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Ashwin Mahesh, Von P. Walden, and Stephen G. Warren

Abstract

One full year of twice-daily longwave atmospheric emission spectra measured from the surface at 1-cm−1 resolution are used to infer optical thicknesses and ice crystal sizes in tropospheric clouds over the Antarctic Plateau. The method makes use of the cloud's emissivity at 10- and 11-μm wavelength and the cloud's transmittance of stratospheric ozone emission in the 9.6-μm band. Knowledge of the cloud-base temperature and the vertical distributions of ozone and temperature is required; these are available at South Pole Station from radiosondes and ozonesondes. The difference in emissivity between 10 and 11 μm is sensitive to ice particle size because the absorption coefficient of ice varies greatly between these two wavelengths. The retrieval of optical depth (expressed as its value in the geometric-optics limit τ g) is limited to τ g < 5, and the effective particle radii r eff are distinguished only for r eff < 25 μm, but 80% of the clouds observed have τg and r eff in the retrievable range. These clouds over the Antarctic interior are found to be optically thin, usually with τ g < 1, in contrast to coastal clouds, which usually have τ g > 20. Most have r eff in the range of 5–25 μm, with a mode at 15 μm. The retrieved r eff is larger in summer than in winter, in agreement with in situ measurements. From November to April, r eff was usually at least 10 μm, whereas, for a 3-month period in winter (July–September), no r eff values greater than 25 μm were retrieved. The particle sizes retrieved from the infrared spectra are compared with dimensions of ice crystals falling to the surface and measured on photomicrographs. Effective spherical radii are computed from the photographs in three ways: equal area, equal volume, and equal volume-to-area ratio (V/A). Agreement with the r eff derived from radiation measurements is best for equal-V/A spheres. The optical thicknesses and base heights inferred from the emission spectra agree qualitatively with the visual reports of the weather observers, in that the optically thicker clouds are usually reported as nimbostratus and clouds with the highest retrieved bases are reported as cirrus or cirrostratus. Stratus clouds tend to be reported as low; altostratus and altocumulus are intermediate in height.

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Ashwin Mahesh, Von P. Walden, and Stephen G. Warren

Abstract

Very steep shallow temperature inversions occur during most of the year in the near-surface layer on the Antarctic Plateau. A radiosonde carried by a balloon rising at a few meters per second does not measure such inversions accurately because the response time of the thermistor is several seconds. To quantify this error, the authors flew a radiosonde on a tethered kite on several occasions in winter at South Pole Station immediately prior to the routine launch of the same sonde on a weather balloon. In all cases, the equilibrated temperatures measured by the tethered sonde at a given pressure level were higher than those from the balloon-borne sonde throughout most of the inversion layer. Assuming that the tethered sonde data represent the true atmospheric temperature profile, a procedure can be developed to correct the temperature data from routine radiosonde soundings for the finite response time of the thermistor. The authors devise an accurate deconvolution method to retrieve the true atmospheric temperature profile from the radiosonde data when the thermistor response time is known. However, a simple technique of shifting the profile a few seconds back in time gives results that are nearly equivalent to the deconvolution. Additional temperature errors result at the South Pole because the radiosonde is launched immediately after being brought out of a warm room, making it necessary to further adjust data from the lowest few tens of meters. It is found that the temperature errors cause a 0.3 W m−2 error in the computed downward longwave radiation flux in winter at the South Pole, most of which is in spectral regions dominated by emission from water vapor and carbon dioxide. This is similar to the 0.5 W m−2 change induced by the increase in carbon dioxide concentration from preindustrial to present values. The thermal lag is shown to be significant also for winter profiles in Alaska. A correction for thermal lag is recommended for all situations where radiosondes are used to measure steep temperature gradients in the boundary layer: in polar regions throughout the year, at midlatitude continental stations in winter, and at the tops of subtropical marine stratocumulus clouds.

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Ashwin Mahesh, Von P. Walden, and Stephen G. Warren

Abstract

A Fourier-transform interferometer, operated throughout 1992 at South Pole Station, measured downward spectral longwave radiance from 550 to 1500 cm−1 (7–18 μm) at a resolution of 1 cm−1. Radiance measurements were usually made twice daily, coincident with routine launches of radiosondes made by the South Pole Weather Office; 223 radiance measurements (40% of the observations) were of cloudy-sky conditions. Cloud-base heights are retrieved from these data using a ground-based version of the radiance-ratioing method, which was originally developed to retrieve cloud-top heights from satellite data. Frequencies in the R branch of the 15-μm carbon dioxide band are used, exploiting the variation of atmospheric opacity with wavenumber. The annual cycle of cloud-base heights shows a bimodal distribution in all seasons except during the brief summer (December–January). Cloud-base heights are typically higher in the summer than in winter. Although retrieved cloud-base heights are uncorrelated with heights estimated by visual observers, both the retrieved and observed data indicate that base heights are bimodal. Most clouds have bases in the lowest few hundred meters, within the surface-based temperature inversion. The other mode is of higher clouds with base heights 1.5–3 km above the surface. Even the highest tropospheric clouds are within 6 km of the surface. Radiance ratioing can be used to detect polar stratospheric clouds, but their base heights are not reliably determined by the method.

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Von P. Walden, Stephen G. Warren, and Elizabeth Tuttle

Abstract

Falling ice crystals were collected daily on a gridded glass slide at South Pole Station, Antarctica, during the Antarctic winter of 1992 and were photographed through a microscope. Nine types of ice crystals are identified, which fall into three main categories: “diamond dust,” blowing snow, and snow grains. The dimensions of about 20 000 crystals were measured on scanned images of the photomicrographs. The predominant crystal types are hexagonal columns and plates (diamond dust) and rounded particles of blowing snow. Diamond-dust crystals have a large range of lengths (2–1000 μm) and aspect ratios (0.1–100). Diamond-dust crystals can usually be classified as either columns or plates; nearly equidimensional crystals are rare. “Long prism” crystals with aspect ratios greater than 5 were collected often, and very long prisms (“Shimizu” crystals), 1000 μm long but only 10 μm thick, were collected occasionally. The extreme Shimizu crystals were predominant on only one winter day, but the meteorological conditions on that day were not unusual. Some precipitation was observed on every day; even when the dominant crystal type was blowing snow, there were always, in addition, some snow grains or diamond dust. Blowing-snow particles dominate by number and contribute nearly one-half of the total surface area. Bullet clusters and blowing snow each contribute about one-third of the total volume of atmospheric ice. Size distributions of the equivalent spherical radius are obtained for each of the nine crystal types, as well as for the three main categories of crystals, using the volume-to-area ratio to specify the equivalent spheres. In addition, the effective radius for each day when crystals were sampled is computed. Many of the distributions are approximately lognormal. The effective radius (area-weighted mean radius) of the entire size distribution of diamond dust is 12 μm in winter, somewhat smaller than in summer (15 μm). The small size of wintertime blowing snow allows it to reach heights of tens of meters in winter, as compared with only a few meters in summer. The average effective radius was 11 μm for blowing snow and 24 μm for snow grains. The most probable effective radius for any given day in winter is about 11 μm.

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Michael S. Town, Von P. Walden, and Stephen G. Warren

Abstract

Annual cycles of downwelling broadband infrared radiative flux and spectral downwelling infrared flux were determined using data collected at the South Pole during 2001. Clear-sky conditions are identified by comparing radiance ratios of observed and simulated spectra. Clear-sky fluxes are in the range of 110–125 W m−2 during summer (December–January) and 60–80 W m−2 during winter (April–September). The variability is due to day-to-day variations in temperature, strength of the surface-based temperature inversion, atmospheric humidity, and the presence of “diamond dust” (near-surface ice crystals). The persistent presence of diamond dust under clear skies during the winter is evident in monthly averages of clear-sky radiance.

About two-thirds of the clear-sky flux is due to water vapor, and one-third is due to CO2, both in summer and winter. The seasonal constancy of this approximately 2:1 ratio is investigated through radiative transfer modeling. Precipitable water vapor (PWV) amounts were calculated to investigate the H2O/CO2 flux ratio. Monthly mean PWV during 2001 varied from 1.6 mm during summer to 0.4 mm during winter. Earlier published estimates of PWV at the South Pole are similar for winter, but are 50% lower for summer. Possible reasons for low earlier estimates of summertime PWV are that they are based either on inaccurate hygristor technology or on an invalid assumption that the humidity was limited by saturation with respect to ice.

The average fractional cloud cover derived from the spectral infrared data is consistent with visual observations in summer. However, the wintertime average is 0.3–0.5 greater than that obtained from visual observations. The annual mean of longwave downwelling cloud radiative forcing (LDCRF) for 2001 is about 23 W m−2 with no apparent seasonal cycle. This is about half that of the global mean LDCRF; the low value is attributed to the small optical depths and low temperatures of Antarctic clouds.

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Michael S. Town, Von P. Walden, and Stephen G. Warren

Abstract

Estimates of cloud cover over the South Pole are presented from five different data sources: routine visual observations (1957–2004; C vis), surface-based spectral infrared (IR) data (2001; C PAERI), surface-based broadband IR data (1994–2003; C pyr), the Extended Advanced Very High Resolution Radiometer (AVHRR) Polar Pathfinder (APP-x) dataset (1994–99; C APP-x), and the International Satellite Cloud Climatology Project (ISCCP) dataset (1994–2003; C ISCCP). The seasonal cycle of cloud cover is found to range from 45%–50% during the short summer to a relatively constant 55%–65% during the winter. Relationships between C pyr and 2-m temperature, 10-m wind speed and direction, and longwave radiation are investigated. It is shown that clouds warm the surface in all seasons, 0.5–1 K during summer and 3–4 K during winter. The annual longwave cloud radiative forcing is 18 W m−2 for downwelling radiation and 10 W m−2 for net radiation. The cloud cover datasets are intercompared during the time periods in which they overlap. The nighttime bias of C vis is worse than previously suspected, by approximately −20%; C ISCCP shows some skill during the polar day, while C APP-x shows some skill at night. The polar cloud masks for the satellite data reviewed here are not yet accurate enough to reliably derive surface or cloud properties over the East Antarctic Plateau. The best surface-based source of cloud cover in terms of the combination of accuracy and length of record is determined to be C pyr. The use of the C pyr dataset for further tests of satellite retrievals and for tests of polar models is recommended.

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Jinxue Wang, John C. Gille, Henry E. Revercomb, and Von P. Walden

Abstract

The Measurement of Pollution in the Troposphere (MOPITT) instrument is an eight-channel gas correlation radiometer selected for the Earth Observing System (EOS) Terra spacecraft launched in December 1999. Algorithms for the retrieval of tropospheric carbon monoxide (CO) profiles from MOPITT measurements have been developed. In this paper, validation studies of the MOPITT CO retrieval algorithm using observations by the Interferometric Monitor for greenhouse Gases (IMG) during the Winter Clouds Experiment (WINCE) conducted from 23 January to 13 February 1997 are described. Synthetic radiance spectra calculated by a line-by-line radiative transfer model, FASCOD3, using the retrieved CO profile agrees well with IMG-measured radiance spectra. Observations by the Moderate Resolution Imaging Spectrometer (MODIS) Airborne Simulator (MAS) from the NASA ER-2 platform during WINCE were successfully used to assist in the identification of clear and cloudy IMG observations.

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Penny M. Rowe, Larry M. Miloshevich, David D. Turner, and Von P. Walden

Abstract

Middle to upper tropospheric humidity plays a large role in determining terrestrial outgoing longwave radiation. Much work has gone into improving the accuracy of humidity measurements made by radiosondes. Some radiosonde humidity sensors experience a dry bias caused by solar heating. During the austral summers of 2002/03 and 2003/04 at Dome C, Antarctica, Vaisala RS90 radiosondes were launched in clear skies at solar zenith angles (SZAs) near 83° and 62°. As part of this field experiment, the Polar Atmospheric Emitted Radiance Interferometer (PAERI) measured downwelling spectral infrared radiance. The radiosonde humidity profiles are used in the simulation of the downwelling radiances. The radiosonde dry bias is then determined by scaling the humidity profile with a height-independent factor to obtain the best agreement between the measured and simulated radiances in microwindows between strong water vapor lines from 530 to 560 cm−1 and near line centers from 1100 to 1300 cm−1. The dry biases, as relative errors in relative humidity, are 8% ± 5% (microwindows; 1σ) and 9% ± 3% (line centers) for SZAs near 83°; they are 20% ± 6% and 24% ± 5% for SZAs near 62°. Assuming solar heating is minimal at SZAs near 83°, the authors remove errors that are unrelated to solar heating and find the solar-radiation dry bias of 9 RS90 radiosondes at SZAs near 62° to be 12% ± 6% (microwindows) and 15% ± 5% (line centers). Systematic errors in the correction are estimated to be 3% and 2% for microwindows and line centers, respectively. These corrections apply to atmospheric pressures between 650 and 200 mb.

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Stephen R. Hudson, Michael S. Town, Von P. Walden, and Stephen G. Warren

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The response of radiosondes to an instantaneous change of environment was studied by taking the instruments from a warm building into the cold environment at South Pole Station. After being initialized inside, the radiosondes were carried outside and placed on the snow surface, where they were left until they reported stable values of temperature, pressure, and relative humidity. Three models of radiosondes were tested: Vaisala RS80, Atmospheric Instrumentation Research (AIR) 4A, and AIR 5A.

The reported temperature equilibrated to the outside conditions within 30 s. However, it frequently took 30 min before the relative humidity outside was accurately reported. Additionally, the reported pressure rose by several hectopascals over a 5-min period when the sonde was taken outside. In the RS80s this bias was as large as 10 hPa, and disappeared in about 30 min. In the AIR sondes, the maximum pressure bias was never much over 2 hPa, but seemed not to diminish with time.

The RS80s were also tested to see if, once equilibrated to the outside conditions, they could respond to smaller changes that would be encountered in flight. The results in this case indicate that, with some corrections for time lag, the RS80 can provide accurate data at low temperatures if allowed to equilibrate initially.

The results of these tests together indicate that the quality of upper-air data in cold regions could be improved if radiosondes are stored and prepared at ambient temperature or are given at least 30 min to equilibrate outside after being prepared inside.

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Christopher J. Cox, David D. Turner, Penny M. Rowe, Matthew D. Shupe, and Von P. Walden

Abstract

The radiative properties of clouds are related to cloud microphysical and optical properties, including water path, optical depth, particle size, and thermodynamic phase. Ground-based observations from remote sensors provide high-quality, long-term, continuous measurements that can be used to obtain these properties. In the Arctic, a more comprehensive understanding of cloud microphysics is important because of the sensitivity of the Arctic climate to changes in radiation. Eureka, Nunavut (80°N, 86°25′W, 10 m), Canada, is a research station located on Ellesmere Island. A large suite of ground-based remote sensors at Eureka provides the opportunity to make measurements of cloud microphysics using multiple instruments and methodologies. In this paper, cloud microphysical properties are presented using a retrieval method that utilizes infrared radiances obtained from an infrared spectrometer at Eureka between March 2006 and April 2009. These retrievals provide a characterization of the microphysics of ice and liquid in clouds with visible optical depths between 0.25 and 6, which are a class of clouds whose radiative properties depend greatly on their microphysical properties. The results are compared with other studies that use different methodologies at Eureka, providing context for multimethod perspectives. The authors’ findings are supportive of previous studies, including seasonal cycles in phase and liquid particle size, weak temperature–phase dependencies, and frequent occurrences of supercooled water. Differences in microphysics are found between mixed-phase and single-phase clouds for both ice and liquid. The Eureka results are compared with those obtained using a similar retrieval technique during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment.

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