The dynamics of heat lows in simple background flows

Abstract

SUMMARYA 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.

A review of research on the dry season mesoscale meteorology of northern Australia

“…Rácz and Smith (1999) investigated also the effects of differing land area and Coriolis parameter on various aspects of the heat low, but gave little attention to the overlying anticyclone. In a subsequent paper, the two authors extended their model to examine the effects of orography on heat low formation and structure (Reichman and Smith (2003)) and the model was extended further by Spengler et al (2005) to examine the effects of simple background flows on the heat low.…”

“…Calculations for a heat low in simple background flows were carried out by Spengler et al (2005). Like the case without a background flow, 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.…”

The dynamics of heat lows over flat terrain

“…Other changes are those incorporated by Spengler et al (2005), who used a bi-harmonic (∇ 4 ) diffusion and higher horizontal resolution (25 km instead of 100 km) to provide an improved representation of the sea-breeze circulation. In contrast to RS99, we consider flow over a circular island instead of a square one, where the radius of the island is chosen to be 400, 800 or 1200 km.…”

“…Over the last two and a half decades there have been several modelling studies of heat lows, some in idealized flow configurations (Leslie, 1980;Fandry and Leslie, 1984;Kepert and Smith, 1992;Gaertner et al, 1993;Adams, 1986Adams, , 1993RS99;Reichmann and Smith, 2003;Spengler et al, 2005;Zängl and Chico, 2006) and others related to the heat low in specific regions (Leslie and Skinner, 1994;Alonso et al, 1994;Portela and Castro, 1996;Hoinka and Castro, 2003).…”

“…Recently, Spengler et al (2005) extended the RS99 model to investigate the effects of simple basic flows on the dynamics of heat lows. During the course of that study, the first author discovered some errors in the model initialization and in the numerical implementation of the radiation scheme by RS03 and also errors in R94.…”