Jonathan D. W. Kahl

20th-Century Industrial Black Carbon Emissions Altered Arctic Climate Forcing

Black carbon (BC) from biomass and fossil fuel combustion alters chemical and physical properties of the atmosphere and snow albedo, yet little is known about its emission or deposition histories. Measurements of BC, vanillic acid, and non–sea-salt sulfur in ice cores indicate that sources and concentrations of BC in Greenland precipitation varied greatly since 1788 as a result of boreal forest fires and industrial activities. Beginning about 1850, industrial emissions resulted in a sevenfold increase in ice-core BC concentrations, with most change occurring in winter. BC concentrations after about 1951 were lower but increasing. At its maximum from 1906 to 1910, estimated surface climate forcing in early summer from BC in Arctic snow was about 3 watts per square meter, which is eight times the typical preindustrial forcing value.

Air mass trajectories to Summit, Greenland: A 44-year climatology and some episodic events

The seasonal variation in atmospheric transport patterns to Summit, Greenland, is examined using a 44-year record of daily, 10-day, isobaric back trajectories at the 500-hPa level. Over 24,000 modeled trajectories are aggregated into distinct patterns using cluster analysis. Ten-day trajectories reaching Summit are longest during winter, with 67% extending upwind (westward) as far back as Asia or Europe. Trajectories are shortest during summer, with 46% having 10-day origins over North America. During all seasons a small percentage (3–7%) of trajectories originate in west Asia/Europe and follow a meridional path over the Arctic Ocean before approaching Summit from the northwest. Trajectories at the 700-hPa level tend to be shorter than at 500 hPa, with many of the 700-hPa trajectories from North America tracking over the North Atlantic and approaching Summit from the south. The long-range transport climatology for Summit is similar to a year-round climatology prepared for Dye 3, located 900 km to the south [Davidson et al., 1993b]. An analysis of several aerosol species measured at Summit during summer 1994 reveals examples of the usefulness and also the limitations of using long-range air trajectories to interpret chemical data.

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...

Analysis of 10-day isentropic flow patterns for Barrow, Alaska : 1985-1992

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 Earths 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).

A cautionary note on the use of air trajectories in interpreting atmospheric chemistry measurements

Abstract The limitations of air trajectory models are reviewed, and suggestions are made regarding their appropriate use in interpreting atmospheric chemistry measurements. Single trajectories are shown to be potentially unrepresentative in the presence of large wind shears.

Uncertainty in Trajectory Calculations Due to Low Resolution Meteorological Data

Abstract Meteorological observations conducted during the Cross Appalachian Tracer Experiment (CAPTEX) were utilized to quantify the uncertainty in boundary layer trajectory calculations due to low-resolution meteorological data [the current National Weather Service (NWS) rawinsonde network). Evaluation of several spatial and temporal interpolation techniques against high-resolution measurements revealed mean absolute errors of 2–4 m s−1 in estimation of horizontal wind components. A trajectory of errors procedure is introduced that allows the quantification of probable errors in transport calculation due to imprecise interpolation. Our results, based on the observed distributions of spatial and temporal interpolation errors during CAPTEX, indicate dust boundary layer trajectories calculated using the current NWS network with 12 h resolution contain a 50% chance of exceeding horizontal displacement errors of 350 km after 72 h travel time. An increase in spatial resolution is shown to improve the accuracy ...

On the prediction of trajectory model error

A methodology for the development of an empirical, predictive model for forecasting trajectory model error is described. Trajectory model error is assumed to be a function of the complexity of the meteorological scenario under study. This complexity is quantified using a meteorological complexity factor (MCF), defined by the dispersion of a set of stochastic trajectories generated by random perturbations superimposed upon an observed wind field. Comparison between published trajectory model error data for specific cases and the corresponding MCFs would yield a means of forecasting trajectory model error. A weak relationship is found between MCFs and trajectory model errors for 22 cases, but a large degree of scatter precludes the development of a predictive model based on the MCF. Several explanations for this result are presented, including possible limitations in both the definition of MCFs as well as in the trajectory error data. Suggestions are given for further efforts to develop a predictive model for trajectory errors.

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.

Spatial resolution of a transport inversion technique

A spatial calibration for conditional frequency analysis, the inversion technique applied to routine back trajectories to study the association between geographical regions and conditions at a receptor, is discussed for trajectories arriving at Hopi Point, Arizona. Several 0.5° × 0.5° cells were selected at three different distances from Hopi Point. These cells were used to mimic sources, with air residing over them considered contaminated by imaginary emissions. The distribution of back trajectory segment endpoints over a latitude-longitude grid was then examined as a function of the trajectories sojourn over each imaginary source cell on their way to the receptor. The areal extent of significant associations, evaluated for different seasons and the various ideal source positions, suggests that this inversion technique can resolve the direction of potential sources but has limited resolution for their radial distance from the receptor.

A tracer calibration of back trajectory analysis at the Grand Canyon

The potential of routine back trajectory analyses to locate sources of contaminants in air at Grand Canyon is investigated with data on methylchloroform (CH 3 CCl 3 ) concentrations collected during the Subregional Cooperative Electric Utility, Department of Defense, National Park Service, and Environmental Protection Agency Study(SCENES). Following a now-standard approach, the distribution of back trajectory segment endpoints over a latitude-longitude grid is examined as a function of measured concentrations at the fixed monitoring site. Grid cells in which segment endpoints are preferentially associated with high concentrations are then identified as candidate emissions sources. The method correctly identifies southern California as a source of CH 3 CCl 3 . An objective rationale is developed for screening out chance associations, attributable to statistical fluctuations, and the results are evaluated for real and hypothetical tracers with known, simple distributions.