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Atmospheric boundary-layer structure over the Mediterranean during a Tramontane event

Abstract : Observations of the unstable atmospheric boundary layer were made over the Mediterranean by the airborne backscatter lidar LEANDRE I during a cold air outbreak related to a Tramontane event. These measurements performed during the Pyrénées Experiment show a development of the internal boundary-layer over the sea influenced by the atmospheric flow-perturbation caused by the vicinity of the Pyrénées. A one-dimensional model is used to analyse the internal boundary-layer growth in relation with mixed-layer temperature, specific humidity, surface fluxes and vertical velocity. It includes bulk formulae for surface-flux determination taking into account surface and mixed-layer values of these parameters. Transfer coefficient values are obtained which are smaller than those computed in the model of Stage and Businger (1981). Results obtained using nomograms of mean column-heating computed by Chou and Atlas (1982) lead, in the case of the Pyrénées Experiment to overestimated surface-fluxes. Comparison with our model reveals that the discrepancy originates from the higher heat-transfer coefficient values used by Chou and Atlas. Model results are compared to lidar and in-situ data (internal thermal boundary-layer growth, temperature, specific humidity, average fluxes) and excellent agreement is obtained. The comparison between model results and lidar data shows that the growth rate of the internal thermal-boundary-layer is very sensitive to the vertical velocity w̄, which is not the case for surface fluxes. Comparisons between measured and modelled parameters thus allow the derivation of an estimation of the vertical-velocity field in the vicinity of the Pyrénées. It is finally shown that a simplified equation could be used to estimate surface buoyant heat-flux far from the shoreline during such cold air outbreaks provided accurate correction of vertical velocity impact on internal thermal-boundary-layer growth can be achieved.
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Submitted on : Thursday, August 12, 2021 - 11:21:01 AM
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Cyrille Flamant, Jacques Pelon. Atmospheric boundary-layer structure over the Mediterranean during a Tramontane event. Quarterly Journal of the Royal Meteorological Society, Wiley, 1996, 122 (536), pp.1741-1778. ⟨10.1002/qj.49712253602⟩. ⟨insu-03319362⟩



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