Stephen G. Warren

Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate

The major source of cloud-condensation nuclei (CCN) over the oceans appears to be dimethylsulphide, which is produced by planktonic algae in sea water and oxidizes in the atmosphere to form a sulphate aerosol Because the reflectance (albedo) of clouds (and thus the Earths radiation budget) is sensitive to CCN density, biological regulation of the climate is possible through the effects of temperature and sunlight on phytoplankton population and dimethylsulphide production. To counteract the warming due to doubling of atmospheric CO2, an approximate doubling of CCN would be needed.

Optical constants of ice from the ultraviolet to the microwave

A compilation of the optical constants of ice Ih is made for temperatures within 60 K of the melting point. The imaginary part mim of the complex index of refraction m is obtained from measurements of spectral absorption coefficient; the real part mre is computed to be consistent with mim by use of known dispersion relations. The compilation of mim requires subjective interpolation in the near-ultraviolet and microwave, a temperature correction in the far-infrared, and a choice between two conflicting sources in the near-infrared. New measurements of the spectral absorption coefficient of pure ice are needed, at temperatures near the melting point, for 185–400-nm, 1.4–2.8-μm, 3.5–4.3-μm, 33–600-μm, and 1–100-mm wavelengths.

A Model for the Spectral Albedo of Snow. I: Pure Snow

Abstract We present a method for calculating the spectral albedo of snow which can be used at any wavelength in the solar spectrum and which accounts for diffusely or directly incident radiation at any zenith angle. For deep snow, the model contains only one adjustable parameter, an effective grain size, which is close to observed grain sizes. A second parameter, the liquid-equivalent depth, is required only for relatively thin snow. In order for the model to make realistic predictions, it must account for the extreme anisotropy of scattering by snow particles. This is done by using the “delta-Eddington” approximation for multiple scattering, together with Mie theory for single scattering. The spectral albedo from 0.3 to 5 μm wavelength is examined as a function of the effective grain size, the solar zenith angle, the snowpack thickness, and the ratio of diffuse to direct solar incidence. The decrease in albedo due to snow aging can be mimicked by reasonable increases in grain size (50–100 μm for new snow...

A Model for the Spectral Albedo of Snow. II: Snow Containing Atmospheric Aerosols

Abstract Small highly absorbing particles, present in concentrations of only 1 part per million by weight (ppmw) or less, can lower snow albedo in the visible by 5–15% from the high values (96–99%) predicted for pure snow in Part I. These particles have, however, no effect on snow albedo beyond 0.9 μm wavelength where ice itself becomes a strong absorber. Thus we have an attractive explanation for the discrepancy between theory and observation described in Part I, a discrepancy which seemingly cannot be resolved on the basis of near-field scattering and nonsphericity effects. Desert dust and carbon soot are the most likely contaminants. But careful measurements of spectral snow albedo in the Arctic and Antarctic paint to a “grey” absorber, one whose imaginary refractive index is nearly constant across the visible spectrum. Thus carbon soot, rather than the red iron oxide normally present in desert dust, is strongly indicated at these sites. Soot particles of radius 0.1 μm, in concentrations of only 0.3 pp...

Global distribution of total cloud cover and cloud type amounts over the ocean

This is the fourth of a series of atlases to result from a study of the global cloud distribution from ground-based observations. The first two atlases (NCAR/TN-201+STR and NCAR/TN-241+STR) described the frequency of occurrence of each cloud type and the co-occurrence of different types, but included no information about cloud amounts. The third atlas (NCAR/TN-273+STR) described, for the land areas of the earth, the average total cloud cover and the amounts of each cloud type, and their geographical, diurnal, seasonal, and interannual variations, as well as the average base heights of the low clouds. The present atlas does the same for the ocean areas of the earth.

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.

Snow Depth on Arctic Sea Ice

Snow depth and density were measured at Soviet drifting stations on multiyear Arctic sea ice. Measurements were made daily at fixed stakes at the weather station and once- or thrice-monthly at 10-m intervals on a line beginning about 500 m from the station buildings and extending outward an additional 500 or 1000 m. There were 31 stations, with lifetimes of 1‐7 yr. Analyses are performed here for the 37 years 1954‐91, during which time at least one station was always reporting. Snow depth at the stakes was sometimes higher than on the lines, and sometimes lower, but no systematic trend of snow depth was detected as a function of distance from the station along the 1000-m lines that would indicate an influence of the station. To determine the seasonal progression of snow depth for each year at each station, priority was given to snow lines if available; otherwise the fixed stakes were used, with an offset applied if necessary. The ice is mostly free of snow during August. Snow accumulates rapidly in September and October, moderately in November, very slowly in December and January, then moderately again from February to May. This pattern is exaggerated in the Greenland‐Ellesmere sector, which shows almost no net accumulation from November to March. The Chukchi region shows a steadier accumulation throughout the autumn, winter, and spring. The average snow depth of the multiyear ice region reaches a maximum of 34 cm (11 g cm22) in May. The deepest snow is just north of Greenland and Ellesmere Island, peaking in early June at more than 40 cm, when the snow is already melting north of Siberia and Alaska. The average snow density increases with time throughout the snow accumulation season, averaging 300 kg m23, with little geographical variation. Usually only two stations were in operation in any particular year, so there is insufficient information to obtain the geographical pattern of interannual variations. Therefore, to represent the geographical and seasonal variation of snow depth, a two-dimensional quadratic function is fitted to all data for a particular month, irrespective of year. Interannual anomalies for each month of each year are obtained relative to the long-term mean snow depth for the geographical location of the station operating in that particular year. The computed interannual variability (IAV) of snow depth in May is 6 cm, but this is larger than the true IAV because of inadequate geographical sampling. Weak negative trends of snow depth are found for all months. The largest trend is for May, the month of maximum snow depth, a decrease of 8 cm over 37 yr, apparently due to a reduction in accumulation-season snowfall.

Effect of surface roughness on bidirectional reflectance of Antarctic snow

The angular pattern of sunlight reflected by snow is altered by surface roughness, which in the interior of Antarctica is usually in the form of meter-scale longitudinal erosional features (sastrugi), whose axes align with the direction of strong winds. The bidirectional reflectance distribution function (BRDF) changes over the course of a day as the solar azimuth changes relative to the sastrugi axis. The normalized BRDF, or “anisotropic reflectance factor” R, was measured at South Pole Station from a 22-m tower at 600, 660, and 900 nm wavelengths. The R pattern was similar at the three wavelengths; it probably varies little from 300 to 900 nm. Measurements were made at solar zenith angles θ0 from 67° to 90°, over the full range of viewing zenith angle (θr), azimuth angle between Sun and view (ϕ), and azimuth angle between Sun and sastrugi (ϕsas). Variation of R with ϕsas was notable; sastrugi oriented perpendicular to the solar beam cause a reduction of the forward peak, and sastrugi at an oblique angle cause R to lose its symmetry about the solar azimuth. However, the effects of sastrugi are mostly restricted to large viewing zenith angles, so remote sensing of albedo and atmospheric properties can be carried out accurately without knowledge of sastrugi height and orientation if only near-nadir views are used. This recommendation is opposite that for observations of broken clouds over dark surfaces, for which large θr is preferred. A parameterization of R is developed, valid for viewing angles θr ≤ 50°. Sastrugi can cause a reduction of the snow albedo by altering the angle of incidence and by trapping of photons. For the small sastrugi of the Antarctic Plateau, the albedo is unaffected at visible wavelengths but can be reduced by a few percent at near-infrared wavelengths when the Sun is low.

Snow on Antarctic sea ice

Snow on Antarctic sea ice plays a complex and highly variable role in air-sea-ice interaction processes and the Earths climate system. Using data collected mostly during the past 10 years, this paper reviews the following topics: snow thickness and snow type and their geographical and seasonal variations; snow grain size, density, and salinity; frequency of occurrence of slush; thermal conductivity, snow surface temperature, and temperature gradients within snow; and the effect of snow thickness on albedo. Major findings include large regional and seasonal differences in snow properties and thicknesses; the consequences of thicker snow and thinner ice in the Antarctic relative to the Arctic (e.g., the importance of flooding and snow-ice formation); the potential impact of increasing snowfall resulting from global climate change; lower observed values of snow thermal conductivity than those typically used in models; periodic large-scale melt in winter; and the contrast in summer melt processes between the Arctic and the Antarctic. Both climate modeling and remote sensing would benefit by taking account of the differences between the two polar regions.