Robert S. Stone

High-latitude surface temperature estimates from thermal satellite data

Abstract The surface temperature of the polar regions controls sea ice growth, snow melt, and surface-atmosphere energy exchange. However, our limited knowledge of polar surfaces and atmospheres has hampered the development of methods to estimate surface temperature with satellite data. In this article, clear-sky surface-temperature retrieval algorithms for rise with the Advanced Very High Resolution Radiometer (AVHRR) and the Along Track Scanning Radiometer (ATSR) for the Arctic and the Antarctic, over ocean and land, are presented. The methods are similar to those used in estimating sea and land surface temperatures but are developed with. data specific to the polar regions. An extensive validation analysis using an annual cycle of .surface meamarements gives accuracies in the range of 0.3–2.1 K, the larger errors being attributable to the spatially variable .surface of the validation area. For homogeneous surfaces the expected accuracy is sufficient for many climate process studies.

An Arctic Springtime Mixed-Phase Cloudy Boundary Layer Observed during SHEBA

Abstract The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, ...

Variations in western Arctic temperatures in response to cloud radiative and synoptic‐scale influences

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.

Recent Interannual Variations in Solar Radiation, Cloudiness, and Surface Temperature at the South Pole

Abstract Incoming global solar irradiance measured at the surface at the South Pole unexpectedly decreased steadily by 15% from 1976 through 1987 during the late austral summer season, whereas no trend is apparent for September through December. Februarys irradiance trend, − 1.24% yr−1 on the average, is statistically significant at greater than the 99.9% confidence level. The irradiance observations were made continuously with the same calibrated sensor and are confirmed by a second simultaneous solar irradiance measurement series. Associated changes in seasonal sky cover (clouds) and surface air temperature were also observed. Seasonally increasing cloud cover is directly associated with the decreasing irradiance trends, whereas temperatures show a warming trend significant only in March, followed by a cooling trend significant only in May. Cloudiness and temperature records for 32 years suggest that the observed cloudiness trend began in the late 1970s, while the temperature trends become apparent onl...

Evaluation of surface radiative flux parameterizations for use in sea ice models

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.

Expected uncertainty in satellite‐derived estimates of the surface radiation budget at high latitudes

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 ability 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.

Properties and decay of stratospheric aerosols in the Arctic following the 1991 eruptions of Mount Pinatubo

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 stratosphere. 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 dimodal, having a large-pArcticle mode radius of about 0.50 [mu]m and a small-particle mode of higher concentration with radii less than 0.18 [mu]. 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 stratospheres opacity decays exponentially; this estimate is larger than decay times observed following the major volcanic eruptions. These results suggest that any climate perturbations in the Artic 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.

Tropospheric temperature trends in the Arctic: 1958–1986

Arctic temperature trends in four tropospheric layers during the period 1958–1986 are examined through analysis of a comprehensive archive of Arctic upper air meteorological data. The goals of the study are to describe the trends in Arctic tropospheric temperatures and to provide verification data for model simulations of Arctic and global climate. This analysis extends the work of previous researchers by examining rawinsonde time series from a much denser distribution of stations than was previously available and by resolving the vertical distribution of tropospheric temperatures as well. Absolute trends of 3°C/30yr or larger were found, with both cooling and warming tendencies observed in all layers. The majority of the trends, however, are not statistically significant. Considerable regional and seasonal variability is observed. Trends at many stations in Eurasia and Greenland are highly sensitive to large positive anomalies during the period 1958–1963, which may be artifacts of the data. On the basis of our analysis, we conclude that greenhouse-induced warming is not detectable in the Arctic troposphere for the 1958–1986 period.

An analysis of 25 Years of balloonborne aerosol data in search of a signature of the subsonic commercial aircraft fleet

The University of Wyoming balloonborne condensation nuclei (CN) record for 1973–1997 was analyzed to determine possible effects of commercial aircraft on the 8.6–12.7 km altitude range of the atmosphere, which includes the primary commercial airlanes between 29 and 41 kft. Thin layers of highly concentrated CN are often observed in this region of the atmosphere. Generally, aircraft flight information is not available for past balloon soundings making it impossible to ascribe a source to any specific observed CN layers. However, a CN layer observed in March 1997 was traced to a particular aircraft thus supporting the hypothesis that at least some of the observed layers are contrail remnants. Using the Laramie data set, a method was developed to quantify the enhancement in CN concentration induced by aircraft in comparison with natural background levels. We estimate conservatively that the contribution of the commercial aircraft fleet in the vicinity of Laramie, Wyoming, averaged over the period of record, amounts to about 10% of the background CN concentration. The frequency of occurrence of the CN layers approximately doubled from 1980 to 1992.

No significant increase in long‐term CH4 emissions on North Slope of Alaska despite significant increase in air temperature

Continuous measurements of atmospheric methane (CH4) mole fractions measured by NOAAs Global Greenhouse Gas Reference Network in Barrow, AK (BRW), show strong enhancements above background values when winds come from the land sector from July to December from 1986 to 2015, indicating that emissions from arctic tundra continue through autumn and into early winter. Twenty-nine years of measurements show little change in seasonal mean land sector CH4 enhancements, despite an increase in annual mean temperatures of 1.2 ± 0.8°C/decade (2σ). The record does reveal small increases in CH4 enhancements in November and December after 2010 due to increased late-season emissions. The lack of significant long-term trends suggests that more complex biogeochemical processes are counteracting the observed short-term (monthly) temperature sensitivity of 5.0 ± 3.6 ppb CH4/°C. Our results suggest that even the observed short-term temperature sensitivity from the Arctic will have little impact on the global atmospheric CH4 budget in the long term if future trajectories evolve with the same temperature sensitivity.