Numerical Modeling of the Dynamics and Microphysics of Warm Cumulus Convection

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  • 1 Geophysical Fluid Dynamics Program, Princeton University, Princeton, N. J. 08540
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Abstract

Two cumulus cloud models in two space dimensions are presented; one is a bulk physical model without microphysics and the other includes the microphysical processes such as nucleation, diffusional growth of cloud droplets, stochastic coalescence, and fallout of raindrops. Some numerical aspects of the models are discussed; in particular, a method is described which allows long time steps to be used for the calculation of condensation in regions of relatively old droplet populations. The bulk-physical and microphysical models are compared for a non-precipitation case. The numerical results revealed that the inclusion of microphysics had little effect on the whole cloud dynamics, a result which considerably differs from that of Árnason and Greenfield.

The microphysical model produced hi-modal spectra of droplets through the interaction of a mid-level nucleation region, which corresponds to the base of an upper intense thermal, and the vertical advection and diffusion of existing droplets from a lower thermal. Supersaturations calculated ranged from approximately 0.4 to 1% in non-nucleation regions to slightly over 2% in nucleation regions for a non-coalescence run in which Warner's nuclei distribution was assumed. Some calculations with coalescence included resulted in unrealistically high values of supersaturation, which were caused by the model's inability to replenish scavenged droplets fast enough. The results indicate not only that breakup will have to be included (as is physically clear), but that the nuclei resolution may have to be extended to relatively high values of critical supersaturation to account for droplet replenishment in the presence of scavenging raindrops.

Abstract

Two cumulus cloud models in two space dimensions are presented; one is a bulk physical model without microphysics and the other includes the microphysical processes such as nucleation, diffusional growth of cloud droplets, stochastic coalescence, and fallout of raindrops. Some numerical aspects of the models are discussed; in particular, a method is described which allows long time steps to be used for the calculation of condensation in regions of relatively old droplet populations. The bulk-physical and microphysical models are compared for a non-precipitation case. The numerical results revealed that the inclusion of microphysics had little effect on the whole cloud dynamics, a result which considerably differs from that of Árnason and Greenfield.

The microphysical model produced hi-modal spectra of droplets through the interaction of a mid-level nucleation region, which corresponds to the base of an upper intense thermal, and the vertical advection and diffusion of existing droplets from a lower thermal. Supersaturations calculated ranged from approximately 0.4 to 1% in non-nucleation regions to slightly over 2% in nucleation regions for a non-coalescence run in which Warner's nuclei distribution was assumed. Some calculations with coalescence included resulted in unrealistically high values of supersaturation, which were caused by the model's inability to replenish scavenged droplets fast enough. The results indicate not only that breakup will have to be included (as is physically clear), but that the nuclei resolution may have to be extended to relatively high values of critical supersaturation to account for droplet replenishment in the presence of scavenging raindrops.

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