The mistral and its effect on air pollution transport and vertical mixing

Within the framework of ESCOMPTE, the influence of local wind systems like land-sea/mountain-valley winds on the distribution of air pollutants in the southern part of the Rhone valley and the coastal regions of southern France was investigated. In addition, the influence of the mistral on the long-range transport and vertical mixing of such substances on July 1, 2001 was analyzed. The results of the measurements of this mistral situation show high concentrations of O-3 and NO2 in the layer just above the PBL at the southern exit of the Rhone valley near Avignon. By measurements from airborne and ground-based platforms and numerical simulations with the "Local Moder'(LM) of the German Weather Service (DWD), it is shown that the mistral develops according to the theory conceived by Pettre [J. Atmos. Sci. 39 (1982) 542-554]. The synoptic-scale northerly flow through the Rhone valley is accelerated up to a Fronde number (Fr) of 2.1, while the valley widens. Then, near the Mediterranean coast, a hydraulic jump occurs and Fr drops down to values below 1.0. High ozone concentrations of 112 ppb measured above the mistral layer disappear due to enhanced mixing after the flow has passed the hydraulic jump. There is some evidence that the ozone-rich air originates from the source region of greater Paris or upwind. The results confirm that regional wind systems associated with transport of trace gases in the high-grade industrialized Rhone valley can be successfully predicted using data of operational weather forecast models. (C) 2004 Elsevier B.V. All rights reserved.