Thomas C. Grenfell

Reflection of solar radiation by the Antarctic snow surface at ultraviolet, visible, and near-infrared wavelengths

The variation of snow albedo with wavelength across the solar spectrum from 0.3 μm in the ultraviolet (UV) to 2.5 μm in the near infrared (IR) was measured at Amundsen-Scott South Pole Station during the Antarctic summers of 1985–1986 and 1990–1991. Similar results were obtained at Vostok Station in summer 1990–1991. The albedo has a uniformly high value of 0.96–0.98 across the UV and visible spectrum, nearly independent of snow grain size and solar zenith angle, and this value probably applies throughout the interior of Antarctica. The albedo in the near IR is lower, dropping below 0.15 in the strong absorption bands at 1.5 and 2.0 μm; and it is quite sensitive to grain size and somewhat sensitive to zenith angle. Near-IR albedos were slightly lower at Vostok than at South Pole, but day-to-day variations in the measured grain size due to precipitation, drifting, and metamorphism were found to cause temporal variations in near-IR albedo larger than those due to any systematic geographical change from South Pole to Vostok. The spectrally averaged albedos ranged from 0.80 to 0.85 for both overcast and clear skies, in agreement with measurements by others at South Pole and elsewhere in Antarctica. Using a two-layer radiative transfer model, the albedo can be explained over the full wavelength range. Tests were made to correct for systematic errors in determining spectral albedo. Under clear skies at about 3000-m elevation the diffuse fraction of downward irradiance varied from 0.4 in the near UV to less than 0.01 in the near IR; knowledge of this fraction is required to correct the measured irradiance for the instruments deviation from a perfect cosine-response. Furthermore, the deviation from cosine response is itself a function of wavelength. Under clear skies a significant error in apparent albedo can result if the instruments cosine collector is not parallel to the surface; e.g., if the instrument is leveled parallel to the horizon, but the local snow surface is not horizontal. The soot content of the snow upwind of South Pole Station was only 0.3 ng/g. It was somewhat greater at Vostok Station but was still too small to affect the albedo at any wavelength. Bidirectional reflectance at 0.9-μm wavelength, measured from a 23-m tower at the end of summer after the sastrugi (snow dunes) had diminished, showed a pattern remarkably similar to the spectrally averaged pattern obtained from the Nimbus 7 satellite.

Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation

Disclosed herein is a new and unique type of circuit control for an electric guitar. Simply stated, it varies the resonant frequency of the pickup itself in addition to filtering out frequencies which are suppressed or rolled off. The mechanism by which this is brought about includes a potentiometer connected to a center tap of the coil assembly.

Surface Albedo of the Antarctic Sea Ice Zone

In three ship-based field experiments, spectral albedos were measured at ultraviolet, visible, and nearinfrared wavelengths for open water, grease ice, nilas, young “grey” ice, young grey-white ice, and first-year ice, both with and without snow cover. From the spectral measurements, broadband albedos are computed for clear and cloudy sky, for the total solar spectrum as well as for visible and near-infrared bands used in climate models, and for Advanced Very High Resolution Radiometer (AVHRR) solar channels. The all-wave albedos vary from 0.07 for open water to 0.87 for thick snow-covered ice under cloud. The frequency distribution of ice types and snow coverage in all seasons is available from the project on Antarctic Sea Ice Processes and Climate (ASPeCt). The ASPeCt dataset contains routine hourly visual observations of sea ice from research and supply ships of several nations using a standard protocol. Ten thousand of these observations, separated by a minimum of 6 nautical miles along voyage tracks, are used together with the measured albedos for each ice type to assign an albedo to each visual observation, resulting in “ice-only” albedos as a function of latitude for each of five longitudinal sectors around Antarctica, for each of the four seasons. These ice albedos are combined with 13 yr of ice concentration estimates from satellite passive microwave measurements to obtain the geographical and seasonal variation of average surface albedo. Most of the Antarctic sea ice is snow covered, even in summer, so the main determinant of area-averaged albedo is the fraction of open water within the pack.

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.

A radiative transfer model for sea ice with vertical structure variations

A radiative transfer model designed for application to snow-covered sea ice with vertical layering is presented which removes several of the computational problems associated with media whose optical properties are strong functions of wavelength and depth. With this model it is possible to treat the effects of biological activity in the ice as well as vertical variations in brine volume and bubble density. Transmission spectra can also be calculated as a function of depth in the water column beneath the ice. Accurate results are achieved for both direct and diffuse incident irradiance for single scattering albedos ranging from 0.0 to 0.99999999. Calculated albedos and extinction coefficients for snow and sea ice are consistent with both observations and previous models, and the model results resolve apparent inconsistencies among certain observational data sets. Refraction is included for all layer interfaces and is found to reduce the albedo, while the transmissivity responds to changes in the optical depth in addition to redirection of the radiation. For bare sea ice, ignoring refraction can introduce albedo errors as great as 24% and transmissivity errors of more than 70%. Absorption by algae in the ice is shown to reduce the transmitted radiation by more than a factor of 10 at certain visible wavelengths. The predicted azimuthal dependence of the emergent radiances is very weak even for large solar incidence angles.

Spectral albedos of an alpine snowpack

Abstract 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 Bidirectional Reflectance of Antarctic Snow: Measurements and Parameterization

The bidirectional reflectance distribution function (BRDF) of snow was measured from a 32-m tower at Dome C, at latitude 75°S on the East Antarctic Plateau. These measurements were made at 96 solar zenith angles between 51° and 87° and cover wavelengths 350–2400 nm, with 3- to 30-nm resolution, over the full range of viewing geometry. The BRDF at 900 nm had previously been measured at the South Pole; the Dome C measurement at that wavelength is similar. At both locations the natural roughness of the snow surface causes the anisotropy of the BRDF to be less than that of flat snow. The inherent BRDF of the snow is nearly constant in the high-albedo part of the spectrum (350–900 nm), but the angular distribution of reflected radiance becomes more isotropic at the shorter wavelengths because of atmospheric Rayleigh scattering. Parameterizations were developed for the anisotropic reflectance factor using a small number of empirical orthogonal functions. Because the reflectance is more anisotropic at wavelengths at which ice is more absorptive, albedo rather than wavelength is used as a predictor in the near infrared. The parameterizations cover nearly all viewing angles and are applicable to the high parts of the Antarctic Plateau that have small surface roughness and, at viewing zenith angles less than 55°, elsewhere on the plateau, where larger surface roughness affects the BRDF at larger viewing angles. The root-mean-squared error of the parameterized reflectances is between 2% and 4% at wavelengths less than 1400 nm and between 5% and 8% at longer wavelengths.

A model for retrieving total sea ice concentration from a spaceborne dual-polarized passive microwave instrument operating near 90 GHz

Abstract An algorithm has been developed for estimating total ice concentration from spaceborne high-frequency passive microwave instrumentation. The algorithm is intended for use with the coming Special Sensor Microwave/Imager (SSM/I) data giving a spatial resolution of 12 km. It is based on radiation physics and detailed millimetre wave surface signature measurements and can therefore be applied to other similar data. However, due to large effects on the signals caused by time varying atmospheric conditions and radiation properties of the ice, the algorithm is made self-adjusting. The atmospheric effects are implicitly treated as a smooth function of the ice concentration with tie points over open ocean and 100 per cent ice for each orbit. This means that the main errors are due to patches of heavy clouds and ice floes with atypical radiation properties. An error analysis indicates possible errors of the order of 5 percent for concentrations representative for the Arctic Basin, increasing with decreasin...

The effect of included participates on the spectral albedo of sea ice

Sediments and other participates are often entrained into sea ice formed over shallow shelves in the Arctic, causing significant changes in the albedo of the ice and in the amount of shortwave radiation absorbed and transmitted by the ice. A structural-optical model was used in conjunction with a four-stream radiative transfer model to examine the effects of such particulates on the optical properties of sea ice. Albedo data from well-characterized ice with moderate particulate loading were combined with model calculations to infer a spectral absorption coefficient and effective size for the particulates. Results indicate that sediment particles contained in the ice have an effective radius (R) of ∼9 μm, assuming absorption coefficients similar to those of Saharan dust. With these values, model predictions are in close agreement with spectral albedo observations over a broad range of particulate loading. For particle size distributions commonly observed in sea ice, the calculations indicate that particles with R>30 μm have little effect on the bulk optical properties of the ice. The albedo data also suggest that even apparently “clean” ice contains trace amounts (5–10 g m−3) of particulates that reduce albedos by as much as 5–10% in the visible part of the spectrum. The calculations show that particulates in sea ice primarily affect radiative transfer at visible wavelengths, whereas apparent optical properties in the near-infrared tend to be governed by ice structure rather than by the presence of particulates. Particle-bearing layers occurring below ∼20–30 cm are found to have little effect on albedo, although they can still have a substantial effect on transmission. Estimates of total particle loading cannot be obtained from reflectance data without some additional information on particle size, vertical distribution, and ice structure.

Multifrequency Passive Microwave Observations of First-Year Sea Ice Grown in a Tank

Microwave brightness temperatures of new, young, and optically opaque sea ice grown in a large tank were obtained in the course of a joint microwave experiment at CRREL in Hanover, New Hampshire, during the winters of 1983-1984 and 1984-1985. Dual-polarized observations were taken at frequencies of 10, 18, 37, and 90 GHz over a range of incidence angles, and the concurrent temperature and ice thickness were obtained. Bulk salinities as well as radar and dielectric properties were also measured concurrently by other investigators. Emissivity and degree of polarization were observed in detail during the early stages of ice growth and variations were found indicating that the ice became optically opaque at 10 GHz for ice thickness between 30 and 50 mm. The addition of a snow cover reduced the brightness temperature at the higher frequencies with little effect at 10 GHz. Artificial roughening of the surface reduced the degree of polarization considerably but changed the emissivity at vertical polarization only slightly. Cluster plots of the data shown six distinguishable surface types: optically opaque bare ice, thin ice (less than 15 mm), roughened ice, ridged ice, rotting wet ice, and open water.