Trough-Merger Characteristics over North America

Abstract

Split flow, defined by the presence or two separate westerly airstreams in the mid- and upper troposphere, is common in middle latitudes. Occasionally, individual troughs in the separate branches of the westerlies will merge into one trough with a single vorticity maximum. Illustrative examples are used to demonstrate the significance of the trough merger process to the cyclogenesis process. The results of a subjective climatological study revealed the existence of 21 merger events across North America based upon the amalgamation of separate 500-mb vorticity centers. Eighteen of these occurred during the 1978?85 sampling period and three were added from other years. The trough merger cases were found to occur only in central and eastern North America east of 100°W and north of 35°N. Trough merger locations varied seasonally with the position of the principal tropospheric baroclinic zone. A composite of 17 trough-merger events demonstrated that the orientation of the 500-mb trough axis changed from northern-southwest to northwest-southeast (negative tilt) while the trough amplitude nearly doubled in the 24 hours preceding trough merger. Explosive cyclogenesis occurred in 14 out of 21 events in response to the development of the negatively tilted 500-mb trough associated with the merger process. The trough merger process was not well simulated by the then operational limited fine mesh (LFM) prediction model with 12 (2) of the 21 events correctly predicted 24 h (48 h) in advance. Finally, the physical implications of the climatological findings are discussed.