Ice-Bearing Cumulus Cloud Evolution: Numerical Simulation and General Comparison Against Observations

L. Randall Koenig The Rand Corporation, Santa Monica, Calif. 90406

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Francis W. Murray The Rand Corporation, Santa Monica, Calif. 90406

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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.

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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