UHF radio propagation experiment through the inversion capping Santiago (Chile) during an air pollution episode

Most of the air pollution episodes in Santiago (Chile) occur during the aus tral winter in connection with downslope easterly winds forced by upper-air troughs crossing the Andes and the associated poleward propagation of coas tal 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 co nditions associated with those episodes have relied mostly upon winds and t emperatures measured at hilltop automatic stations located around the typic al heights of the subsidence inversion over the Santiago basin. In search o f alternative ways to track in real time the presence of the inversion at c ritical heights and strengths, the trapping of radio waves through the inve rsion should provide a way to monitor basin scale properties. Ray-tracing t echniques to model radio signals propagating in an 'atmospheric duct' assoc iated with strong subsidence inversions are discussed together with experim ental results from a 65 km UHF radio link encompassing the Santiago basin a t a typical height associated with air pollution episodes. It is concluded that the shape of the received signal during an air pollution episode in Se ptember 1997 reproduces the expected transition from multi-ray constructive interference to the shadow area as the inversion moves vertically. It is a lso demonstrated that a simple vertically-staggered set of receiving antenn as could monitor the inversion drift at the onset and exit of episodic cond itions once the inversion has leached the critical strength to generate an atmospheric duct.