Development of the Upper Level Synoptic Index

This study developed a methodology to temporally classify large scale, upper level atmospheric conditions over North America, utilizing the newly-developed Upper Level Synoptic Index (ULSI). Four meteorological variables: geopotential height, specific humidity, and uand vwind components, at the 500 mb level over North America were obtained from the NCEP/NCAR Reanalysis Project data set for the period 1965–1974. These data were subjected to principal components analysis to standardize and reduce the data set, and then an average linkage clustering algorithm identified groups of observations with similar flow patterns. The procedure yielded 16 clusters. These flow patterns identified by the ULSI typify all patterns that are anticipated to be observed over the study area. Additionally, the resulting cluster calendar for the period 1965–1974 showed that the clusters are generally temporally continuous. The ULSI calendar of synoptic conditions can be used to identify situations that lead to periods of extreme weather and to explore transport of airborne particles and biota across North America.