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 reached the critical
strength to generate an atmospheric duct.