An Objective Technique for Detecting Large-Scale Tilted Ridges and Troughs and Its Application to an East Asian Cold Event

The present study identified the large-scale tilted ridge and trough (LSTR and LSTT, respectively) axes of the midtropospheric circulation over mid- and high-latitude regions using an objective identification technique that has not previously been applied. In this method, the identification procedure classified contours of 500-hPa height (Z500) fields into three types: the circumpolar wavy contour, the circumpolar contour containing the meridionally overturned (or wave breaking) part, and the locally closed contour. The ridge and trough points were defined on these three types of contours and, subsequently, the ridge or trough axes were identified after connecting successively the nearest ridge or trough points of the neighboring contours under a minimum distance constraint. The performance of the identification method was tested in the daily Z500 fields during 15 November 2011-15 March 2012. The hit rate, false-alarm ratio, and threat score of the method in test reveal that it performs well with a reasonably good skill. An examination of the wave-breaking features during the same period also suggests that the method performs well in the identification of LSTRs and LSTTs for the meridionally overturned parts of the Z500 contours. This objective technique was also applied to an extensive and persistent cold event over East Asia. Results show that the horizontal extent of the Siberian high corresponds well with the zonal extents of the identified LSTR and LSTT. The identification method in the present study might be useful for identifying the key circulation systems associated with extensive and persistent cold air outbreaks during winter.