Appalachian Cold-Air Damming

Abstract

Appalachian cold-air damming is investigated by means of 1) a 50-yr monthly climatology, 2) a synoptic case study of the event of 21–23 March 1985 and 3) an investigation of the flow structure and force balance within the cold dome. The climatology reveals cold-air damming is a year-round phenomenon in the southern Appalachians with the most frequent, prolonged and intense events occurring in winter (particularly December and March) when three-five events per month can be expected. Cold-air damming is least frequent and intense in July. The synoptic case study reveals that cold-air damming is critically dependent upon the configuration of the synoptic-scale flow. The cold dome can be identified by a “U” shaped ridge (trough) in the sea level isobar (thermal) patterns and the 930-mb height (temperature) fields representative of conditions at the base of the inversion overlying the cold dome. Differential horizontal and vertical thermal advection, as well as adiabatic and evaporative cooling, are responsible for the configuration of a strongly sloping inversion at the top of the cold dome and the pronounced baroclinic zone along the eastern edge of the cold dome. Evaporative cooling accounts for roughly 30% of the total cooling in parts of the dome, while adiabatic cooling explains a similar percentage of the cooling adjacent to the mountain slopes. An accelerated flow nearly parallel to the mountains within the cold dome is identified and shown to be linked to the evolution of the synoptic-scale pressure field. The mountain-parallel component of the pressure gradient force is the primary acceleration source. The force balance on the accelerated flow after cold dome formation is geostrophic in the cross-mountain direction and antitriptic in the along-mountain direction. A geostrophic adjustment process is triggered from the formation of a region of small-scale ridging against the mountain slopes as cold air is constrained by the mountains to remain along the eastern slopes. The tendency for the Coriolis force to turn the flow toward the mountain is negated and the flow within the cold dome is directed ready parallel to the mountains and down the large-scale pressure gradient. Cold dome drainage occurs with the advection of the cold air toward the coast in response to synoptic-scale pressure falls accompanying coastal cyclogenesis.