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Mesoscale Cellular Convection

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle
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Abstract

A linear stability model for mesoscale cellular convection in the atmosphere is developed. The model includes a forcing term which is a parameterization of the net heating due to small-scale cumulus convection. A sub-cloud and cloud layer are defined, with the forcing term having non-zero values only in the cloud layer. Positive static stability is assumed in both layers so that the only source of buoyant energy is the forcing term.

The parameterization of latent heating due to cumulus convection is accomplished by assuming that the heating is proportional to the cloud-environment temperature difference and the vertical flux of moisture by the perturbation vertical velocity.

Normal mode horizontal dependence and exponential time dependence is assumed for the vertical velocity, and the forcing term is defined as proportional to the vertical velocity at the interface between the two layers. The solutions in the two layers are matched across the interface and the particular solution associated with the forcing term is expressed in terms of the arbitrary constants contained in the homogeneous solutions. This yields a homogeneous solution matrix which is solved.

Solutions are found for a wide range of values of atmospheric static stability, system depth, mean temperature and relative humidity, as well as varying degrees of anisotropy of the eddy mixing coefficients. The observed flattening of atmospheric cells, with diameter-to-depth ratios an order of magnitude greater than predicted by the stability analysis of classical Rayleigh convection, is duplicated by the model. Anisotropy of the eddy mixing coefficients is not a requirement for flattened cells in the model. The choice of boundary conditions is also of minor importance in producing cell flattening. Growth rates and preferred cell diameters are most sensitive to the relative humidity and static stability of the atmosphere. These two parameters represent, respectively, the source and sink of buoyant energy in the model. Positive static stability is responsible for cell flattening because it suppresses very strongly the relatively large vertical velocities associated with smaller cells.

Abstract

A linear stability model for mesoscale cellular convection in the atmosphere is developed. The model includes a forcing term which is a parameterization of the net heating due to small-scale cumulus convection. A sub-cloud and cloud layer are defined, with the forcing term having non-zero values only in the cloud layer. Positive static stability is assumed in both layers so that the only source of buoyant energy is the forcing term.

The parameterization of latent heating due to cumulus convection is accomplished by assuming that the heating is proportional to the cloud-environment temperature difference and the vertical flux of moisture by the perturbation vertical velocity.

Normal mode horizontal dependence and exponential time dependence is assumed for the vertical velocity, and the forcing term is defined as proportional to the vertical velocity at the interface between the two layers. The solutions in the two layers are matched across the interface and the particular solution associated with the forcing term is expressed in terms of the arbitrary constants contained in the homogeneous solutions. This yields a homogeneous solution matrix which is solved.

Solutions are found for a wide range of values of atmospheric static stability, system depth, mean temperature and relative humidity, as well as varying degrees of anisotropy of the eddy mixing coefficients. The observed flattening of atmospheric cells, with diameter-to-depth ratios an order of magnitude greater than predicted by the stability analysis of classical Rayleigh convection, is duplicated by the model. Anisotropy of the eddy mixing coefficients is not a requirement for flattened cells in the model. The choice of boundary conditions is also of minor importance in producing cell flattening. Growth rates and preferred cell diameters are most sensitive to the relative humidity and static stability of the atmosphere. These two parameters represent, respectively, the source and sink of buoyant energy in the model. Positive static stability is responsible for cell flattening because it suppresses very strongly the relatively large vertical velocities associated with smaller cells.

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