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Titre : How does synoptic-scale descent influence the planetary boundary layer?
Nom du conférencier : Shira Raveh-Rubin
Son affiliation : Department of Earth and Planetary Sciences, Weizmann Institute of Science, Israel
Laboratoire organisateur : LMD
Date et heure : 28-11-2019 11h00
Lieu : LMD - École Polytechnique - 91 Palaiseau - Conference room - salle Marie-Pierre Lanceau - aile 5, 2d floor
Résumé :

Organized synoptic-scale descent occurs in association with the passage of midlatitude weather systems. Typically, air descends slantwise from the upper troposphere behind troughs, while moving equatorward behind cold fronts trailing from extratropical cyclones. Thus, within these so-called dry intrusions (DIs), air parcels connect the upper-tropospheric flow with the lower troposphere downstream in a Lagrangian sense. It is expected that upon their arrival into the lower-troposphere, DI air interacts with the planetary boundary layer (PBL) by modifying the atmospheric column temperature, humidity and wind patterns. Thus, DIs potentially affect the static stability, vertical mixing and turbulent heat fluxes in the air-sea interface, having implications on e.g., cloud regimes. However, the link between DIs and boundary-layer characteristics has not been climatologically quantified, and the underlying mechanisms linking the descent in the free troposphere to the PBL are yet not well understood.

Here, using the ERA Interim global reanalysis from ECMWF, we employ a Lagrangian perspective and feature-based diagnostics to study the occurrence of DIs and their impact on the boundary layer. Illustrated with individual case studies, and generalized climatologically, we find that DI occurrence peaks in winter, with preferred location in and outside the midlatitude storm tracks. When they occur, the arrival of the relatively cold air to the PBL top induces potential instability in the majority of the cases. At the same time, the PBL deepens and surface sensible and latent heat fluxes into the atmosphere are enhanced. Using field measurements from the ACE-ENA Azores site, we demonstrate the DI impact on the lower-troposphere, the inversion layer and clouds, and highlight the DI role in strongly modifying the PBL regime.

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