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The climatological characteristics of low-level tropospheric temperature inversion along the Alaskan Arctic coast are examined from a 10-year record of surface and upper-air meteorological data at Barrow and Barter Island, Alaska. The meteorology at the two stations is found to be remarkably similar; surface-based inversions occur nearly half of the time, but even in the winter months inversions are frequently based up to 200 m above the surface. Median inversion depths range from 250 m to 850 m, with median temperature differences across the inversion ranging from 2°C to 11°C. In March and April when the inversion is strongest, the lowest levels frequently erode. This indicates that the transient character of the inversion reduces its effectiveness as a barrier to vertical mixing. The inversion depth closely follows the annual cloud-cover cycle, demonstrating that the development and maintenance of the inversion is a result of complex interactions between radiative forcing, synoptic activity, and sea-ice dynamics.KEY WORDS Temperature inversion Arctic meteorology Arctic boundary layer.

Abstract. 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.

The climatology of long-range atmospheric transport to the Mauna Loa Observatory (MLO) is examined through an analysis of 10-day isobaric back trajectories for the period 1981-1988, computed using the National Meteorological Center's analyzed gridded winds. The trajectories are classdied into distinct transport patterns through the use of cluster analysis, a procedure that avoids the somewhat arbitrary classification by compass sector and instead searches for meaningful groupings within the data themselves. The clustering procedure is performed separately for yeafly and monthly sets of 500-and 700-hPa trajectories to examine both year-to-year and month-to-month variability in flow characteristics. The dominant transport features are quite similar from year to year, and include frequent summertime easterly flow associated with the trade winds and strong, wintertime westerly flow close to the mean axis of the subtropical jet stream. Veering westerly patterns that approach MLO from the north and northeast dominate during winter/summer transition periods. Other noteworthy features include recurring air mass transport from Saharan and Asian desert regions, infrequent interhemispheric flow from as far as 10øS, and occasional flow from high northern latitudes. Ten-day back trajectories that pass over anthropogenic pollution source regions in western North America are rare.

Climatological characteristics of thé law-level tropospheric température inversion in thé Canadian

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