The dynamics of heat lows over elevated terrain

Orographic effects on the dynamics of heat lows are investigated in an idealized flow configuration using a hydrostatic numerical model. The behaviour of the heat low that forms over a plateau‐like orography on a circular island is compared with that when the island is flat, and that when the plateau is surrounded by land instead of sea. In all cases, a broad‐scale, negative radial gradient of potential temperature forms in the daytime mixed layer over land. The presence of orography enhances the broad‐scale baroclinicity over the orographic slope due to the identical heating of a column of air with a reduced mass, i.e. lower surface pressure. In the absence of sea, the baroclinicity is solely confined to the slope of the orography. The broad‐scale potential temperature gradient results in an overturning circulation in the lowest few kilometres, which is separate from the shallower and more intense sea breeze circulation in the island cases. The presence of orography leads to a stronger overturning circulation via enhanced baroclinicity. In the case without sea, both the overturning circulation and tangential circulation are closely tied to the orography. The overturning circulation advects absolute angular momentum inwards to spin up the low‐level circulation, despite some frictional loss of angular momentum en route. During the night, radiative cooling over the land leads to a strong nocturnal low‐level jet that amplifies the spin‐up process. During the daytime, the cyclone weakens as the angular momentum is convectively mixed through a deep layer. The study extends an earlier one of Reichmann and Smith and corrects some details in their model. The results offer a refined interpretation of the Atlantic inflow to the Saharan heat low, described recently by Grams et al. , and emphasize the role of orography in the formation of the inland baroclinic zone, which is a feature thereof. Copyright © 2011 Royal Meteorological Society