水滴衝突を最適化するための対流・雲チャンバーの調整(Tailoring a Convection-Cloud Chamber for Optimizing Droplet Collisions)

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2024-03-12 パシフィック・ノースウェスト国立研究所(PNNL)

微粒子の衝突凝結は、微小な液滴の初期形成に重要な役割を果たすが、乱流雲でのこのプロセスの定量的理解は不足している。暖かく湿った底部と冷たく湿った上部の間に乱流雲を生成する対流雲チャンバーは、実験室で霧の形成を研究するための制御された環境を提供できる。この研究は、数値シミュレーションを使用して、衝突凝結を研究するための雲チャンバーの設計機能を探るものであり、高濃度でエアロゾルを注入することで液滴の数濃度と衝突頻度が増加することがわかった。

<関連情報>

衝突合体のための対流・雲チャンバーの設計と粒子物理学による大規模渦シミュレーション Designing a Convection-Cloud Chamber for Collision-Coalescence Using Large-Eddy Simulation With Bin Microphysics

Aaron Wang, Mikhail Ovchinnikov, Fan Yang, Silvio Schmalfuss, Raymond A. Shaw
Journal of Advances in Modeling Earth Systems  Published: 27 January 2024
DOI:https://doi.org/10.1029/2023MS003734

水滴衝突を最適化するための対流・雲チャンバーの調整(Tailoring a Convection-Cloud Chamber for Optimizing Droplet Collisions)

Abstract

Collisional growth of cloud droplets is an essential yet uncertain process for drizzle and precipitation formation. To improve the quantitative understanding of this key component of cloud-aerosol-turbulence interactions, observational studies of collision-coalescence in a controlled laboratory environment are needed. In an existing convection-cloud chamber (the Pi Chamber), collisional growth is limited by low liquid water content and short droplet residence times. In this work, we use numerical simulations to explore various configurations of a convection-cloud chamber that may intensify collision-coalescence. We employ a large-eddy simulation (LES) model with a size-resolved (bin) cloud microphysics scheme to explore how cloud properties and the intensity of collision-coalescence are affected by the chamber size and aspect ratio, surface roughness, side-wall wetness, side-wall temperature arrangement, and aerosol injection rate. Simulations without condensation and evaporation within the domain are first performed to explore the turbulence dynamics and wall fluxes. The LES wall fluxes are used to modify the Scalar Flux-budget Model, which is then applied to demonstrate the need for non-uniform side-wall temperature (two side walls as warm as the bottom and the two others as cold as the top) to maintain high supersaturation in a tall chamber. The results of LES with full cloud microphysics reveal that collision-coalescence is greatly enhanced by employing a taller chamber with saturated side walls, non-uniform side-wall temperature, and rough surfaces. For the conditions explored, although lowering the aerosol injection rate broadens the droplet size distribution, favoring collision-coalescence, the reduced droplet number concentration decreases the frequency of collisions.

Key Points

  • Collision-coalescence effects on a steady-state droplet size distribution are stronger in a taller chamber
  • Wet side walls are essential for maintaining cloud liquid water in a chamber with a low width-to-height aspect ratio
  • Rougher surfaces increase surface heat and moisture fluxes, leading to larger liquid water content that promotes collision-coalescence

Plain Language Summary

A convection-cloud chamber is useful in understanding how turbulence affects the interaction between aerosols and cloud droplets. The current convection-cloud chamber (the Pi Chamber) is likely too small to explore how turbulence affects the collision-coalescence among cloud droplets. To see whether collisional growth may be observable in a larger cloud chamber, we use numerical simulations to model the cloud droplet size distributions under several different configurations of the cloud chamber. The results suggest that the likelihood of detectable collisional growth increases significantly in a tall chamber with two warm and two cold saturated side walls and rough wall surfaces.

1702地球物理及び地球化学
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