Stratification of the lower troposphere during the ESCOMPTE campaign

The ESCOMPTE 2001 measurements were performed during anticyclonic conditions, since those are likely to increase the photochemical pollution events that are observed during the campaign. The vertical structure of the low troposphere was studied, in conjunction with a thorough analysis of the boundary layer, during a period of intense pollution. The period could be split into two parts: Intensive Observation Period A (IOP A) with a few days of strong to moderate orographic wind, and IOP B with a very low flow, propitious to sea-breeze conditions. In both cases, a clear vertical stratification of staggered stable layers appeared between the boundary layer top and 3000m. This stratification revealed spatially homogeneous and steady during IOP A. These characteristics were also observed during IOP B but the steadiness was not as good.

The objective of the presentation is an attempt to explain the origin of the diverse layers using backward trajectories in connection with the study of vertical profiles of chemical compounds and particles. In a second step, we will highlight the conditions under which these stable layers may interact with the polluted boundary layer, through local mechanisms such as turbulence and entrainment at the top of the boundary layer, or mesoscale transport such as slope and valley winds, subsidence or ascendence linked to synoptic or mesoscale conditions.

We will use radiosounding, aircraft data, measurements from a network of UHF radars and sodars as well as a simulation from the non hydrostatic Mésonh-C mesoscale model.