Nonlocal Factors of the Convective Boundary Layer and Its Evening Transition Observed with Fixed and Mobile Ceilometers in the Santiago Valley

Le résumé fourni par la source

Abstract Results are presented of a 3-day intensive observational period carried out during a wintertime air pollution episode in the Santiago Valley in central Chile. The objective was to characterize nonlocal factors of the convective boundary layer (CBL) and evening transition (ET), including advective effects and the possible role of internal waves. The principal measurements were performed with a mobile ceilometer that probed the fine vertical structure of aerosols along two ∼70-km paths designed to assess the horizontal variation of the CBL inside the valley and the meridional structure of complex ET aerosol layers documented by previous studies. Ancillary observations included four fixed ceilometers, 3-hourly radiosondes, one tethered balloon system, occasional pilot balloons, and two automatic meteorological stations measuring wind above the standard 10 m AGL height. The CBL showed a terrain-following mode of horizontal variation in the center of the valley and a growth rate affected by a surface-detached early morning warming. A minimum-advection (MA) estimate of the energy and water CBL budgets suggests a 3.2 Bowen ratio for the valley surface fluxes. The complex ET aerosol layers were associated with southerly winds that brought clean air to the center of the valley displacing the CBL residual layer. During one ET, intense wind and pressure oscillations were observed presumably related to an internal gravity wave, with significant effects in surface winds, stability, and turbulence. The observations enhanced the understanding of this complex terrain boundary layer and provided data for the validation of its numerical modeling.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.