An Analysis of the Impact of a Split-Front Rainband on Appalachian Cold-Air Damming

Abstract

Appalachian cold-air damming (CAD) is characterized by the development of a cool, stable air mass that is advected southwestward along the eastern slopes of the Appalachian Mountains by low-level ageostrophic flow. Operational forecasters have identified the demise of CAD as a major forecasting challenge, in part because numerical weather prediction models have a tendency to erode the cold air too quickly. Previous studies have considered the role of clouds and precipitation in the initiation and maintenance of CAD; generally, precipitation is thought to reinforce CAD due to the cooling and stabilization resulting from evaporation. Here, the impact of precipitation on CAD during a situation where the lower-tropospheric air mass was near saturation prior to the arrival of precipitation is considered. Previous studies have indicated that the passage of a cold front can bring about CAD demise, as the synoptic-scale flow becomes northwesterly behind the front and low-level stable air is scoured. Additional complexity is evident in the case of split cold fronts (or cold fronts aloft). In these situations, precipitation bands are found well to the east of the surface cold front and may be accompanied by severe weather. Here, the impact of a split-front rainband on a mature CAD event from 14 February 2000 is investigated. The coastal front, marking the eastern boundary of the CAD region, made significant inland progress as the split-front rainband passed. Computations from Eta Model forecast fields revealed substantial latent heat release above the cold dome during the passage of the rainband. The CAD cold dome persisted longer in an MM5 model numerical simulation in which the effects of latent heat were withheld relative to both a full-physics control run and to observations. A third model simulation where the low levels of the cold dome were initially dried showed that once saturation occurred, the cold dome began to erode. Analysis of model output and observations suggests that, in this case, precipitation contributed to the retreat of the cold dome through lower-tropospheric pressure falls, an isallobaric wind response, and a resultant inland jump of the coastal front.