The dynamics of heat lows in simple background flows

Abstract

AbstractA three‐dimensional numerical model is used to study the evolution of an idealized heat low that develops over an isolated level land mass surrounded by sea. Calculations are carried out without a background flow and for simple background flows. In all cases, the flow shows a significant diurnal variation in which the sea‐breeze circulations are the prominent feature during the daytime and the nocturnal low‐level jet is the prominent feature at night. If the land area is large enough so that the sea breezes do not cover it entirely during the diurnal cycle, strong convergence associated with the low‐level jet leads to the formation of intense, but shallow, cold fronts along the inland boundaries of sea‐breeze air. These fronts decay rapidly after sunrise when surface heating leads to renewed vertical mixing, which destroys the low‐level stable layer.The presence of a uniform easterly flow leads to the formation of a west‐coast trough, similar to the situation commonly observed over Western Australia. The trough broadens during the day due to the heating of the land, while at night it sharpens and again frontogenesis occurs at low levels near the leading edge of the sea breezes to form shallow cold fronts. In these calculations there is an east–west asymmetry in the sea‐breeze circulations: the cool air behind the west‐coast sea breeze is shallower than that behind the east‐coast sea breeze, but the vertical circulation associated with the west‐coast sea breeze is deeper. The east‐coast sea‐breeze front penetrates further inland, but is more diffuse and is recognizable more by its signature in the relative vorticity than by that in the horizontal temperature gradient.The presence of a horizontal shear‐flow leads to the deformation of the heat trough, a process that appears to play an important role in the formation of cold fronts over central Australia. Again the calculations show the formation of shallow fronts over the land during the night, which frontolyse rapidly after sunrise. Despite the idealized nature of the calculations, the associated patterns of low‐level vorticity, divergence and horizontal temperature gradient that develop overnight show remarkable similarity to those observed over central Australia. Indeed, the calculations help to interpret the observations of nocturnal frontogenesis over this region. Copyright © 2005 Royal Meteorological Society.