Jean-Claude André

Modeling the 24-Hour Evolution of the Mean and Turbulent Structures of the Planetary Boundary Layer

Abstract A high-order model is proposed for the study of the 24 h evolution of clear planetary boundary layers. The model includes the rate equations of correlations up to the third order, as required for an accurate description of daytime convective phenomena, but it also takes into account interactions between radiative transfer and turbulence in order to achieve a physically reasonable description of the nocturnal structure of the boundary layer. This numerical model is tested against the Wangara boundary layer data of Day 33 and Night 33–34 (Clarke et al., 197l). The computed daytime mean structure of the boundary layer compares favorably with the Wangara data, while the daytime turbulent structure, expressed in the framework of the convective similarity theory, is in particularly good quantitative agreement with a number of experimental and numerical data concerning convection in the boundary layer, with particular concern to the production of turbulence at the top of the mixed layer. The computed no...

Regional estimates of heat and evaporation fluxes over non-homogeneous terrain. Examples from the HAPEX-MOBILHY programme

Questions related to the spatial integration of turbulent surface fluxes of heat and moisture up to horizontal scales of 100km are discussed. Results taken from the HAPEX-MOBILHY programme are presented, concerning either comparisons between surface-based and aircraft measurements, or estimates derived from numerical simulations of the large-scale hydrological balance, or finally area-averaged values obtained from atmospheric numerical mesoscale models. Conclusions are drawn from these results on how to estimate spatially-averaged surface fluxes efficiently.

Turbulence Approximation for Inhomogeneous Flows: Part II. The Numerical Simulation of a Penetrative Convection Experiment

Abstract The quasi-normal theory modified by the clipping approximation is used to construct a numerical model of vertically inhomogeneous turbulence in a Boussinesq fluid. The pressure-correlation terms are expressed according to recently proposed formulations. With the aid of the horizontal homogeneity assumption the model is applied to the description of a laboratory experiment on penetrative convection and the numerical results are compared to the experimental data. It is found that the model does reproduce the measured vertical variations of mean temperature, turbulent beat flux, eddy kinetic energy and temperature variance, as well as a third-order correlation such as the vertical flux of eddy kinetic energy. The stationarity of the vertical profiles of various turbulent quantities, when scaled by the convective velocity and temperature, is also verified.

On the effective roughness length for use in numerical three-dimensional models

We present analytical and numerical calculations of the effective roughness length (ERL) over a flat surface with varying roughness elements, for use in large-scale models. It is shown that ERL is mostly determined by the roughest elements present inside the averaging domain and that, more surprisingly, the ERL increases as the first level of the numerical model gets closer to the surface and its altitude approaches the value of the largest local roughness length. This effect further increases the drag coefficient, in addition to the well-known increase due to the lowering of the first model level.

Similarity studies of entrainment in convective mixed layers

A similarity study of entrainment at the top of convectively driven mixed layer is presented. The similarity framework is used for a comparison between various parameterized models of mixed-layer growth rate and between closely related models for the ratio of heat fluxes at ground and inversion levels. These various models are also tested, in dimensionless form, against data from laboratory, field experiments and numerical higher-order-modeling of the convective layer. It is concluded that a rather accurate prediction of mixed-layer growth can be achieved with the simple constant flux ratio model, but that more refined studies of entrainment are required to account for the decrease of the heat flux ratio with increasing convective intensity.

The Clipping Approximation and Inhomogeneous Turbulence Simulations

A new treatment of third-order moments is proposed for the study of inhomogeneous turbulence: the equations for the rate of change of third-order correlations are used in conjunction with the quasi-normal approximation and with the enforcement of certain generalized Schwarz inequalities (the “clipping” mechanism) relating third- and second-order moments, the clipping mechanism being intended to preserve realizability. Pressure and dissipative effects on double correlations are parametrized with the aid of classical formulations while the corresponding effects on triple correlations are expressed by a suitable generalization of this approach. Together with these parametrizations, the clipping approximation is used for numerical modeling of a penetrative convection experiment and of an asymmetric channel flow experiment.

ON THE INTEGRAL MODELLING OF KATABATIC FLOWS

The integral model for katabatic flows proposed by Manins and Sawford (1979) is solved numerically. It is shown that numerical solutions can be approximated by Balls (1956) model in the upper part of the slope, while they tend toward Manins and Sawfords simplified solution farther downslope. The importance of entrainment and ambiant stable stratification is shown. Some limitations of Manins and Sawfords model are discussed.

On the similarity functions A and B as determined from the ‘VOVES’ experiment

Data collected during the french ‘VOVES-1977’ experiment are used for the determination of the functionsA(μ) andB(μ) of the stability parameter μ. It is found that, although pertaining to different geographic conditions, they lead to the same kind of dependency ofA andB upon μ as found previously considered data. The influence of baroclinicity onA andB is also studied and it is found in that, statistically, consideration of this parameter does not lead either to a decrease of the large scatter of experimental points or to a significant improvement of the similarity theory. It is also shown that a satisfactory estimation of the surface geostrophic wind can be made from the network of meteorological stations, to compute such functions.

HAPEX-MOBILHY: results from the special observing period

The HAPEX-DBILHY program is studying the hydrological budget arid evapotranspiration (ET) flux at the scale of the GG4 grid square i.e. 1O4km2. For the program several surface and subsurface networks operated from mid 1985 to early 1987 to monitor soil moisture, surface energy budget, and surface meteorological parameters. During the Special Observing Period (SOP) from 7 May to 15 July 1986, there were additional measurements including detailed observations of atmospheric fluxes at the surface and with two well instrumented aircraft: the NCPR King-Air for eddy correlation flux measurements and the NTSA C-l30 for remote sensing observations. Brief descriptions of measurement systems and some results from the SOP will be presented here concentrating on the remote sensing of surface tetrerature.