Discrete Frontal Propagation in a Nonconvective Environment

Abstract

Surface discrete frontal propagation in a wintertime, nonconvective environment is documented using conventional surface and upper-air data and simulated using the PSU?NCAR mesoscale model. Synoptic and mesoscale surface analyses show a cold front associated with a synoptic-scale low-pressure system propagating from northwest to southeast across the central United States. Apparently discrete frontal propagation occurs when the surface front dissipates and a new front forms approximately 500 km ahead of the original front, with no compelling evidence of frontal passage in the intervening space. Upper-air analyses indicate the infusion of three different airstreams into the frontal region, resulting in the formation of a ribbon of low static stability air parallel to and several hundred kilometers in advance of the original front. This static stability structure appears to be involved in the observed evolution of the front. The development of precipitation over the intervening zone between the old and new frontal positions suggests that precipitation-induced diabatic processes also played a role in the discrete frontal propagation. The numerical simulation captures the essential surface, upper-air, and precipitation features associated with the discrete propagation. Cross-section analyses of the simulated atmospheric fields indicate that the front propagated discretely only at the surface and in the lowest 200 hPa of the atmosphere, while the midtropospheric trough associated with the surface front propagated continuously though the region. The cross sections also indicate that the vertical winds associated with the frontal system adjust very quickly to the new frontal location while the horizontal winds and mass fields adjust more slowly. Analysis of frontogenetical forcing verifies that the new surface front forms at the expense of the original front. A careful examination of the temperature budgets within the simulation shows that the mass field redistribution associated with the discrete frontal propagation occurred as a result of the lifting of a strong temperature inversion in the prefrontal environment combined with precipitation induced diabatic cooling. Based on the results of the model simulation, a conceptual model of discrete frontal propagation is presented that incorporates the observed and simulated sequence of events.