Summaries - Office of Research & Innovation
Back Dynamics of Shallow to Deep Convective Transitions During CACTI
|Division||Graduate School of Engineering & Applied Science|
|Sponsor||Department of Energy (Other-Fed)|
Cumulus clouds are commonplace. They frequently exist in most tropical and subtropical atmospheric regimes on Earth. Most cumuli are rooted in the boundary layer. They draw moist static energy from the boundary layer and displace it upward into upper parts of the boundary layer or into the lower free troposphere. Most cumulus clouds do not develop into cumulonimbus clouds, meaning that they never produce precipitation. However, given favorable conditions, a cumulus cloud that does grow into a cumulonimbus cloud can grow upscale vertically and horizontally into expansive mesoscale convective systems that may produce heavy rain, large hail, and lightning, especially over continents and/or where upward motion is enhanced by terrain.
The ARM CACTI field campaign, conducted in central Argentina during austral summer 2018-19, was executed near an isolated, narrow mountain range, where shallow cumulus clouds were regularly focused. Many afternoons, these clouds grew into deep cumulonimbi, then moved eastward where they sometimes grew upscale into large systems. This work will investigate the environmental conditions, both in the boundary layer and lower free troposphere, that are responsible for the timing of the growth of the cloud population as a whole and for the mechanisms responsible for the growth of individual clouds. C-band scanning radar data will be used to characterize the growth of the entire cloud population during cases when convection appeared to be enhanced by mesoscale boundaries such as cold pools and separately during cases when convection appeared to grow over terrain without any external influence. Radar data will also be used to further examine relationships between rain rate and various thermodynamic and kinematic variables derived from rawinsonde data.
The radar data will also serve as a verification tool for numerical modeling experiments that will explore the impact of vertical accelerations due to buoyancy and vertical pressure gradients on the vertical growth of cumulus clouds in early stages of their life cycles. Output from numerical model experiments will be used to calculate quantities such as in-cloud vertical accelerations that cannot be readily determined using observations alone. Specifically, we will investigate the potential importance of cloud base vertical velocity and lower free tropospheric total vertical acceleration on a cloud's growth by following individual cloud evolution in the model through time. The importance of these quantities on the timing and duration of observed shallow to deep convective transitions will be investigated. Finally, simulated vertical velocity and acceleration will be connected to quantities currently used to attempt prediction of rainfall intensity, such as tropospheric relative humidity, convective available potential energy, and formulations of convective organization or clustering.
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