐射状况的基础上,提出了一种新的测温模型,并在此基础上建立了多谱段红外辐射高温计系统。实验证明,在环境辐射较强时,这种新型高温计系统达到了令人满意的结果。     

AC-Mode Short-Wavelength IR Radiation Thermometers for Measurement of Ambient Temperatures

Recent improvements in the fabrication of short-wave infrared (SW-IR) quantum detectors have opened a new era in radiation thermometry. Ambient and higher temperatures can be measured with low uncertainties using thermoelectrically (TE) cooled extended-InGaAs (E-IGA) and short-wave photovoltaic-HgCdTe (SW-MCT) detectors. Since these detectors have low cut-off wavelengths (2.5 μm and 2.8 μm, respectively), they do not respond past cut-off and are less sensitive to the background infrared radiation, resulting in orders of magnitude lower background noise than traditional broad-band infrared detectors such as cryogenically cooled quantum detectors or thermal detectors. At the same time, the cut-off is far enough in the infrared to obtain a large enough signal from the source of interest. Because of the low detector cut-off wavelength, traditional glass-based optics can be used in the radiation thermometers. A chopper-produced alternating-current (AC) signal was used to measure low temperatures by separating the AC signal from the background-radiation-produced direct-current (DC) signal and its fluctuations. Design considerations and characteristics of a newly developed SW-IR radiation thermometer are discussed. A noise-equivalent temperature difference (NETD) of < 3mK for a 50°C blackbody was measured. At the human body temperature of 36°C, the obtained NETD of ~10mK indicates that these detectors can be used in non-contact temperature measurements to replace thermopile- or pyroelectric-based radiation thermometers.     

Upward-looking ground-penetrating radar for monitoring snowpack stratigraphy

Operational remote monitoring of snowpack stratigraphy, melt water intrusions and their evolution with time for forecasting snowpack stability is not possible to date. Determination of the spatial variability of snowpack conditions on various scales requires a number of point measurements with various methods. These methods are either destructive or do not provide information about the internal structure of the snowpack. The application of a remotely controlled non-destructive sensor system would help to gain a higher spatio-temporal resolution about information of the snowpack. In this study we present results from upward-looking ground-penetrating radar (GPR) surveys from horizontal caves dug in the front wall of snow pits at the bottom of the snowpack. GPR data are compared with vertical profiles of snow hardness and density, obtained in the snow pit. Data were acquired in different areas with varying snow conditions with various GPR impulse systems, frequencies and polarizations. Radar experiments with high frequencies (> 1 GHz) detect internal layers in the snowpack in dry snow, but fail to provide clear reflections at the upper snow-air transition because of attenuation. In wet snow, the radar signals < 1 GHz are capable to penetrate a meter-thick snowpack and detect the snow surface, although the signal is strongly attenuated. Analysis of reflection phases and magnitudes allows interpretation of their physical origin in terms of changes in electrical permittivity. Varying antenna polarization causes a strongly different signal response, likely caused by the snow-pit wall present in our set-up. Forward calculation of density-based reflection coefficients between neighboring layers of varying hardness yields ambiguous results in terms of correspondence with observed radar reflections apart except for interferences of neighboring reflections. Moreover, we identify several pitfalls for future applications. The system set-up used here represents a basis for further developments towards a system, which is capable of improving information on the spatial and temporal snowpack characteristics.     

Modified two-flux approximation for identification of radiative properties of absorbing and scattering media from directional-hemispherical measurements

A modified two-flux approximation is suggested for calculating the hemispherical transmittance and reflectance of a refracting, absorbing, and scattering medium in the case of collimated irradiation of the sample along the normal to the interface. The Fresnel reflection is taken into account in this approach. It is shown that the new approximation is rather accurate for the model transport scattering function. For an arbitrary scattering medium, the error of the modified two-flux approximation is estimated by comparison with the exact numerical calculations for the Henyey-Greenstein scattering function in a wide range of albedos and optical thicknesses. Possible applications of the derived analytical solution to identification problems are discussed.     

Light absorption from particulate impurities in snow and ice determined by spectrophotometric analysis of filters

Light absorption by particulate impurities in snow and ice can affect the surface albedo and is important for the climate. The absorption properties of these particles can be determined by collecting and melting snow samples and extracting the particulate material by filtration of the meltwater. This paper describes the optical design and testing of a new instrument to measure the absorption spectrum from 400 to 750 nm wavelength of the particles collected on filters using an "integrating-sandwich" configuration. The measured absorption is shown to be unaffected by scattering of light from the deposited particulates. A set of calibration standards is used to derive an upper limit for the concentration of black carbon (BC) in the snow. The wavelength dependence of the absorption spectra from 450 to 600 nm is used to calculate an absorption ?ngstrom exponent for the aerosol. This exponent is used to estimate the actual BC concentration in the snow samples as well as the relative contributions of BC and non-BC constituents to the absorption of solar radiation integrated over the wavelength band 300 to 750 nm.     

Effect of topography on average surface albedo in the ultraviolet wavelength range

The reflectivity of the 22 km x 24 km region surrounding Sonnblick Observatory near Salzburg, Austria (3104-m altitude, 47.05 degrees N, 12.95 degrees E), was calculated with a three-dimensional albedo model. The average albedo of the region was calculated at 305 and 380 nm for different solar zenith angles, ground reflectances, and solar azimuth angles. To determine geometrical effects, we first carried out the simulations without taking account of the effects of the atmosphere. The ratio to the reflectivity of a corresponding flat surface area (area with the same ground characteristics) was always less than 1 and showed a decrease with increasing solar zenith angle and with diminishing ground reflectance. Even when the ground reflectance was 100%, the average albedo was less than 1. The effect of the atmosphere was then taken into consideration in these calculations and was found to diminish the reflected components. This diminishing effect was compensated for, however, by the scattered irradiance. Finally, simulations of real conditions (nonhomogeneous ground reflectivities) were performed for different snow lines in the Sonnblick region. The average albedos obtained when all the surroundings were covered with snow were 0.32-0.63 with low solar zenith angles and 0.38-0.77 with a 40 degrees solar zenith angle.     

Modeling the spatial distribution of surface hoar in complex topography

Buried surface hoar is a well-known weak snowpack layer, often associated with snow avalanches. Knowledge about the spatial distribution of surface hoar is therefore of great importance for avalanche forecasting. We investigate if spatial variations of surface hoar in mountainous terrain can be modeled based on terrain characteristics. Using a detailed radiation balance model, distributed radiation over an ensemble of 1800 simulated topographies, covering a wide range of terrain characteristics, was computed. Light winds and increased relative humidity were assumed to be favorable for surface hoar formation. To describe surface hoar formation, we derived a sky view factor threshold associated with the minimum snow surface cooling necessary for surface hoar formation based on laboratory measurements. To describe surface hoar destruction, as a first approach, we assumed that surface hoar only survives on shaded slopes. Applying two simple thresholds to our spatial radiation modelings, our results show that the spatial distribution of surface hoar is greatly affected by large-scale terrain roughness and sun elevation angle. Spatial correlation ranges for surface hoar, on the order of several hundred meters, were closely related to the typical spacing between mountains. Furthermore, correlation ranges of surface hoar decreased with increasing sun elevation angle. Overall, the modeled spatial patterns of surface hoar were in line with previously published spatial field observations, suggesting that simple terrain parameters can very well be used to describe the predominant surface hoar layer patterns in complex topography.     

An integrated platform for observing the radiation budget of sea ice at different spatial scales

An integrated instrument package for measuring and understanding the surface radiation budget of sea ice is presented, along with results from its first deployment. The setup simultaneously measures broadband fluxes of upwelling and downwelling terrestrial and solar radiation (four components separately), spectral fluxes of incident and reflected solar radiation, and supporting data such as air temperature and humidity, surface temperature, and location (GPS), in addition to photographing the sky and observed surface during each measurement. The instruments are mounted on a small sled, allowing measurements of the radiation budget to be made at many locations in the study area to see the effect of small-scale surface processes on the large-scale radiation budget. Such observations have many applications, from calibration and validation of remote sensing products to improving our understanding of surface processes that affect atmosphere-snow-ice interactions and drive feedbacks, ultimately leading to the potential to improve climate modeling of ice-covered regions of the ocean. The photographs, spectral data, and other observations allow for improved analysis of the broadband data. An example of this is shown by using the observations made during a partly cloudy day, which show erratic variations due to passing clouds, and creating a careful estimate of what the radiation budget along the observed line would have been under uniform sky conditions, clear or overcast. Other data from the setup's first deployment, in June 2011 on fast ice near Point Barrow, Alaska, are also shown; these illustrate the rapid changes of the radiation budget during a cold period that led to refreezing and new snow well into the melt season.     

Detection of transient surface acoustic waves of nanometric amplitude with double-pulsed TV holography

We describe the detection of bursts of surface acoustic waves by a double-pulsed TV holography technique. We describe mathematically the long- and short-wave bursts in the output correlograms and validate theoretical results with experimental images. The use of short-wave bursts permits us to scan the surface and makes it easier to distinguish, for purposes of nondestructive testing, the disturbances produced by flaws.     

The use of microwave FMCW radar in snow and avalanche research

This paper describes a frequency modulated, continuous wave (FMCW) microwave radar system used for different types of investigations in snow and avalanche research. Different semi-empirical equations describing transmission and backscatter of electromagnetic energy in snow are compared and applied to analyse the frequency domain spectra of the backscattered radiation. The FMCW scatterometers are either buried in the ground looking upward into the snow cover or are towed on skis looking downward into the snow. The backscatter of electromagnetic radiation from avalanche snow moving perpendicular to the radar beam is analysed to estimate the height of dense flow in the avalanche. The geometrical layering, density, water equivalence, settlement, total snow height, percolation of water through the snow cover and moisture content of the snow are determined from the backscatter of the stratigraphy of a static snow pack.     

Variation of the reflection coefficient of semiconductors in a wavelength range from 0.2 to 20 μm under the action of ultrasonic waves

The effect of ultrasonic waves on the spectral coefficient of radiation reflection from the surface of semiconductors used in solar energy converters is considered. A change in the reflectance of semiconductors before and after ultrasonic treatment is determined. It is shown that acoustic stimulation of the semiconductor surface and subsurface layers determines the radiation reflection conditions.     

Some strength features of natural snow surfaces that affect snow drifting

The load, travel distance, and work index needed to break surface elements on 21 natural snow surfaces were measured with a motor-driven, spherical probe. The strongest and least brittle surfaces were fresh deposits of wind transported snow. The weakest surface was a new surface formed during snowfall with no sun and no wind. The most brittle surfaces were two cases of sun-softened snow that had refrozen before measurement. In new snow that had aged $\text{6–7}\text{1}\text{2}\text{h}$ with no sun and no wind, strength increased by a factor of 2–3, work index increased 10 times, and brittleness decreased by 4–9 times. On surfaces with both erosional and depositional features, the latter were stronger, less brittle, and had work indexes 10–100 times greater than the erosional features. The strength of snow surface elements as measured with the current hardness gage corresponded closely to the forces generated by saltating snow particles.