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Multiscale and unsteady aspects of a deep föhn event during MAP
Author(s) -
Jaubert Genevieve,
Stein Joel
Publication year - 2003
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/qj.02.38
Subject(s) - mesoscale meteorology , geology , turbulence , flow (mathematics) , meteorology , hydrostatic equilibrium , hydraulic jump , climatology , downscaling , jet (fluid) , front (military) , atmospheric sciences , climate change , mechanics , physics , oceanography , quantum mechanics
Abstract A strong föhn event occurring during the Mesoscale Alpine Programme Intensive Observing Period 2b (19–20 September) is simulated with the non‐hydrostatic model Meso‐NH. The grid‐nesting techniques and a suitable turbulence scheme allow a coherent downscaling from the analysed scales to the movements inside the Rhine Valley. The flow regime around the Alps changes from a flow‐around regime to a flow‐partly‐over regime, when the wind increases ahead of the cold front. Simplified simulations allow us to estimate a non‐dimensional height for the Alps which controls the flow regime. The föhn characteristics and its temporal evolution are discussed using both measurements and simulation. Backward trajectories show that the föhn air comes from an altitude around 2 km above the Po Valley. Nonlinear phenomena, such as wave breaking near 5 km above sea level just behind some secondary crests, and hydraulic jumps in the foothills, are found in the highest‐resolution simulation, using a mesh of 625 m. This mesh clearly improves the comparison of the numerical simulation with the observations collected in the Rhine Valley. Mass budgets detail the progressive building of the föhn jet in the Rhine Valley. Copyright © 2003 Royal Meteorological Society.