THE INFLUENCE OF LARGE CONVECTIVE EDDIES ON THE SURFACE LAYER TURBULENCE
S.S. Zilitinkevich, J. C. R. Hunt, A. A. Grachev, I. N. Esau, D. P.Lalas, E. Akylas, M. Tombrou, C. W. Fairall, and H. J. S. Fernando
Geophysical Research Abstracts, Vol. 6, 00794, 2004
Large-scale coherent structures in the shear-free convective boundary layers consist of narrow strong plumes and wider but weaker downdraughts. In the surface layer they cause local ¸convective winds blowing towards the plume axes. Their typical life-times are much larger that the overturning time scale. As a result, the convective wind shears act similarly to the mean-wind shears, generate turbulence in addition to its buoyancy generation, and by this means strongly enhance the turbulent fluxes of heat, water vapour and other scalars near the surface. Earlier heat/mass transfer models accounting for this mechanism were insufficiently advanced to accurately reproduce the role of the surface roughness in different flow-roughness interaction regimes. In the present paper, an advanced theoretical model overcoming this drawback is developed and comprehensively validated against data from measurement in different sites over the sea and the land and also through large-eddy simulation of convective boundary layers over a range of surfaces from very smooth to very rough. Excellent correspondence between model results, field observations and large-eddy simulations is achieved over the very wide range of the surface roughness lengths z0u and boundary layer heights h: 103 < h/z0u < 109. The obtained resistance and heat/mass transfer coefficients are recommended for practical use in weather-prediction, climate and other environmental models.
S.S. Zilitinkevich, J. C. R. Hunt, A. A. Grachev, I. N. Esau, D. P.Lalas, E. Akylas, M. Tombrou, C. W. Fairall, and H. J. S. Fernando
Geophysical Research Abstracts, Vol. 6, 00794, 2004
Large-scale coherent structures in the shear-free convective boundary layers consist of narrow strong plumes and wider but weaker downdraughts. In the surface layer they cause local ¸convective winds blowing towards the plume axes. Their typical life-times are much larger that the overturning time scale. As a result, the convective wind shears act similarly to the mean-wind shears, generate turbulence in addition to its buoyancy generation, and by this means strongly enhance the turbulent fluxes of heat, water vapour and other scalars near the surface. Earlier heat/mass transfer models accounting for this mechanism were insufficiently advanced to accurately reproduce the role of the surface roughness in different flow-roughness interaction regimes. In the present paper, an advanced theoretical model overcoming this drawback is developed and comprehensively validated against data from measurement in different sites over the sea and the land and also through large-eddy simulation of convective boundary layers over a range of surfaces from very smooth to very rough. Excellent correspondence between model results, field observations and large-eddy simulations is achieved over the very wide range of the surface roughness lengths z0u and boundary layer heights h: 103 < h/z0u < 109. The obtained resistance and heat/mass transfer coefficients are recommended for practical use in weather-prediction, climate and other environmental models.