Linking air mass analysis to daily and monthly mid‐tropospheric flow patterns

Air mass analysis, though widely used in early atmospheric studies, has seldom been employed in recent decades. In part, this may be due to the lack of agreement on a quantitative air mass classification technique. Synoptic analyses (of which air mass analysis is one example) have much potential for examining the relationships among a variety of surface processes and climate. Schwartz (1991) proposed an integrated air mass classification system that was able to overcome some of the limitations of previous methods. This study further validates that approach by quantitatively linking these air masses to 500‐hPa height and surface pressure patterns. Cluster analysis was used with daily and average monthly air mass distributions in the north‐central USA to identify similar groups. For the daily analyses, average 500‐hPa height, surface pressure, and air mass percentage coverage patterns were determined for the groups in each season. In the monthly analyses, transects of 500‐hPa height difference were examined with the multiple analysis of variance (MANOVA) technique in order to detect significant changes relative to the clusters. Our results show that: (i) integrated‐method air mass percentage coverage distributions can be related quantitatively to a small number of meaningful 500‐hPa height and surface pressure patterns in all seasons; (ii) key transects can be used to describe changes in average monthly air mass dominance and 500‐hPa flow patterns from one year to another; and (iii) for the period 1958–1981, cool air masses decreased in frequency over the study area during spring and summer.