A DETERMINATION OF CHARACTER AND FREQUENCY CHANGES IN AIR MASSES USING A SPATIAL SYNOPTIC CLASSIFICATION

Of the numerous climate change studies which have been performed, few of these have analyzed recent trends using an air mass-based approach. The air mass approach is superior to simple trend analysis, as it can identify patterns which may be too subtle to influence the entire cli mate record. The recently-developed 'spatial synoptic classification' (SSC) is thus used to identify trends over the contiguous United State s for summer and winter seasons from 1948 to 1993. Both trends in air mass frequency and character have been assessed. The most noteworthy t rend in frequency is a decline in air mass transitional days (TR) duri ng both seasons. In winter, decreases of up to 1% per decade are noted in parts of the central U.S. Other notable trends include a decrease in moist tropical (MT) air in winter, and an increase in MT in summer over the southeastern states. Numerous national and local air mass cha racter changes have been uncovered. A large overall, upward trend in c loudiness is noted in summer. All air masses feature an overnight incr ease, yet afternoon cloudiness increases are generally limited to the three 'dry air masses'. Also in summer, a significant warming and incr ease in dew point of MT air has occurred at many locales. The most pro found winter trend is a large decrease in dew point (up to 1.5 degrees C per decade) in the dry polar (DP) air mass over much of the eastern states. (C) 1998 Royal Meteorological Society.