Colloquium: Dr. Raymond Shaw, Michigan Tech University
Off Campus: via Webex
Location
Online
Colloquium: Dr. Raymond Shaw, Michigan Tech University – Online Event
Date & Time
September 2, 2020, 3:30 pm – 4:30 pm
Description
TITLE: Stratocumulus in a box: Resolving "sub-grid-scale" aerosol-cloud interactions in a convection-cloud chamber
ABSTRACT:
Cloud-aerosol interactions in a turbulent mixed layer are studied by creating moist Rayleigh-Benard convection in a laboratory chamber (the Pi Chamber). In this several-cubic-meter turbulent cloud, all microphysical processes that are usually considered "sub-grid-scale" even in detailed models, are represented. Cloud formation is achieved by injecting aerosols into the water-supersaturated environment created by the isobaric mixing of saturated air at different temperatures. In steady state, the injection and activation of aerosol particles to form cloud droplets is balanced by cloud droplet growth through vapor condensation and loss by gravitational settling. A range of steady-state cloud droplet number concentrations is achieved by supplying aerosols at different rates. The experiments strikingly reproduce empirical microphysical signatures observed in stratocumulus clouds, such as the linear relationship between mean volume and effective radius. Current explorations are focusing on the influence of turbulence on cloud droplet activation and onset of collision-coalescence in warm clouds, and degree of glaciation in mixed-phase clouds.
Aerosol particles, such as sea salt, dust and anthropogenic pollution influence the optical properties of clouds and the tendency of a cloud to form precipitation through droplet collisions. We have investigated cloud droplet growth in a turbulent environment under varying levels of aerosol concentration. The results reveal a surprising role of turbulence in cloud droplet growth that leads to two regimes: a polluted cloud regime in which thermodynamic conditions are rather uniform and cloud droplet sizes are similar, and a clean cloud regime in which thermodynamic conditions are highly variable and cloud droplet sizes are very diverse. The narrowing of droplet size range under polluted conditions introduces a new stabilizing factor by which increased aerosol concentration can suppress precipitation and enhance cloud brightness.
Cloud-aerosol interactions in a turbulent mixed layer are studied by creating moist Rayleigh-Benard convection in a laboratory chamber (the Pi Chamber). In this several-cubic-meter turbulent cloud, all microphysical processes that are usually considered "sub-grid-scale" even in detailed models, are represented. Cloud formation is achieved by injecting aerosols into the water-supersaturated environment created by the isobaric mixing of saturated air at different temperatures. In steady state, the injection and activation of aerosol particles to form cloud droplets is balanced by cloud droplet growth through vapor condensation and loss by gravitational settling. A range of steady-state cloud droplet number concentrations is achieved by supplying aerosols at different rates. The experiments strikingly reproduce empirical microphysical signatures observed in stratocumulus clouds, such as the linear relationship between mean volume and effective radius. Current explorations are focusing on the influence of turbulence on cloud droplet activation and onset of collision-coalescence in warm clouds, and degree of glaciation in mixed-phase clouds.
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