Abstract

Most of the air pollution episodes in Santiago (Chile) occur during the austral winter in connection with downslope easterly winds forced by upper-air troughs crossing the Andes and the associated poleward propagation of coastal lows along central Chile. During this process the subsidence inversion strengthens and lowers with a severe reduction of the ventilation factor of the city. Efforts to produce short-term forecasts of the onset and exit conditions associated with those episodes have relied mostly upon winds and temperatures measured at hilltop automatic stations located around the typical heights of the subsidence inversion over the Santiago basin. In search of alternative ways to track in real time the presence of the inversion at critical heights and strengths, the trapping of radio waves through the inversion should provide a way to monitor basin scale properties. Ray-tracing techniques to model radio signals propagating in an 'atmospheric duct' associated with strong subsidence inversions are discussed together with experimental results from a 65 km UHF radio link encompassing the Santiago basin at a typical height associated with air pollution episodes. It is concluded that the shape of the received signal during an air pollution episode in September 1997 reproduces the expected transition from multi-ray constructive interference to the shadow area as the inversion moves vertically. It is also demonstrated that a simple vertically-staggered set of receiving antennas could monitor the inversion drift at the onset and exit of episodic conditions once the inversion has leached the critical strength to generate an atmospheric duct.