THE EASTERN UNITED-STATES SIDE-DOOR COLD-FRONT OF 22 APRIL 1987 - A CASE-STUDY OF AN INTENSE ATMOSPHERIC DENSITY-CURRENT

A case study is presented of an unusual cold front that affected the east coast of the United States on 22 April 1987. Noteworthy aspects of this front are its genesis behind a preexisting back-door cold front, its propagation to the southwest, the shallow nature of the cold air associated with it (<600 m), and the absence of precipitation. This front has been termed a ''side-door'' cold front because of the significant differences between it and typical back-door cold fronts, such as its direction of propagation and vertical structure. Weather conditions in the mid-Atlantic states changed abruptly from partly cloudy skies, light winds, and 25-degrees-30-degrees-C surface temperatures ahead of the front to a windy (gusts to 20 m s-1), low overcast regime with temperatures 10-degrees-15-degrees-C cooler behind the front. Analysis of satellite imagery, surface data, and sounding data revealed that the front possessed density-current structure during a portion of its lifetime. Observed front-relative flow, as well as agreement between computed and observed frontal velocity, support this conclusion. Results from this study demonstrate that the cold front evolved without the blocking and channeling effects of a major mountain range, as is typically the case for similar events around the world. It is hypothesized that the front formed in response to differential surface heating and friction along the New England coast. Similarly, the differential heating across the coastline contributed to the intensification of the front as well as to the evolution of the density-current structure later in the life history of the front. Results of this study relevant to several numerical simulations of similar events are discussed.