Dynamics of Shallow to Deep Convective Transitions During CACT

Abstract

Cumulus clouds are commonplace. They frequently exist in most tropical and subtropical atmospheric regimes on Earth. Most cumuli are rooted in the atmospheric boundary layer. They transport energy upward from the lowest layer of the atmosphere, the atmospheric boundary layer. 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. Determining where and when specific clouds develop can provide insight on how to improve numerical models to predict more closely exactly where and when adverse weather events might occur.

The Atmospheric Radiation Measurement Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign, conducted in central Argentina during austral summer 2018–19, took place near an isolated, narrow mountain range, where shallow cumulus clouds regularly formed. Many afternoons, these clouds grew into deep cumulonimbi, then moved eastward where they sometimes grew into large precipitation-producing systems. This work will use a variety of data from the field campaign to investigate the environmental conditions that were 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 (weather balloon) data. High-resolution numerical model simulations of selected days featuring growing convection will be leveraged as an extension to observations to investigate fine-scale features of dynamic importance that observations themselves cannot capture.

Specific objectives of this research involve determining physical processes in the atmosphere that control the transition of continental non-precipitating shallow clouds into deep cumulonimbi near terrain. More specifically, the research seeks answers to the following questions:

1. In a population of numerous shallow cloud elements, what determines which specific cumulus clouds grow vertically and precipitate and which clouds dissipate without ever producing precipitation? Are boundary layer or free tropospheric processes more important for growth?
2. Evolution of what key quantities (like vertical velocity and acceleration) over the life cycle of individual clouds or a broader cloud population permit deep convection?
3. How quickly do shallow to deep transitions of cloud populations occur?