Connections Between Sub-Cloud Coherent Updrafts and the Life Cycle of Maritime Shallow Cumulus Clouds in Large Eddy Simulation

We develop a novel approach to detect cloud-subcloud coupling during the cloud life cycle and analyze a large eddy simulation of marine shallow cumulus based on the Barbados oceanographic and meteorological experiment campaign. Our results demonstrate how the activity of sub-cloud coherent updrafts (SCUs) affect the evolution of shallow cloud properties during their life cycles, from triggering to development, and through to dissipation. Most clouds (similar to 80%) $(\sim 80\%)$ are related to SCUs during their lifetime but not every SCU (similar to 20% ${\sim} 20\%$ for short-lived ones) leads to cloud formation. The fastest growing SCUs in a relatively moist region are most likely to initiate clouds. The evolution of cloud base mass-flux depends on cloud lifetime. Compared with short-lived clouds, longer lived clouds have longer periods of development, even normalized by the full lifetime, and tend to increase their cloud base mass-flux to a stronger maximum. This is consistent with the evolution of mass flux near the top of SCU, indicating that the development of clouds is closely related to the sub-cloud activity. When the SCUs decay and detach from the lifting condensation level, the corresponding cloud base starts to rise, signifying the start of cloud dissipation, during which the cloud top lowers to approach the rising cloud base. Previous studies have described similar conceptual pieces of this relationship but here we provide a continuous framework to cover all the stages of cloud-subcloud coupling. Our findings provide quantitative evidence to supplement the conceptual model of shallow cloud life cycle and is critical to improve the steady-state assumption in parameterization. The coupling between shallow trade wind cumulus clouds and sub-cloud processes is not well understood and remains one of the roadblocks to improving our climate modeling capability. As model resolution increases, the steady-state assumption adopted in conventional convection schemes no longer holds and the knowledge about the evolution of clouds is necessary to improve the parameterization. Observations have provided evidence of sub-cloud coherent updrafts being the roots of shallow clouds, but it is currently not practical to continuously detect how the shallow clouds evolve alongside the sub-cloud coherent structures. Our study takes a step forward to demonstrate how the sub-cloud coherent structures interact with the shallow clouds. It is shown that the evolution of sub-cloud coherent structures plays an important role from cloud initiation, development and through to dissipation. The comprehensive physical picture of the whole cloud life cycle uncovered in this study provides useful insights to improve the representation of cloud evolution in convection parameterizations with an explicit treatment of the cloud life cycle in the future. The evolution of maritime shallow cloud properties during the life cycle is connected to the activity of sub-cloud coherent structures Clouds are likely to be triggered by the humid faster growing sub-cloud coherent structures and start to dissipate without their ongoing support The evolution of cloud base mass-flux depends on cloud lifetime and is asymmetric around the middle of life for short and mid-lived clouds