Aircraft observations of the structure of the lower boundary layer over the sea

The turbulent characteristics of the neutral boundary layer developing over rough surfaces are not well predicted with operational weather-forecasting models. The problem is attributed to inadequate mixing-length models, to the anisotropy of the flow and to a lack of controlled experimental data against which to validate numerical studies. Therefore, in order to address directly the modelling difficulties for the development of a neutral boundary layer over rough surfaces, and to investigate the turbulent momentum transfer of such a layer, a set of hydraulic flume experiments were carried out. In the experiments, the mean and turbulent quantities were measured by a particle image velocimetry (PIV) technique. The measured velocity variances and fluxes (u i u j ) in longitudinal vertical planes allowed the vertical and longitudinal gradients (∂/∂z and ∂/∂ x) of the mean and turbulent quantities (fluxes, variances and third-order moments) to be evaluated and the terms of the evolution equations for ∂e/∂t, ∂u 2 /∂t, ∂w 2 /∂t and ∂u w /∂t to be quantified, where e is the turbulent kinetic energy. The results show that the pressure-correlation terms allow the turbulent energy to be transferred equitably from u 2 to w 2 . It appears that the repartition between the constitutive terms of the budget of e, u 2 , w 2 and u w is not significantly affected by the development of the rough neutral boundary layer. For the whole evolution, the transfers of energy are governed by the same terms that are also very similar to the smooth-wall case. The PIV measurements also allowed the spatial integral scales to be computed directly and to be compared with the dissipative and mixing length scales, which were also computed from the data.

Section: Turbulent Quantitiessupporting

confidence: 66%

Experimental Investigation of Turbulent Momentum Transfer in a Neutral Boundary Layer over a Rough Surface

Tomas

Eiff

Masson

2010

“…The values of Co extrapolated from 50 to 10 m are generally larger than the quoted GATE surface values but within one or two standard deviations of the GATE Workshop result. Evidence seems to be accumulating that aircraft measurements of drag coefficients tend to be larger than comparable measurements at the surface by as much as 40% (Nicholls and Readings, 1979;McBean and Paterson, 1975) and the present results are no exception. The reasons for this presumably have to do with the turbulent mechanisms by which momentum is transported and the influence that these mechanisms have on detection by instruments which either are or are not stationary with respect to the surface of the earth.…”

Section: Vertical Profilesmentioning

confidence: 33%

Analysis of aircraft measurements of momentum flux in the subcloud layer over the tropical atlantic ocean during gate

Greenhut¹

1981

The momentum flux data obtained by the gust probe aboard the NOAA DC-6 aircraft during GATE are analyzed. Vertical profiles are obtained for Phases I and III and correlated with vertical wind velocity profiles using the geostrophic departure method. Reasonable agreement is obtained using the horizontal equations of motion with negligible advective acceleration. The vertical profiles of momentum flux and wind speed variance compare well with the numerical model results of Deardorff (1972) and Wyngaard et al. (1974). Vertical distributions of power spectra for vertical eddy motion and cospectra corresponding to the momentum flux components are obtained along with the height variation of the dominant length scales of vertical eddy motion and the dissipation rate of turbulence kinetic energy. When normalized by mixed-layer similarity, these results agree well with previous determinations in the boundary layer over tropical oceans and over land.

“…It is an extended version of a similar figure presented by Nicholls and Readings (1979). Three length scales are used in defining the axes, namely the boundary-layer height H, the observation height z, and the Monin-Obukbov length L. The axes are given by the stability parameter z/L and the dimensionless height z/H.…”

Section: Scaling Considerationsmentioning

confidence: 99%

“…Atmospheric conditions falling into this region are rare over land, but rather frequent over sea. The region is not so well explored as the others, but some work has been done (e.g., by Nicholls and Readings, 1979). A dashed line for zJL = -10 is drawn in Figure 1.…”

Section: Scaling Considerationsmentioning

confidence: 99%

Modelling velocity spectra in the lower part of the planetary boundary layer

Olesen

Larsen

Højstrup

1984

151

Principles used when constructing models for velocity spectra are reviewed. Based upon data from the Kansas and Minnesota experiments, simple spectral models are set up for all velocity components in stable air at low heights, and for the vertical spectrum in unstable air through a larger part of the planetary boundary layer. Knowledge of the variation with stability of the (reduced) frequency f, for the spectral maximum is utilized in this modelling. Stable spectra may be normalized so that they adhere to one curve only, irrespective of stability, and unstable w-spectra may also be normalized to fit one curve.The problem of using filtered velocity variances when modelling spectra is discussed. A simplified procedure to provide a first estimate of the filter effect is given.In stable, horizontal velocity spectra, there is often a 'gap' at low frequencies. Using dimensional considerations and the spectral model previously derived, an expression for the gap frequency is found.