A Migratory Anticyclone Event during Project GALE

Abstract

A case study of a transitory anticyclone is conducted as part of the Genesis of Atlantic Lows Experiment (GALE) for the Intensive Observing Period (IOP) of 7?9 March 1986. The special GALE data networks were activated in anticipation of possible Appalachian cold air damming. Cold air damming did not occur and the purpose of the case study is to diagnose a nonevent so that processes associated with cold air damming can be better isolated. An important finding is that the configuration of the antecedent synoptic-scale flow is crucial in determining whether cold air damming will evolve and be sustained. In the 7?9 March 1986 case a surface anticyclone moved steadily southeastward from west of the Great Lakes to the Middle Atlantic coast and offshore as part of a progressive westerly flow regime aloft. The typical cold air damming signature consisting of 1) a slow moving surface anticyclone along the United States-Canada border with a ?U? shaped pressure ridge in the sea level isobars east of the Appalachian Mountains, 2) a split-flow regime in the middle and upper troposphere with a leading northern trough and a lagging southern trough, 3) a tendency for widespread precipitation within the shallow dome of trapped cold air with cooling locally reinforced by evaporation, 4) a mountain-parallel low-level jet down the pressure gradient. and 5) a moist southeasterly flow from the Atlantic Ocean above the trapped cold air dome?was not observed. Instead, a transient sea-level anticyclone crossed the central Appalachians and moved rapidly offshore in response to synoptic-scale forcing aloft. This prevented the establishment of a sustained low-level northerly flow of cool air cast of the mountains. While the lower third of the troposphere stabilized along the coast in response to initial cold air advection near the surface and subsidence warming near 700 mb, further cooling and stabilization was precluded by the absence of sufficient moisture to trigger the generation of widespread precipitation and resultant rain-cooled air.