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G. Reverdin and G. Sommeria

Abstract

During Summer 1979, 88 super-pressure constant-level balloons were launched at the nominal flight level of 900 mb from the Seychelles Islands and from the northern tip fo the Malagasy Republic as part of the French participation in the MONEX experiment. Balloons tracked by the Argos system on board TIROSN and NOAA-6 satellites provide good estimates of the wind velocity and of the Lagrangian acceleration of air masses within the planetary boundary layer (PBL). With additional wind and pressure data, a study of the balance of forces and of the vertical wind structure in the planetary boundary layer has been performed, including the computation of the vertical transport of horizontal momentum by turbulent mixing. The accuracy of this estimate is marginal in equatorial regions, but sufficient in the Northern Hemisphere to derive a friction coefficient. The meridional variation of the wind veering within the planetary boundary layer is interpreted using a simple one-dimensional model. Veering is weak in the vicinity of the equator and appears to be of the Ekman type more than 10° away. If one follows a cross-equatorial trajectory, the transition in the direction of the veering occurs ∼5° north of the equator. This provides a measure of the relative importance of advective terms compared to local forcing terms in the dynamics of the tropical boundary layer.

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Martin G. Beniston and G. Sommeria

Abstract

Use is made of a three-dimensional model of the planetary boundary layer to investigate features of non-precipitating convection and its parameterization. The fine-grid mesh (50 m) model was first developed by J.W. Deardorff (1973) and modified by G. Sommeria (1976) to include possible cloud formation; the model and data set are briefly described in Section 2.

In Section 3 the model data are utilized in order to test the validity of certain hypotheses concerning individual cloud features as well as those of a cloud population. These hypotheses, for the most part unverified in the real atmosphere, are frequently used in cumulus parameterization schemes.

Sections 4 and 5 present the results of three recent cumulus parameterization schemes by Betts (1975, 1976) and Fraedrich (1976), and tests are made of these schemes with the present model.

In order to illustrate the versatility of such a model when compared to the possibilities of intensive studies using atmospheric data, an attempt is made in Section 6 to link turbulent quantities with directly observable cloud features. The turbulent fluxes of heat and moisture, for example, are not easily observable but are important to cloud development and evolution. The empirical relations presented in this section present a reasonable alternative to atmospheric observations for the estimation of various important turbulent quantities.

The study presented here emphasizes that despite many drawbacks, a numerical model is a useful tool to increase our understanding of a particular atmospheric phenomenon and can be used as a complement to observations for the improvement of parameterization methods in larger scale models.

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J. L. Redelsperger and G. Sommeria

Abstract

This article presents the main features of a three-dimensional model for deep convection developed with special care given to the formulation of subgrid turbulent processes. It explicitly simulates the dynamics of turbulent eddies, including condensation and precipitation processes. Second-order moments are expressed as a function of the grid-averaged field of variables and of a prognostic turbulent kinetic energy. The formulation includes a simple statistical treatment of subgrid condensation and subgrid conversion of cloud water into rain water. The coherence and relative importance of the various closure hypotheses are tested in an idealized case of precipitating cloud.

Results indicate the extent that features of the computed field are dependent on hypotheses used in the turbulence closure, choice of the basic turbulent variables, and formulation of the second-order moments. Significant benefits are obtained from the use of variables that are conserved in the condensation process. The computation of grid-scale condensation and precipitation is mostly dependent on the hypotheses made respectively for subgrid condensation and precipitation. Finally, it is shown that an advanced subgrid turbulence parameterization can partially compensate for the effects of a low spatial resolution.

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G. Sommeria and J. W. Deardorff

Abstract

One of the shortcomings of present condensation schemes is the assumption that a computational grid volume is either entirely saturated or entirely unsaturated, which is a crude approximation in some instances even with a relatively fine mesh. The concept of a statistical distribution of the points where condensation occurs inside a given grid volume is discussed and results are presented for diagnosing both the fraction of the grid volume containing saturated air and the liquid water content when that fraction is less than unity.

An abbreviated procedure is tested in a boundary layer model of 50 m mesh for a case of nonprecipitating tropical moist convection. Two main effects upon the simulated turbulent field are found: first, the cloud activity measured by the total liquid water content or by the vertical moisture transport is increased and extends farther up, at the expense of subcloud layer moisture; second, the temporal variations within the model domain of the calculated turbulent properties of the cloud layer are decreased, which allows a given time sample of the simulated atmosphere to be more representative of the ensemble mean.

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