18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Thursday, 2 August 2001: 11:30 AM
Detection and Evaluation of the Effects of Split Fronts on the Evolution of Appalachian Cold Air Damming
Michael J. Brennan, North Carolina State University, Raleigh, NC; and S. E. Koch and G. M. Lackmann
Poster PDF (1.1 MB)
The conceptual model of the split front (SF) marks a departure from the more traditional model of the precipitation distribution near a mid-latitude cyclone. A split front occurs as a mid-level baroclinic zone moves ahead of the surface cold front. The SF is associated with a rainband (sometimes convective) that is often located 100-300 km ahead of the surface cold front. Research has been undertaken with the goal of identifying optimum SF detection techniques, including detailed analysis of surface and upper-air data, numerical model output, satellite and Doppler radar. Thermally retrieved geostrophic temperature advections from velocity azimuth display (VAD) winds from WSR-88D radar are demonstrated to have utility in SF detection.

It is hypothesized that the precipitation associated with the split front can influence the evolution of the cold-air damming (CAD) east of the Appalachian Mountains. Hydrostatic pressure fluctuations associated with latent heat release from the SF rain band may affect the vertical and horizontal cold dome structure, and can affect the movement of the coastal front that often marks the eastern boundary of the cold dome during damming events. Evaporative cooling associated with a SF rain band can reinforce the cold dome, slowing erosion. These hypotheses have been investigated by computing latent heat release and pressure tendencies via two independent methods, utilizing observed surface rainfall, and a parameterized latent heating computed from Eta model grids. Preliminary results demonstrate that surface pressure falls are sufficiently large to explain observed alterations in cold dome structure during three cases. Due to the strong sinking motion and mid-level drying behind the SF, the passage of the surface cold front is often without precipitation. Thus, the potential impacts of the SF and its interactions with CAD on the forecasting of sensible weather are significant. Primary forecasting considerations relating to the SF include timing of precipitation, the potential for severe weather with elevated convection above the cold air (where it may not normally be expected), and the potential for rapid surface temperature fluctuations accompanying the movement of the coastal front.

The importance of the SF in a CAD event is demonstrated with a representative case from 13-14 February 2000. During this event, a well defined SF and is associated rain band developed in the Southern Plains and moved across the Southeastern U.S. The SF reached the Appalachians as a strong CAD event was beginning to erode and weaken. The SF rainband produced strong to severe convection well ahead of the surface cold front in the Mississippi Valley and Deep South, and thunderstorms were observed in the CAD region. During the time of the SF rainband passage, large pressure falls were observed along the margin of the CAD region and the coastal front made significant inland progress. The VAD wind profile thermal retrieval technique showed a large area of mid-level cold advection at nearby WSR-88D radar sites as the split front passed overhead. The surface cold front passed these areas several hours later without precipitation.

Supplementary URL: