Journal Article

Scaling of Turbulence and Microphysics in a Convection–Cloud Chamber of Varying Height

Cantrell, Will, Ovchinnikov, Mikhail, Shaw, Raymond A., Yang, Fan, Thomas, Subin

2023

Overview

The convection–cloud chamber enables measurement of aerosol and cloud microphysics, as well as their interactions, within a turbulent environment under steady‐state conditions. Increasing the size of a convection–cloud chamber, while holding the imposed temperature difference constant, leads to increased Rayleigh, Reynolds and Nusselt numbers. Large–eddy simulation coupled with a bin microphysics model allows the influence of increased velocity, time, and spatial scales on cloud microphysical properties to be explored. Simulations of a convection–cloud chamber, with fixed aspect ratio and increasing heights of H = 1, 2, 4, and (for dry conditions only) 8 m are performed. The key findings are: Velocity fluctuations scale as H1/3, consistent with the Deardorff expression for convective velocity, and implying that the turbulence correlation time scales as H2/3. Temperature and other scalar fluctuations scale as H−3/7. Droplet size distributions from chambers of different sizes can be matched by adjusting the total aerosol injection rate as the horizontal cross‐sectional area (i.e., as H2 for constant aspect ratio). Injection of aerosols at a point versus distributed throughout the volume makes no difference for polluted conditions, but can lead to cloud droplet size distribution broadening in clean conditions. Cloud droplet growth by collision and coalescence leads to a broader right tail of the distribution compared to condensation growth alone, and this tail increases in magnitude and extent monotonically as the increase of chamber height. These results also have implications for scaling within turbulent, cloudy mixed‐layers in the atmosphere, such as fog layers. Plain Language Summary In a convection–cloud chamber, turbulent convection is generated by heating the bottom surface and cooling the top surface. Water‐supersaturated conditions are achieved by maintaining the bottom and top surfaces wet. When aerosols are injected into the resulting turbulent, supersaturated flow, cloud droplets are formed and grow by vapor condensation, and possibly by collision and coalescence. The relative roles of condensation due to mean properties and fluctuating properties, as well as the role of collisional growth, depend on the time and velocity scales of the turbulence, all of which depend on the height of the chamber. In this work, turbulence and cloud properties in a convection–cloud chamber are simulated for several chamber heights. It is found that time and velocity scales increase with chamber height; cloud droplet size distributions can be approximately matched by appropriately increasing the aerosol injection rate; and the relevance of collisional growth increases with chamber height. These findings will help guide future computational and laboratory implementations of cloud formation in thermal, moist convection. Key Points Increasing the height of a convection‐cloud chamber leads to an increase in characteristic velocity and time scales Cloud droplet size distributions can be approximately matched by increasing the total aerosol injection rate as the square of the height Concentration of large cloud droplets due to collision and coalescence increases monotonically with increasing height

• Publisher: John Wiley & Sons, Inc,American Geophysical Union (AGU)
• Subject: Aerosols / Boundary conditions / cloud chamber / Cloud chambers / Cloud droplet collision / Cloud droplet growth / Cloud droplet size / Cloud droplet size distribution / Cloud droplets / Cloud microphysics / Clouds / Coalescence / Convection / Droplets / ENVIRONMENTAL SCIENCES / Fluctuations / Fog / Height / Laboratories / Microphysics / Rayleigh number / Reynolds number / Scaling / Size distribution / Temperature differences / Turbulence / turbulence scaling / Velocity