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Abstracts
Western Arctic

Dutton, E. G., and D. J. Endres, Date of snowmelt at Barrow, Alaska, U.S.A., Arctic Alp. Res., 23, 115-119. 1991.

The date of snowmelt near Barrow, Alaska, for recent years is determined from radiometric in situ measurements of the tundra solar albedo. The snowmelt dates determined from the albedo measurements dispute recently published values based on routine visual observations at the Barrow National Weather Service (NWS) Office. Local heat- budget- altering effects of the village and its recent abrupt growth are suggested as the cause for the disagreement between the rural albedo and NWS "in town" observations. The open tundra albedo measurements combined with historical NWS observations suggest that there is no significant trend in the date of snowmelt near Barrow.

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Harris, J. M., and J. D. W. Kahl, Analysis of 10-day isentropic flow patterns for Barrow, Alaska: 1985-1992, J. Geophys. Res., 99, 25,845-25,855, 1994.

Atmospheric transport patterns to Barrow, Alaska, during 1985-1992 were investigated using a newly developed isentropic air trajectory model. The new model features a layer-averaged mode that is activated whenever an air parcel traveling isentropically approaches the Earth's surface. A dynamic preprocessing program ensures that trajectories always arrive at a constant, predetermined altitude. Ten-day back trajectories arriving twice daily at 500, 1500, and 3000 m above sea level revealed no long-term trends in flow patterns during the 8-year period. Frequency of transport type was fairly stable from year to year, except in the anomalously warm year of 1989 when increased numbers of trajectories from the Aleutian region were observed. During the Arctic haze season, trajectories suggest that transport of pollution from north central Russia occurs near the surface (about 20% frequency), whereas that from northern Europe occurs at higher elevations (about 10% frequency).

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Stone, R. S., Variations in western Arctic temperatures in response to cloud radiative and synoptic-scale influences, J. Geophys. Res., 102, 21,769-21,776, 1997.

The analysis focuses on Barrow, Alaska, a site that is sensitive to changing conditions because it is located near cryospheric boundaries and is influenced by both extratropical and Arctic synoptic activity. Surface and upper air meteorological data for a 31-year period (1965-1995) are used to evaluate temperature variations as they relate to dynamical and radiative processes. Both annual and monthly analyses indicate a tendency toward warming overall. However, the annual warming is not monotonic over time and varies seasonally. Comparisons of temperature time series from four sites along the Siberian-Alaskan coastline show that Barrow is a representative site to evaluate climate change in the western Arctic coastal zone. Regionally, the warming is dominated by significant temperature increases during winter and spring, but cooling is indicated for autumn. These results are not entirely consistent with model predictions of a more uniform high-latitude warming during the cold season in response to increasing concentrations of greenhouse gases in the atmosphere. Rather, the observed changes are attributed to well-known natural processes that affect regional cloud distributions in response to changing circulation patterns. Coincident daily and hourly meteorological and radiation data are also used to demonstrate empirically how clouds modulate Arctic temperatures.

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Stone, R. S., Properties of austral winter clouds derived from radiometric profiles at south pole, J. Geophys. Res., 98, 12961-12971, 1993.

Although clouds are known to radiatively modulate the energy budget of Antarctica, particularly during the polar night, very little is known about their physical, radiative or microphysical properties. A unique set of measurements consisting of infrared flux profiles (radiometersonde data) collected at the south pole is analyzed in conjunction with ancillary meteorological observations to gain a better understanding of austral winter clouds. Distinct radiometric features associated with cloud boundaries are used to estimate the heights, thicknesses, and temperatures of selected cloud systems. The physical cloud properties are combined with infrared flux measurements to formulate the thermal energy budgets of the cloud layers and to derive their bulk radiative properties. By comparing these derived radiative properties with theoretically computed values for model clouds varying in microphysical characteristics the clouds' ice contents and effective particle sizes are also inferred. Austral winter clouds are found to be moderately thick and on average extend to heights greater than 3 km above the top of the surface-based inversion layer. They are optically thin, however, and nonblack having mean effective emissivities of about 0.6. Because they are cold ( -40°C) these clouds are composed primarily of small ice particles, have low total ice contents and contain little or no liquid water. Overall, they are similar to high-level cirrus clouds observed in the midlatitudes. The characteristic radiometric features that distinguish overcast from clear-sky conditions at south pole are discussed, mean profiles of infrared fluxes, temperatures, vector winds, and heating rates are presented, and a summary is provided of cloud properties that may be of use to climate modelers. (Polar meteorology, radiative processes, instruments and techniques, transmission and scattering of radiation.)

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Stone, R. S., and J. D. Kahl, Variations in boundary layer properties associated with clouds and transient weather disturbances at the South Pole during winter, J. Geophys. Res., 96, 5137-5144, 1991.

Changes in the South polar atmosphere are linked to dynamical processes that control the transport of mass, heat, and moisture from lower latitudes. Radiation and meteorological data collected at the South Pole during the 1986 austral winter are analyzed to gain a better understanding of the relationships between cloud radiative effects, transport processes and the vertical distribution of temperature and wind. The quasi-permanent surface-based temperature inversion and the "warm" radiatively active layer above it are characterized. Mean winter temperature and wind profiles for clear and overcast conditions are combined with surface radiation measurements and synoptic-scale circulation patterns to study the mechanisms that cause periodic weakening of the inversion. Results support previous studies that ascribe this weakening to (1) warm air advection, (2) downward vertical mixing of sensible and latent heat, and (3) longwave cloud radiative heating. The integrity of the inversion depends on the combined effects of all three mechanisms. Parameters representing the intensity of the inversion and the bulk wind shear through the lower troposphere are suggested as appropriate indices for the detection of climate change in the region of the Antarctic Plateau. (Antarctic, clouds, radiation, circulation.)

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Pinatubo effects in the Arctic

Dutton, E. G., and J. R. Christy, Solar radiative forcing at selected locations and evidence for global lower tropospheric cooling following the eruptions of El Chichon and Pinatubo, Geophys. Res. Lett., 19, 2313-1216, 1992.

As a result of the eruption of Mt. Pinatubo (June 1991), direct solar radiation was observed to decrease by as much as 25-30% at four remote locations widely distributed in latitude. The average total aerosol optical depth for the first 10 months after the Pinatubo eruption at those sites is 1.7 times greater than that observed following the 1982 eruption of El Chichon. Monthly-mean clear-sky total solar irradiance at Mauna Loa, Hawaii, decreased by a much as 5% and averaged 2.4% and 2.7% in the first 10 months after the El Chichon and the Pinatubo eruptions, respectively. By September 1992 the global and northern hemispheric lower tropospheric temperatures had decreased by 0.5C and 0.7C, respectively compared to pre-Pinatubo levels. The temperature record examined consists of globally uniform observations from satellite microwave sounding units.

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Dutton, E. G., J. J. Deluisi, and G. Herbert, Shortwave aerosol optical depth of Arctic haze measured on board the NOAA WP-3D during AGASP-II, April 1986, J. Atmos. Chem., 9, 71-79, 1989.

Measurements of spectral aerosol depth in the Alaskan and Canadian Arctic were made from the NOAA Lockheed WP-3D aircraft as part of the second Arctic Gas and Aerosol Sampling Program (AGASP-II) during April 1986. The flight tracks and altitudes flown enabled measurements of the vertical and horizontal distribution of aerosol optical depth in the troposphere as well as direct determination of the stratospheric component. Tropospheric aerosol optical depth ranged from about 0.1 to 0.7. The factor of 7 variability sometimes occurred within 60 km horizontally; comparable variability occurred within less than 1 to 2 km vertically. The Angstrom exponents of the spectral optical depths ranged from 0.5 to 2.0, and some of the variability was apparently related to distinct aerosol regimes.

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Dutton, E. G., J. J. Deluisi, and B. A. Bodhaine, Features of aerosol optical depth observed at Barrow, March 10-20, 1983, Geophys. Res. Lett., 11, 385-388, 1984.

Total vertical aerosol optical depth over Barrow, Alaska, during March 1983 was up to four times greater than the average for recent years, with part of the excess being due to stratospheric debris from El Chichon. The variability in optical depth during a 10-day period spanning the aircraft flights of the Arctic Gas and Aerosol Sampling Program (AGASP) suggest a major tropospheric aerosol event on March 12 and 13, which accounts for the maximum observed optical depths. Occurrence of the tropospheric event is substantiated with independent aerosol data from aircraft, surface sampling, and synoptic scale meteorological data. Analysis of the Barrow optical depth data yields information on the climatic effects of both the stratospheric aerosol from El Chichon and the tropospheric aerosol commonly called Arctic haze.

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Herbert, G. A., P. J. Sheridan, R. C. Schnell, M. Z. Bieniulis, B. A. Bodhaine, and S. J. Oltmans, Analysis of meteorological conditions during AGASP-IV: March 30-April 23, 1992, NOAA Tech. Memo. ERL CMDL-5, Climate Monitoring and Diagnostics Laboratory, Boulder, CO, 118 pp., 1993.

The fourth Arctic Gas and Aerosol Sampling Program (AGASP-IV) was conducted over Alaska and the Beaufort Sea during March and April 1992. The NOAA WP-3D aircraft made nine flights. On the first eight flights special aerosol and gas sampling instrumentation was installed, and extensive time was spent over the pack ice. Measurements of wind, pressure, temperature, relative humidity, ozone, and condensation nucleus (CN) concentration were used to identify the air mass type, recent origin, and existence of pollution-derived aerosols, i.e., haze. While small patches of elevated CN concentrations and higher aerosol scattering coefficients were observed, significantly large regions, of the type found in previous AGASP missions, were not observed during this series. On most flights the CN concentrations in the troposphere were representative of the "clean" background conditions at this latitude. Significant concentrations of CN (CN>7000 cm-3) found above the tropopause on three fights indicated the presence of volcanic aerosol probably from the Pinatubo volcanic plume at high latitudes. In four instances low ozone concentrations suggest the destruction of ozone in the surface layer.

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Hofmann, D. J., and J. M. Rosen, On the prolonged lifetime of the El Chichon sulfuric acid aerosol cloud, J. Geophys. Res., 92, 9825-9830, 1987.

The observe decay of the aerosol mixing ratio following the eruption of El Chichon appears to have been 20-30% slower than that following the eruption of Fuego in 1974, even though the suluric acid droplets were observed to grow to considerably larger sizes after El Chichon. This suggests the possible presence of a condensation nuclei and sulfuric acid vapor source and continued growth phenomena occurring well after the El Chichon eruption. It is proposed that the source of these nuclei and the associated vapor may be derived from annual evaporation and condensation of aerosol in the high polar regions during stratospheric warming events, with subsequent spreading to lower latitudes.

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Stone, R. S., E. G. Dutton, and J. R. Key, Properties and decay of Pinatubo aerosols in polar regions compared with tropical observations, Extended abstracts for the AMS 8th Conference on Atmospheric Radiation, 23-28 Jan. 1994, Nashville, TN, 1994.

The June 1991 eruptions of Mount Pinatubo ejected massive amounts of debris and sulfur dioxide gas into the stratosphere that were dispersed globally by upper-level winds. By March 1992 the spread of volcanic aerosols had reached the high latitudes. The initial radiative effects of Pinatubo were to cool the troposphere and warm the stratosphere. Though the stratosphere's opacity had peaked and is now diminishing, the climatic impacts of Pinatubo must still be fully assessed. The geographical distributions of the properties and time decay of the volcanic aerosols need to be known before accurate simulations of any volcanically-induced climate perturbations can be made on a global scale.

During April 1992 in situ measurements were made of the Arctic atmosphere using airborne techniques as part of the Fourth Arctic Gas and Aerosol Sampling Program (AGASP-IV). Spectral measurements of solar irradiance were made from near the surface into the stratosphere using handheld sunphotometers during several flights of the NOAA WP-3D aircraft. As part of the continuing program of NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL), routine measurements are made of the atmosphere's opacity at four baseline observatories located in Alaska, Hawaii, Western Samoa and at the South Pole. In this paper we use the AGASP-IV data to quantify the spectral optical depth and infer effective size distributions for the Pinatubo aerosols present in the Arctic, and compare these results with similar estimates made for Mauna Loa (MLO), Hawaii. In addition, we analyze the time series of the CMDL optical depth records to evaluate the decay of Pinatubo aerosols geographically.

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Stone, R. S., J. R. Key, and E. G. Dutton, Properties and decay of stratospheric aerosols in the Arctic following the 1991 eruptions of Mount Pinatubo, Geophys. Res. Lett., 20, 2359-2362, 1993.

Sunphotometer observations made from an aircraft several months after the June 1991 eruptions of Mount Pinatubo are used to quantify the spectral opacity of the Arctic stratrosphere. Ancillary surface-based measurements are presented in support of the aircraft data that show large increases in stratospheric optical depth attributed to the presence of volcanic aerosols. Visible optical depths greater than 0.2 were observed during flight segments flown above the tropopause. An inversion algorithm and the optical depth data are used to infer effective aerosol size distributions. The distributions tend to be bimodal, having a large-particle mode radius of about 0.50 m and a small-particle mode of higher concentration with radii less than 0.18 m. Surface measurements made during spring 1992 and 1993 are also used to estimate a time constant (e-folding time) of about 13.5 months assuming that the Arctic stratosphere's opacity decays exponentially; this estimate is larger than decay times observed following other major volcanic eruptions. Our results suggest that any climate perturbations in the Arctic caused by the eruptions of Pinatubo may be significant and will very likely persist longer than any volcanically-induced changes observed there during the past century.

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Barrow Radiation Climatology

Dutton, E. G., R. S. Stone, and J. J. DeLuisi, South Pole surface radiation balance measurements, April 1986 to February 1988, NOAA Data Rep. ERL ARL-17, Air Resources Laboratory, Silver Spring, MD, 49 pp., 1989.

This report presents details of, and data from, an ongoing solar radiation measurement program administered by NOAA/Geophysical Monitoring for Climatic Change from 1976 through 1983. The data include daily sums of downward global solar flux and discrete measurememts of direct beam solar flux in clear skies. Also included are the data for passive remote sensing of atmospheric aerosol properties, using the sun as a source. The remote sensing measurements were made in the spring of 1982 and 1983. Data are presented in reduced form such that direct application to research studies can be made.

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Dutton, E. G., An extended comparison between LOWTRAN7-computed and observed broadband thermal irradiances: Global extreme and intermediate surface conditions, J. Atmos. Oceanic Technol., 10, 326-336, 1993.

Differences between observed and LOWTRAN7-computed downward longwave irradiances were examined at each of four globally diverse locations for an entire year at each site. The final results are restricted to times determined to be completely or nearly cloud-free. The irradiances from 367 such times range from 60 to 435 W m-2, and results indicate that the modeled irradiances and those measured directly using a pyrgeometer agree to within 5 W m-2 at individual sites to within less than 0.2 W m-2 when averaged over all four sites, neglecting any site-specific biases. The standard deviations and standard errors associated with these results are roughly 10 and 1 W m-2, respectively. An unbiased estimate of the agreement between the model and observations results in a mean difference of 0.62 W m-2 with standard deviation of 5 W m-2 but an even larger 95% confidence interval because of the small sample size. The comparison variance can be logically ascribed to a number of different sources, including atmospheric variability and inhomogeneity, as well as to short-term instrument and LOWTRAN7 input variations. LOWTRAN7 and the observations agree better, in the mean, than the commonly accepted uncertainties for either would suggest. Maximum cloud radiative forcing at the surface for each site is quantified as a by-product of the comparison process.

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Dutton, E. G., J. J. DeLuisi, and D. J. Endres, Solar radiation at the Barrow GMCC baseline observatory 1976-1983, NOAA Data Rep. ERL ARL-6, Air Resources Laboratory, Boulder, CO, 112 pp., 1985.

Measured radiation budget (or balance) components with daily time resolution are presented for the U.S. South Pole station. The measurement project and the data reduction and summary procedures are described. Useful plots and tables are used to present the data in final form. Additionally, meteorological data, which could be useful in interpretation of the radiation measurements, are included. It is seen that, during the three peak solar months, there is a radiative gain of about 20 W m -2 by the surface (except for December 1987 where a small loss was recorded) whereas during the dark months there is an average loss of about 15 to 20 W m-2. Many transitory events of one to several days duration are also seen in the record. Radiation budget measurements are continuing at the site on an ongoing basis in an effort to establish a climatological record.

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Dutton, E. G., and D. J. Endres, Date of snowmelt at Barrow, Alaska, U.S.A., Arctic Alp. Res., 23, 115-119. 1991.

The date of snowmelt near Barrow, Alaska, for recent years is determined from radiometric in situ measurements of the tundra solar albedo. The snowmelt dates determined from the albedo measurements dispute recently published values based on routine visual observations at the Barrow National Weather Service (NWS) Office. Local heat- budget- altering effects of the village and its recent abrupt growth are suggested as the cause for the disagreement between the rural albedo and NWS "in town" observations. The open tundra albedo measurements combined with historical NWS observations suggest that there is no significant trend in the date of snowmelt near Barrow.

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Halter, B. C., and J. T. Peterson, On the variability of atmospheric carbon dioxide concentration at Barrow, Alaska during winter, Atmos. Environ., 15, 1391-1399, 1981.

Atmospheric carbon dioxide data obtained at Barrow, Alaska for the May-September period of 1978 were studied to understand the causes of the day-to-day and within-day variations. Sixteen instances of 24-h change in average CO2 concentration of from 15 to 50% of the annual range (approx. 14 ppm) were identified. Within-day variations of up to 50% of the annual range were noted. The variations were found to be related to local and synoptic scale meteorology interacting with local and regional sources and sinks of CO2. The results are consistent with an overall source of CO2 in the tundra of the Alaskan North Slope and a significant sink for CO2 in the ice-free areas of the seas bordering Alaska. The analysis provides an interpretation of the Barrow CO2 record which can be used in the selection of representative data for studying large scale trends.

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Harris, J. M., and J. D. W. Kahl, Analysis of 10-day isentropic flow patterns for Barrow, Alaska: 1985-1992, J. Geophys. Res., 99, 25,845-25,855, 1994.

Atmospheric transport patterns to Barrow, Alaska, during 1985-1992 were investigated using a newly developed isentropic air trajectory model. The new model features a layer-averaged mode that is activated whenever an air parcel traveling isentropically approaches the Earth's surface. A dynamic preprocessing program ensures that trajectories always arrive at a constant, predetermined altitude. Ten-day back trajectories arriving twice daily at 500, 1500, and 3000 m above sea level revealed no long-term trends in flow patterns during the 8-year period. Frequency of transport type was fairly stable from year to year, except in the anomalously warm year of 1989 when increased numbers of trajectories from the Aleutian region were observed. During the Arctic haze season, trajectories suggest that transport of pollution from north central Russia occurs near the surface (about 20% frequency), whereas that from northern Europe occurs at higher elevations (about 10% frequency).

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Herbert, G. A., E. R. Green, G. L. Koenig, and K. W. Thaut, Monitoring instrumentation for the continuous measurement and quality assurance of meteorological observations, NOAA Tech. Memo., ERL ARL-148, Air Resources Laboratory, Boulder, CO, 44 pp., 1986.

The NOAA/GMCC program was chartered to monitor the trends in those atmospheric constituents that can cause climate change. A four-observatory network was established and a 15-year-long data base has resulted for selected variables. At the inception, a central data-recording system was established at each observatory using minicomputers to compress and record the signals from monitoring instrumentation onto a computer-compatible magnetic tape. A new, distributed recording system using microprocessors has now been developed and is reported here. The STD BUS was selected as a means of internal computer communication, thus allowing a modular design that was tailored to the specific instrumentation. The resulting Control And Monitoring System (CAMS) operates three peripherals: a microterminal, dual cartridge tape drives for data recording, and a printer. An interactive multitasking version of Forth was adapted as the operating system software. Three seperate versions of CAMS were built and programmed. They control and monitor the carbon dioxide analyzer, aerosol and solar radiation instrumentation, and meteorological signals along with surface ozone instrumentation. In a 2-yr period, the three different CAMS were programmed and tested using microprocessor development hardware. During this period, 20 (3 plus a spare for each of the four observatories and a training facility) were assembled and tested. Early results show that CAMS recovers very well from power outages, resulting in minimum data losses. By distributing the system and limiting one CAMS to meteorological sensors, it has been possible to reduce significantly electromagnetic noise pickup. Improved data quality has resulted from use of flags and the display of scaled values.

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Key, J. R., A. J. Schweiger and R. S. Stone, Expected uncertainty in satellite-derived estimates of the surface radiation budget at high latitudes, J. Geophys. Res., 102, 15,837-15,847, 1997.

An analysis of spatial and temporal variations of the polar radiation budget will undoubtedly require the use of multispectral satellite data. How well we can estimate the radiation balance depends on how well we can estimate the physical and microphysical properties of the surface and atmosphere that directly affect it, e.g., surface temperature and albedo, cloud droplet effective radius, cloud optical depth, cloud thickness, and cloud height. Here we examine our current ablility to estimate the high-latitude surface radiation budget using visible and thermal satellite data. The method for estimating radiative fluxes incorporates estimates of surface and atmospheric parameters, so the accuracy with which these can be retrieved from satellite data is first assessed. The effects of errors in the estimates of these parameters on the surface net radiation during summer and winter are quantified, and the relative sensitivity of the net radiation budget to errors in individual parameters is assessed. The combined uncertainty is then determined and examined in light of validation data in the Arctic. The results show upper and lower bounds for the uncertainties between 7.9 and 41 W m-2 for instantaneous retrievals of net radiation. By far, the largest portion of the uncertainty in net radiation is associated with errors in the retrieval of surface temperature and surface albedo. Although improvements in retrievals are desirable, currently available methods can provide surface net radiation in the Arctic with uncertainties similar to those of surface-based climatologies.

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Key, J. R., R. A. Silcox, and R. S. Stone, Evaluation of surface radiative flux parameterizations for use in sea ice models, J. Geophys. Res., 101, 3839-3849, 1996.

The surface radiation budget of the polar regions strongly influences ice growth and melt. Thermodynamic sea ice models therefore require accurate, yet computationally efficient methods of computing radiative fluxes. In this study a variety of simple parameterizations of downwelling shortwave and longwave radiation fluxes at the Arctic surface are examined. Parameterized fluxes are compared to in situ measurements over an annual cycle. Results suggest that existing parameterizations can estimate the downwelling shortwave flux to within 2% in the mean, with a root-mean-square error (RMSE) of about 4% for clear skies and 21% for cloudy conditions. Parameterized longwave fluxes are accurate to within 1% in the mean, with RMSE values of 6% for both clear and cloudy skies. On the basis of these results, two parameterization schemes are recommended to estimate radiation forcings in sea ice models for Arctic applications.

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Stone, R. S., Variations in western Arctic temperatures in response to cloud radiative and synoptic-scale influences, J. Geophys. Res., 102, 21,769-21,776, 1997.

The analysis focuses on Barrow, Alaska, a site that is sensitive to changing conditions because it is located near cryospheric boundaries and is influenced by both extratropical and Arctic synoptic activity. Surface and upper air meteorological data for a 31-year period (1965-1995) are used to evaluate temperature variations as they relate to dynamical and radiative processes. Both annual and monthly analyses indicate a tendency toward warming overall. However, the annual warming is not monotonic over time and varies seasonally. Comparisons of temperature time series from four sites along the Siberian-Alaskan coastline show that Barrow is a representative site to evaluate climate change in the western Arctic coastal zone. Regionally, the warming is dominated by significant temperature increases during winter and spring, but cooling is indicated for autumn. These results are not entirely consistent with model predictions of a more uniform high-latitude warming during the cold season in response to increasing concentrations of greenhouse gases in the atmosphere. Rather, the observed changes are attributed to well-known natural processes that affect regional cloud distributions in response to changing circulation patterns. Coincident daily and hourly meteorological and radiation data are also used to demonstrate empirically how clouds modulate Arctic temperatures.

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Stone, R., T. Mefford, E. Dutton, D. Longenecker, B. Halter, and D. Endres, Surface radiation and meteorological measurements: January 1992 to December 1994, NOAA Data Report, ERL CMDL-11, 81 pp., 1996.

Measured surface radiation budget (or balance) components with daily and monthly time resolution are presented for the Barrow, Alaska, Observatory (BRW) operated by the Climate Monitoring and Diagnostics Laboratory (CMDL) of NOAA. CMDL's monitoring program and the data reduction and summary procedures are described. Tables and corresponding plots are presented showing annual cycles of several measured and derived radiation variables for three years, 1992, 1993, and 1994. In addition, ancillary meteorological data are included, which are useful for interpreting the radiation measurements and how they vary on time scales of days to years.

It is found that each year there is a net gain of surface radiation at BRW from the end of April until mid-September that peaks during July. Once snow covers the tundra in the autumn, the net surface radiation balance becomes negative until solar gain again exceeds the net loss of terrestrial radiation the following spring. Net radiative loss is greatest during November, December, or January depending on the response to changing meteorological conditions.

Transitory events can influence the record quite dramatically on short time scales. Possible trends in cloudiness and/or in sea ice concentration in the Arctic Ocean may be affecting the radiation regime of BRW on longer time scales. Preliminary analysis suggests that the year-to-year variability of parameters that characterize BRW's climate is significant. Radiation and meteorological observations continue to be made at BRW in an effort to establish a long-term, comprehensive data set. Continuous monitoring will provide opportunities to detect climate change in this region of the Arctic to validate remotely sensed geophysical parameters and to improve parameterization schemes used in models to simulate the climate of this region.

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Snowmelt Date

Dutton, E. G., and D. J. Endres, Date of snowmelt at Barrow, Alaska, U.S.A., Arctic Alp. Res., 23, 115-119. 1991.

The date of snowmelt near Barrow, Alaska, for recent years is determined from radiometric in situ measurements of the tundra solar albedo. The snowmelt dates determined from the albedo measurements dispute recently published values based on routine visual observations at the Barrow National Weather Service (NWS) Office. Local heat- budget- altering effects of the village and its recent abrupt growth are suggested as the cause for the disagreement between the rural albedo and NWS "in town" observations. The open tundra albedo measurements combined with historical NWS observations suggest that there is no significant trend in the date of snowmelt near Barrow.

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Foster, J. L., J. W. Winchester, and E. G. Dutton, The date of snow disappearance on the Arctic tundra as determined from satellite, meteorological station and radiometric in situ observations, IEEE Trans. Geosci. Remote Sens., 30, 793-798, 1992.

In this study satellite-derived snow cover maps for sites in Alaska, Canada, Scandinavia, and Siberia were employed to assess the date when snow disappeared on the Arctic tundra and to determine if the snow has been melting earlier in the spring in more recent years. Results show that for three of the four sites there has been a tendency toward earlier snowmelt during the 1980's. In Alaska, the satellite-derived date of snowmelt was compared to the date of snowmelt as observed at the Barrow meteorological station and a site near Barrow where radiometric in situ measurements were made for the last 5 years. The three data sources complement each other even though the satellite site is located 150 km from Barrow. One mechanism which could cause a trend toward earlier snowmelt in Alaska is the deposition of soot and particulates on the snow surface as a result of Arctic haze.

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Stone, R., T. Mefford, E. Dutton, D. Longenecker, B. Halter, and D. Endres, Surface radiation and meteorological measurements: January 1992 to December 1994, NOAA Data Report, ERL CMDL-11, 81 pp., 1996.

Measured surface radiation budget (or balance) components with daily and monthly time resolution are presented for the Barrow, Alaska, Observatory (BRW) operated by the Climate Monitoring and Diagnostics Laboratory (CMDL) of NOAA. CMDL's monitoring program and the data reduction and summary procedures are described. Tables and corresponding plots are presented showing annual cycles of several measured and derived radiation variables for three years, 1992, 1993, and 1994. In addition, ancillary meteorological data are included, which are useful for interpreting the radiation measurements and how they vary on time scales of days to years.

It is found that each year there is a net gain of surface radiation at BRW from the end of April until mid-September that peaks during July. Once snow covers the tundra in the autumn, the net surface radiation balance becomes negative until solar gain again exceeds the net loss of terrestrial radiation the following spring. Net radiative loss is greatest during November, December, or January depending on the response to changing meteorological conditions.

Transitory events can influence the record quite dramatically on short time scales. Possible trends in cloudiness and/or in sea ice concentration in the Arctic Ocean may be affecting the radiation regime of BRW on longer time scales. Preliminary analysis suggests that the year-to-year variability of parameters that characterize BRW's climate is significant. Radiation and meteorological observations continue to be made at BRW in an effort to establish a long-term, comprehensive data set. Continuous monitoring will provide opportunities to detect climate change in this region of the Arctic to validate remotely sensed geophysical parameters and to improve parameterization schemes used in models to simulate the climate of this region.

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Flight Details - AGASP IV

Herbert, G. A., P. J. Sheridan, R. C. Schnell, M. Z. Bieniulis, B. A. Bodhaine, and S. J. Oltmans, Analysis of meteorological conditions during AGASP-IV: March 30-April 23, 1992, NOAA Tech. Memo. ERL CMDL-5, Climate Monitoring and Diagnostics Laboratory, Boulder, CO, 118 pp., 1993.

The fourth Arctic Gas and Aerosol Sampling Program (AGASP-IV) was conducted over Alaska and the Beaufort Sea during March and April 1992. The NOAA WP-3D aircraft made nine flights. On the first eight flights special aerosol and gas sampling instrumentation was installed, and extensive time was spent over the pack ice. Measurements of wind, pressure, temperature, relative humidity, ozone, and condensation nucleus (CN) concentration were used to identify the air mass type, recent origin, and existence of pollution-derived aerosols, i.e., haze. While small patches of elevated CN concentrations and higher aerosol scattering coefficients were observed, significantly large regions, of the type found in previous AGASP missions, were not observed during this series. On most flights the CN concentrations in the troposphere were representative of the "clean" background conditions at this latitude. Significant concentrations of CN (CN>7000 cm-3) found above the tropopause on three fights indicated the presence of volcanic aerosol probably from the Pinatubo volcanic plume at high latitudes. In four instances low ozone concentrations suggest the destruction of ozone in the surface layer.

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