Russell C. Schnell

Low-Level Temperature Inversions of the Eurasian Arctic and Comparisons with Soviet Drifting Station Data

Abstract Seasonal and regional variations in characteristics of the Arctic low-level temperature inversion are examined using up to 12 years of twice-daily rawinsonde data from 31 inland and coastal sites of the Eurasian Arctic and a total of nearly six station years of data from three Soviet drifting stations near the North Pole. The frequency of inversions, the median inversion depth, and the temperature difference across the inversion layer increase from the Norwegian Sea eastward toward the Laptev and East Siberian seas. This effect is most pronounced in winter and autumn, and reflects proximity to oceanic influences and synoptic activity, possibly enhanced by a gradient in cloud cover. East of Novaya Zemlya during winter, inversions are found in over 95% of all soundings and tend to be surface based. For all locations, however, inversions tend to he most intense during winter due to the large deficit in surface net radiation. The strongest inversions are found over eastern Siberia, and reflect the ef...

Long‐range transport of anthropogenic aerosols to the National Oceanic and Atmospheric Administration baseline station at Mauna Loa Observatory, Hawaii

Size-segregated measurements of aerosol mass and composition are used to determine the composition and seasonal variations of natural and anthropogenic aerosols at Mauna Loa Observatory (MLO) from 1993 through 1996. Although the springtime transport of Asian dust to MLO is a well-documented phenomenon, this study shows that fine anthropogenic aerosols, including sulfur, black carbon, and enriched trace metals such as As, Cu, Pb, and Zn, are also routinely transported to MLO each spring. It is estimated that at least one third of the sulfate measured at MLO during the spring is anthropogenic. In addition, indirect measurements indicate that the organic aerosol concentrations are often comparable to the sulfate concentrations. This study also combines size- and time-resolved aerosol composition measurements with isentropic, backward air-mass trajectories and gas measurements of 222Rn, CH4, CO, and CO2 to identify some potential source regions of the anthropogenic aerosols. Three types of long-range transport episodes are identified: (1) anthropogenic aerosols mixed with Asian dust, (2) Asian pollution with relatively small amounts of soil dust, and (3) biomass burning emissions from North America. This study shows that anthropogenic aerosols and gases can be efficiently transported to MLO from both Asia and North America during the spring.

Tropospheric low‐level temperature inversions in the Canadian Arctic

Abstract Climatological characteristics of the low‐level tropospheric temperature inversion in the Canadian Arctic are examined using 10–40 year records of upper‐air meteorological data. Inversions at the northern sites are primarily surface‐based in winter, and elevated from mid‐spring through summer. At the southern sites, a bimodal pattern is observed with surface‐based inversions occurring during late summer, as well as during winter. From comparisons of our results with other published climatologies, it appears that this bimodal pattern reflects interactions between short‐ and long‐wave radiation, synoptic activity and snowmelt. Maxima in inversion depth and temperature difference across the inversion layer occur in February and March; minima occur in August and September. The annual progression of inversion characteristics closely follows the annual pattern of clear‐sky percentages, reflecting the controlling influence of cloud and clear‐sky radiative forcings on the inversion layer.

Aerosol particles in the Kuwait oil fire plumes: Their morphology, size distribution, chemical composition, transport, and potential effect on climate

Airborne aerosol samples were collected with an impactor in the Kuwait oil fire plumes in late May 1991. A transmission electron microscope was used to examine the morphology and size distribution of the particles, and an X ray energy spectrometer was used to determine the elemental composition of individual particles. A chemical spot test was used to identify particles containing sulfate. The results show that the dominant particles were (1) agglomerates of spherical soot particles coated with sulfate, (2) cubic crystals containing NaCl and S04=, (3) irregular-shaped dust containing Si, Al, Fe, Ca, K, and/or S, and (4) very small ammonium sulfate spherules. The concentrations of small sulfate particles increased at higher levels or greater distances from the fire, suggesting the transformation of SO2 gas to sulfate particles by photooxidation followed by homogeneous nucleation. The number of soot, salt, and dust particles that were coated with sulfate increased farther from the fire, and the thickness of the coating increased with altitude. This suggested that gas-to-particle conversion had occurred by means of catalytic oxidation combined with heterogeneous nucleation during the plume dispersion. Because the sulfate coating can modify the hydrophobic surfaces of soot and dust particles to make them hydrophilic, most of the particles in the plume apparently were active cloud condensation nuclei that could initiate clouds, fog, and smog, which in turn could affect regional surface temperature, air quality, and visibility. Long-range air trajectories suggested that some aerosols from the fires could have transported to eastern Asia. It seems possible (but is presently unproven) that a severe flood in China in June was influenced by aerosols from the plumes.

In Situ Meteorological Sounding Archives for Arctic Studies

Abstract Two new databases containing Arctic in situ meteorological soundings have been constructed and are now available for distribution to interested researchers. The Historical Arctic Rawinsonde Archive is a comprehensive collection of over l.2 million rawinsonde soundings north of 65°N.For most stations the record begins in l958 and extends to 1987; however, for some stations the record begins as early as 1948. The Ptarmigan Dropsonde Archive contains more than 10000 lower-tropospheric soundings over the Beaufort Sea and western Arctic Ocean during the period 1950– 1961.

Theoretical heights of buoyant convection above open leads in the winter Arctic pack ice cover

A fetch-dependent boundary-layer model, driven by observed temperature sounding data, is used to examine theoretical heights of buoyant convection (H) above open leads in the wintertime pack ice of the central Arctic. Assuming wet adiabatic ascent with no entrainment or friction, H is estimated as the height at which the model-predicted equivalent potential temperature at saturation above a lead (θel) intersects with the same value of equivalent potential temperature at saturation (θe) derived from vertical sounding profiles. H increases with increasing lead width. For a 1000-m lead, the widest which can be reasonably expected for the central Arctic, the median value of H is approximately 1000 m, slightly below the median top of the low-level Arctic temperature inversion layer. While H shows large variability, events of convection up to 4 km, as recently observed from lidar backscatter data, appear to be fairly rare. First, these events require an open lead of at least 10,000 m. Second, while H tends to be largest under conditions of low surface wind speed, low surface temperature, and a weak low-level temperature inversion, this combination appears to be atypical of Arctic conditions. Third, while the meteorological conditions that should favor the development of open leads tend to minimize H, conditions favoring large H are also those in which any newly developed leads will quickly ice over.

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.

Aerosol and lidar measurements of hazes in mid-latitude and polar airmasses

Abstract Airborne lidar and in situ measurements of hazes were obtained in March 1986 during research flights from the East Coast of the United States to Baffin Island, Canada. Mid-latitude and polar airmasses, clearly separated by a frontal zone, displayed distinct differences in aerosol characteristics and in the vertical structure of aerosol layers. The data illustrate the differences between freshly polluted, mid-latitude air and clean and hazy polar air. Particle size spectra in the mid-latitude airmass contained distinct nucleation (0.01–0.1 μm diameter) and accumulation (0.1–1.0 μm diameter) modes, while in the polar airmass there was generally only an accumulation mode. The modal diameter of particles in the accumulation mode increased with increasing particle concentration in the polar airmass. Differences in the shapes of the particle size spectra measured in the two airmasses can be explained by differences in the sources and lifetimes of the particles. Hazes observed by a 1.064 μm wavelength lidar in the mid-latitude airmass were relatively diffuse, while those in the polar airmass appeared in multiple thin laminae. Measurements of the wind speed profile suggest that the multi-layered structure of the hazes in the polar airmass was due to the advection of thin, hazy regions into the generally clean polar airmass and by the extreme thermal stability of the lower troposphere.

Radiosonde Observations from the Former Soviet “North Pole” Series of Drifting Ice Stations, 1954–90

An historical archive of over 25 000 radiosonde observations from the former Soviet “North Pole” series of drifting ice stations has been compiled and made available to interested researchers. This archive is the only long-term set of meteorological sounding data over the Arctic Ocean. The digital archive is a result of the multiyear, collaborative efforts of a group of United States and Russian scientists and keypunch operators working under the auspices of Working Group VIII, an area of study within the United States–Russian Federation Agreement for Protection of the Environment and Natural Resources. The archive contains soundings from 21 drifting stations over the period 1954–90 and is being distributed by the National Snow and Ice Data Center in Boulder, Colorado.

Measured and calculated optical property profiles in the mixed layer and free troposphere

Nearly simultaneous measurements of the physical and optical properties of mixed layer and free tropospheric aerosols near Boulder, Colorado, were made on several occasions using aircraft, balloon, and ground-based sensors. This effort (Front Range Lidar, Aircraft, and Balloon experiment (FRLAB)) was conducted with the purpose of obtaining a diverse, self-consistent data set that could be used for testing optical model calculations based on measured physical characteristics such as apparent size distribution, composition, and shape. It was found that even with the uncertainties involved, the model predictions are in good agreement with the measurements in the visible and near infrared wavelength regions. At CO2 lidar wavelengths there is considerably more uncertainty in both the calculated and measured values; however, within the estimated errors there appears to be satisfactory agreement except for the highest free tropospheric layer studied. The results also indicate that during FRLAB the aerosol in the boundary layer and free troposphere behaved as spherical particles for optical modeling purposes. The utility of the observations for determining the extinction-to-backscatter ratio relevant to aerosols in the boundary layer and free troposphere is described with typical measured values being in the 20 to 30 sr range.