South foehn and ozone in the Eastern Alps – case study and climatological aspects

The ozone situation in the Eastern Alps was investigated during the south foehn period from 4–6 May 1997. The event was studied in detail using surface measurements, soundings and aircraft measurements of meteorological and chemical parameters. A numerical simulation with a prognostic meteorological model (MM5) and a chemistry-transport model with 6 km resolution provided additional insight. The case study was supplemented by a climatological evaluation of a three-year data set. The foehn period was preceeded by an ozone episode in the Po Basin south of the Alps. Advection of residual-layer air masses from that area caused a maximum of the ozone concentration in the beginning. Later on, the ozone concentration in the foehn area was determined by a mixture of regional-scale advection from the lower free troposphere and boundary-layer air from the south. The contribution of boundary-layer air was especially strong in the lee of the deep gap formed by the Brenner Pass and visible in many parameters. The climatological evaluation showed that during south foehn, ozone concentrations are elevated in the foehn area, especially in the valleys and during nighttime where the usual nocturnal minimum is suppressed.