Abstract
Abstract. This paper presents some results about the behavior of the atmospheric boundary layer observed during the ESCOMPTE experiment. This campaign, which took place in south-eastern France during summer 2001, was aimed at improving our understanding of pollution episodes in relation to the dynamics of the lower troposphere. Using a large data set, as well as a simulation from the mesoscale non-hydrostatic model Meso-NH, we describe and analyze the atmospheric boundary layer (ABL) development during two specific meteorological conditions of the second Intensive Observation Period (IOP). The first situation (IOP2a, from 22 June to 23 June) corresponds to moderate, dry and cold northerly winds (end of Mistral event), coupled with a sea-breeze in the lower layer, whereas sea-breeze events with weak southerly winds occurred during the second part of the period (IOP2b, from 24 June to 26 June). In this study, we first focus on the validation of the model outputs with a thorough comparison of the Meso-NH simulations with fields measurements on three days of the IOP: 22 June, 23 June and 25 June. We also investigate the structure of the boundary layer on IOP2a when the Mistral is superimposed on a sea breeze. Then, we describe the spatial and diurnal variability of the ABL depths over the ESCOMPTE domain during the whole IOP. This step is essential if one wants to know the depth of the layer where the pollutants can be diluted or accumulated. Eventually, this study intends to describe the ABL variability in relation to local or mesoscale dynamics and/or induced topographic effects, in order to explain pollution transport processes in the low troposphere.
Highlights
We investigate the structure of the boundary layer on IOP2a when the Mistral is superimposed on a sea breeze
Prediction of pollution events and implementation of adapted air quality policies require a considerable improvement in the understanding of chemical and dynamical processes which lead to high pollutant concentrations in the atmosphere
If we focus on the spatial variability of the atmospheric boundary layer (ABL) developments for these two days, we can notice that the development is quite homogeneous on J22, with ABL depths ranging between 1000 and 1500 m and a variation that can be linked to the wind velocity distribution (Fig. 4): the ABL is the lowest when the wind is the strongest (12 m s−1), showing that dynamics prevails over thermodynamics
Summary
Prediction of pollution events and implementation of adapted air quality policies require a considerable improvement in the understanding of chemical and dynamical processes which lead to high pollutant concentrations in the atmosphere. Several European projects have been conducted around the Mediterranean basin to document the circulation of air pollutants: among them are the MEso-meteorological Cycles of Air Pollution in the Iberian Peninsula, MECAPIP, 1988– 1991 (Millan et al, 1996, 2000), the REgional Cycles of Air Pollution in the west-central Mediterranean Area, RECAPMA, 1990–1992 (Gangoiti et al, 2001) and the South European Cycles of Air Pollution, SECAP, 1992–1995 (Millan et al, 1997) These experimental projects showed that during daytime and summer conditions, the combination of sea breezes with up-slope winds, due to the strong heating on the mountains, may inject pollutants up to the low (2–3 km) or even middle troposphere (3–5 km), forming an accumulation of ozone layers over the continent.
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