Orographic influences on the dynamics of heat lows

Orographic influences on the dynamics of heat lows

by

Zsuzsanna Rácz

Meteorological Institute, University of Munich, Germany

 

Heat lows or troughs are a prominent climatological feature of many arid land areas of the world during the warmer months, especially at low latitudes where insolation is at its peak. They are shallow disturbances, generally confined below 700 mb. Both types of disturbance may be thought of as low-level cyclonic relative vorticity maxima which are linked to horizontal gradients of diabatic heating. Heat lows and troughs are important for day-to-day forecasting as they influence the location of regions of low-level convergence, which in turn are likely places where convective storms will be triggered if the converging air happens to be sufficiently moist.

 

The weather of north Australia is dominated by two surface heat troughs in the easterlies during the summer. The stronger of the two is normally the one centred over northern Western Australia, typically over the elevated terrain of the Pilbara and Kimberley ranges, while the second trough develops over the Cloncurry area in Queensland. A study of the linear dynamics of these troughs by Adams (1993) shows that the orographic effects and those induced by differential heating are comparable. Over Europe, the heat low that forms over the Iberian Peninsula is a prominent feature of the low-level flow in summer. The strength of the low is closely related to terrain characteristics (Gaertner et al. 1993).

 

A recent numerical study of the basic dynamics of heat lows over a level heated land mass surrounded by sea has been carried out by Rácz and Smith (1999). The study helps to understand some of the observed aspects of heat lows, including their shallow nature, the existence of a convergent cyclonic circulation at low levels and a divergent anticyclonic circulation aloft, and the late afternoon timing of the lowest minimum surface pressure. The same numerical model is used here to investigate the effects of orography on the dynamics of heat lows in an idealized flow configuration. The model has elevated terrain in the centre of the computational domain. Calculations are carried out for the case with or without a basic flow, with no surrounding sea, as well as with the sea present. Sensitivity studies are used to determine dependence of the mature heat low on the basic external parameters including the sea-surface temperature, the deep soil temperature and the shape of the orography.

 

 REFERENCES

Adams, M., 1993: A linear study of the effects of heating and orography on easterly airstreams with particular reference to northern Australia, Aust. Met. Mag., 42, 69-80.

Gaertner, M. A., C. Fernández and M. Castro: 1993. A two-dimensional simulation of the Iberian summer thermal low. Mon. Wea. Rev., 121, 2740-2756.

Rácz Zs. and R. K. Smith, 1999: The dynamics of heat lows. Q. J. R. Meteorol. Soc., 125, 225-252.