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L. Randall Koenig
and
Francis W. Murray

Abstract

A two-dimensional (axisymmetric) numerical cloud model with parameterized microphysics for water drops and ice particles is described. The parameterized liquid-phase processes include condensation, evaporation, autoconversion of small drops to large ones, and collection of small drops by large ones. The solid-phase processes include heterogeneous sorption nucleation, homogeneous contact nucleation, deposition, sublimation, riming and melting. Both liquid and solid particles may precipitate.

The model was used to simulate three different conditions of ice generation as a function of temperature: 1) based on classical concepts of the activity of heterogeneous ice-forming nuclei—suggesting continental cumuli; 2) based on field observations of much greater concentrations of ice particles at warmer temperatures than indicated measurements of heterogeneous ice nuclei—suggesting maritime cumuli; and 3) based on nucleus seeding practice when the goal is to stimulate the growth of cumulus clouds. General comparisons of model simulation against observation are satisfactory and show that the microphysical parameterizations capture many of the observed properties of glaciating clouds with regard to the locations and sizes of liquid and solid hydrometeors. The variation of hydrometeor properties with time is reasonably satisfactory although no single simulation properly captures the sequence of hydrometeor evolution observed in maritime cumuli. The results support the concept of dynamic seeding to stimulate cloud growth but suggest caution with regard to equating greater vertical growth and greater rainfall on the ground.

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L. Randall Koenig
and
Francis W. Murray

Abstract

The sensitivity of a two-dimensional cloud model to changes in microphysical characteristics was examined using two soundings. The dynamical evolution of clouds having relatively warm bases was controlled more by rapidity of the production of liquid-phase precipitation than by the differences in the concentration of ice particles. There was little change in the dynamical properties of the relatively cold-base clouds with changes in either liquid or ice phases.

A study of the components of the forces causing evolution of the clouds indicated that the net force acting at a given location was often quite small compared to several individual components of the force. Anything causing these individual forces to move relative to one another will make changes in the net force and alter the dynamical properties of the cloud. The earlier versus later formation of precipitation causes changes in location of the forces of condensate loading, and these changes apparently are the explanation of the differences in the behavior of the warm-base cloud.

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Francis W. Murray
,
L. Randall Koenig
, and
Paul M. Tag

Abstract

A two-dimensional field-of-flow numerical model of cloud development is used to study a cloud that formed over a refinery as a result of heat dissipated to the atmosphere. The observed vertical structure of the atmosphere provided initial conditions. The wind necessarily was simplified to a unidirectional flow. The cloud-initiating perturbation consisted of sensible and latent heat equal to the waste heat rejected to the atmosphere by the refinery.

When conditions were matched to those reported, the simulated cloud agreed in most particulars with the observations. Sensitivity tests showed that the simulated cloud depends too strongly on ambient wind speed and shear. This perhaps is a generic defect of two-dimensional formulations. The response of the simulated cloud to expected changes in heat flux density appears more realistic than its response to small changes in ambient wind.

The cloud evolution consists of bubbles forming and breaking away from the main cloud mass, then moving downwind and dissipating. This behavior characterizes real clouds associated with a stationary heat source. The simulations also predict that under appropriate conditions secondary clouds form far downwind.

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Paul M. Tag
,
Francis W. Murray
, and
L. Randall Koenig

Abstract

A two-dimensional model is used to test the effects of using several forms of eddy viscosity parameterization to simulate subgrid-scale turbulence. A well-documented observation of a quasi-steady cumulus cloud which formed over a refinery is used for simulation and comparison. The control parameterization of eddy viscosity is one based on both the deformation and buoyancy fields. When compared to observations, this control run overestimates somewhat the liquid water contents and slightly underpredicts the vertical velocities. Parameterizations based on deformation alone, two-dimensional turbulence theory, and several constant values of eddy viscosity result in cloud simulations that are deficient primarily in their significant overprediction of liquid water content. These experiments confirm that a buoyancy term in the prescription of eddy viscosity is necessary when thermal instability plays an active role in the subgrid forcing.

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