Gh. Bryan et Jm. Fritsch, Discrete propagation of surface fronts in a convective environment: Observations and theory, J ATMOS SCI, 57(13), 2000, pp. 2041-2060
A case example demonstrating that surface fronts can propagate in a discret
e manner is presented. The event occurred as a cold front encountered a mes
oscale area of surface-based convectively generated cold air over the centr
al United States. The surface front stalled when it reached the cold anomal
y and underwent rapid frontolysis. At the same time, a prefrontal trough fo
rmed on the downstream boundary of the convective cold dome, about 300 km a
head of the original front, and underwent rapid intensification. Eventually
, the original surface front became impossible to identify while the new bo
undary became the new surface front.
A careful inspection of surface reports shows that a front did not pass thr
ough the area between the original surface front and the new surface front.
However, analysis of rawinsonde and profiler data reveals that the midleve
l frontal trough propagated continously. Thus, as the midlevel front moved
continously over the cold dome, the surface frontal properties (e.g., press
ure 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 anomalou
sly 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 dissipa
tes. Eventually, only the new trough remains. It is argued that discrete fr
ontal propagation can occur in different environmental settings, but that i
t is generally induced by thermal anomalies in the prefrontal environment.