Molecular dissipation of turbulent fluctuations in the convective mixed layer part I: Height variations of dissipation rates |
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| Authors: | B Guillemet H Isaka P Mascart |
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| Institution: | 1. Laboratoire Associé de Météorologie Physique, Université de Clermont II, Aubière, France
2. Centre National d'Etudes Spatiales (CNES), Paris, France
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| Abstract: | During the Limagne and Beauce experiments, the INAG-IGN Aerocommander FL 280 aircraft made extensive ‘in situ’ measurements of turbulent fluctuations in diurnally evolving convective boundary layers. In this paper, these measurements were used to investigate characteristics of the molecular dissipation of turbulent fluctuations through the mixed layer and well into the overlying stable layer. The dimensionless dissipation rates of turbulent kinetic energy, temperature and humidity variances, and temperature-humidity covariance (ψ, ψθ, ψ qand ψ θq) were computed and their height variations analysed. The behaviour of the dissipation rate ψ was found to differ significantly from those observed for the other rates. In the lowest region of the mixed layer, ψ does not obey the local free convection prediction. Instead, it follows practically a relationship similar to the one established in the surface layer by Wyngaard et al. (1971). The dissipation rate ψ remains fairly constant in the bulk of the mixed layer (0.3 ≤ z/Z i≤ 0.8) and shows a very rapid decrease above the inversion. These results confirm those reported previously from the Minnesota and Ashchurch data by Kaimal et al. (1976), Caughey and Palmer (1979), etc. The height variations for the other dissipation rates were found to obey, as expected, the (z/Z i)-4/3 decrease predicted under the local free convection similarity hypothesis in the lowest region of the mixed layer. This region extends to the height z/Z i- 0.4, 0.1, and 0.3, respectively, for ψθ, ψq, and ψθq. Above these levels, the dissipation rates ψθ and ψq show, on average, a slight increase to reach peak-values near the mixed-layer top, while the ‘dissipation’ rate ψ θqchanges sign from positive to negative around the height z/Z i, - 0.7. These characteristics confirm the fact that the structures of temperature and humidity fluctuations are considerably affected by their entrainment-induced fluctuations. Therefore, an attempt has been made to non-dimensionalize the dissipation rates near the mixed-layer top with the interfacial scaling factors. |
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