11.4 Aerosol-Cloud Feedbacks between Supersaturation and Updraft Speed

Thursday, 8 August 2013: 11:15 AM
Multnomah (DoubleTree by Hilton Portland)
Amanda M. Sheffield, Colorado State University, Fort Collins, CO; and S. C. van den Heever, L. D. Grant, and C. McGee

Mesoscale systems are both dynamically and microphysically influenced. As represented in the vertical momentum equation, dynamical control of cloud development includes a balance between the updraft buoyancy and the production of condensate. However, the dynamics and microphysics are inherently linked, since the cloud buoyancy – and hence updraft speed and cloud lifetime – is a function of hydrometeor formation and latent heat release. Hence, a theoretical study of the feedbacks between buoyancy and hydrometeor formation, including their links to supersaturation, is of interest. One factor useful in this assessment is to consider the presence of aerosol particles, which have been shown to influence the formation of hydrometeors and subsequent latent heat release.

Studies investigating the activation of cloud droplets with cloud parcel models have been completed, including advanced techniques for the activation of aerosol particles and cloud droplets assuming a constant updraft speed. However, with the assumption of a constant updraft speed, these studies cannot incorporate the continuous feedback between supersaturation, cloud droplet and/or ice formation, latent heat release, and updraft strength. Though this feedback is simulated by cloud resolving models (most accurately by those without saturation adjustment), the theoretical limits of the influence of cloud water and ice formation processes on cloud updraft speed and supersaturation (and vice versa) has not been investigated. These theoretical boundaries are especially relevant to aerosol-cloud interaction studies, in which aerosol-induced convective invigoration through warm and cold phase processes are investigated.

This study examines the complex relationship between a cloud's supersaturation control of cloud droplet and ice mass formation, updraft speed, subsequent supersaturation generation, and so on. This feedback of processes is key to our understanding of updraft dynamics. Using a formulation of equations, the authors seek to develop a set of theoretical limits on updraft enhancement by (1) the interaction of cloud condensation nuclei (CCN) and warm clouds; and (2) the interaction of CCN and mixed-phase clouds. These theoretical limits are developed using approximate values of hydrometeor mass formation from completed cloud resolving model simulations of several mesoscale systems. This microphysical analysis has implications for a breadth of mesoscale systems because of the updraft's control on the dynamics of a single convective system that can feed back to the larger scale system itself.

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