Discrete Propagation of Surface Fronts in a Convective Environment: Observations and Theory

Abstract

A case example demonstrating that surface fronts can propagate in a discrete manner is presented. The event occurred as a cold front encountered a mesoscale area of surface-based convectively generated cold air over the central United States. The surface front stalled when it reached the cold anomaly and underwent rapid frontolysis. At the same time, a prefrontal trough formed on the downstream boundary of the convective cold dome, about 300 km ahead of the original front, and underwent rapid intensification. Eventually, the original surface front became impossible to identify while the new boundary became the new surface front. A careful inspection of surface reports shows that a front did not pass through the area between the original surface front and the new surface front. However, analysis of rawinsonde and profiler data reveals that the midlevel frontal trough propagated continously. Thus, as the midlevel front moved continously over the cold dome, the surface frontal properties (e.g., pressure trough, wind shift, and thermal gradient) dissipated on one side of the cold air while simultaneously developing on the other side. A conceptual model of the discrete frontal propagation is presented. Simple hydrostatic arguments are applied to explain the sequence of events. Ahead of the front, moist convection generates a surface-based layer of anomalously cold air. Since hydrostatically high pressure is manifested beneath the cold dome, the surface frontal trough is effectively canceled by the high pressure anomaly. Meanwhile, locally lower pressure appears on the downwind side of the cold pool. As the midlevel front moves over the cold dome, the new surface trough deepens and the original surface frontal trough dissipates. Eventually, only the new trough remains. It is argued that discrete frontal propagation can occur in different environmental settings, but that it is generally induced by thermal anomalies in the prefrontal environment.