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S. Cautenet
and
R. Rosset

Abstract

The airflow over Cape of Three Points (Gulf of Guinea: 4.5°N, 2°W) has been simulated using a three-dimensional mesoscale model in order to investigate the sea breeze developing in synoptic vertical wind shears during the 1979 dry season. Two different meteorological situations, characterized by two contrasted wind profiles between 500 and 2000 m have been studied, with two types of transitions between the lower circulation (SW monsoon) and the upper African easterly jet (AEJ). The first one is a veering case (6 January) and the second is a backing case (23 January). Calculations of CAPE (convective available potential energy) show that whereas instability is a maximum at both sides of the cape, the site of enhanced convection is determined by the wind shear in the 500–2000 m layer. Numerical results confirm satellite observations.

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C. Fravalo
,
Y. Fouquart
, and
R. Rosset

Abstract

The low stratiform cloud model presented here appears as a generalization of Lilly's model (1968). Its main new apsects lie 1) in a detailed vertical computation of the longwave and the shortwave radiative flux profiles and 2) in a formulation of the entrainment rate and the turbulent flux profiles which takes into account the nonlinear vertical structure of the radiative fluxes. Furthermore, the radiative divergence is no longer externally prescribed, like in other models; it is determined as function of the mixed-layer and cloud characteristics. All these features allow a more complete coupling between the turbulent mixing and the radiative fluxes. Within the cloud, the turbulent flux profiles of the moist static energy and the virtual dry static energy are nonlinear functions of height, due to the radiative divergence. This nonlinear structure results in a realistic negative entrainment flux at cloud top.

In the sensitivity tests, the stress has been put on the variability of the radiative and turbulent fluxes as functions of the cloud microphysics. The result is that the integrated liquid water content (liquid water path) is the predominant factor in fixing the radiative and turbulent fluxes, with a secondary but non-negligible role played by the drop size distribution.

For optically thin clouds where the shortwave absorption is negligible, the infrared cooling is distributed throughout the whole cloud and is highly sensitive to the liquid water path; thus the turbulent fluxes and the entrainment rate also depend strongly on the liquid water path.

When increasing the liquid water path, the longwave cooling becomes saturated and localized in a layer of progressively reduced thickness at cloud top. Thus for thick clouds, with no solar flux, our model gives results which are similar to those of Lilly-type models.

The solar heating does not saturate as the liquid water path increases. Moreover, since it is distributed throughout the cloud deck, it not only reduces the radiative divergence and the turbulent kinetic energy production at cloud top, but it also acts as a source of this latter energy component near the cloud base. The overall result is a noticeable reduction of the entrainment rate. This suggests a strong diurnal cycle for thick stratocumulus decks.

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Everett C. Nickerson
,
Evelyne Richard
,
Robert Rosset
, and
David R. Smith

Abstract

A three-dimensional meso-β model with parameterized microphysics is presented. The model is capable of simulating orographically forced clouds, rain, and airflow. Tests using a two-dimensional version confirm the ability of the model to replicate the linear and nonlinear mountain wave simulations of previous authors. The model is applied to the Rhine valley and surrounding mountainous areas, the Vosges in France and the Black Forest in Germany. Model-predicted rainfall over the mountainous areas is in good agreement with observations in both magnitude and location; however, an absence of model-predicted cloud cover over the Rhine valley suggests the need for an improved mesoscale initialization procedure.

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J. F. Mahfouf
,
E. Richard
,
P. Mascart
,
E. C. Nickerson
, and
R. Rosset

Abstract

Various parameterizations of the planetary boundary layer (PBL) currently used in three-dimensional (3D) mesoscale models are compared with a more complex scheme including a turbulent kinetic energy (TKE) equation. In the first set of simulations made with a ID model against the classical Wangara data, the mean wind, temperature and moisture calculated in the PBL are nearly insensitive to the choice of the parameterization. In the second set of simulations, the TKE parameterization is used in a 3D mesoscale model to simulate sea breeze flows over south Florida. A comparison is presented with previous simulations of Pielke, and Pielke and Mahrer, for the mean flow, and with the third-order turbulence closure model of Brière for the turbulent variables, including a discussion of the turbulent energy budget, The analysis of the results obtained with the TKE scheme shows that the predicted turbulent fields are qualitatively realistic and interact significantly with the sea breeze circulation. Finally, a comparison is made between the TKE scheme and the simpler parameterization of Pielke and Mahrer. It shows only slight differences as far as the mesoscale structure of the mean variables is concerned.

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