Numerical Assessment of Advective and Diffusive Dynamics of Interacting and Isolated Prototypical Convectively Initiated Circulations

The bulk circulation associated with convective clouds includes not only a region of updraft and cloudy air but also a region of compensating descent and cloud-free air and horizontal motions coupling these regions. The Kinematic Representation of Non-rotating Updraft Tori (KRoNUT) model is a simple representation of this entire flow. First, the skill of the KRoNUT in representing flows from a high resolution full-physics simulation of marine tropical convection is compared to various plume representations of convection. Then the KRoNUT is used to construct bulk descriptions of the dry dynamics of isolated and interacting convective circulations under the influence of advection and diffusion (only). Cross sections of advective and diffusive tendencies show that while vertical advection of the vertical wind is the most important advective tendency in clouds, the horizontal component of the convective circulation and advection thereof plays a crucial role in the evolution of circulations in the absence of buoyancy. Strong curvature of the flow near the surface and near the updraft core results in locally strong diffusive tendencies that depend on scale. Cross sections of tendencies from the KRoNUT compare favourably to results from the simulation. Interacting circulations are shown to exhibit a wide range of dynamics with some cases of interactions leading to unique stability of geometric properties of otherwise evolving flows and some leading to geometric clustering of circulation centers.

💡 Research Summary

This paper presents a numerical investigation into the fundamental dry dynamics of convective cloud circulations, introducing and validating a novel conceptual model called KRoNUT (Kinematic Representation of Non-rotating Updraft Tori). The study aims to bridge the gap between overly simplistic parcel models and statistically averaged plume models by providing a simplified yet holistic representation of the full circulation associated with convection, including the updraft, the compensating subsidence, and the connecting horizontal flows.

The core of the methodology involves two key phases. First, the authors assess the skill of the KRoNUT model against more traditional plume representations. Using high-resolution, cloud-resolving model simulations of tropical marine convection (based on the DYNAMO field campaign), they objectively identify and extract convective circulation cores. By fitting parameters for both KRoNUTs and various plume types (differing in shape and vertical velocity profile) to the simulated wind fields and then incrementally removing the energy associated with these fitted circulations, they evaluate which model best captures the organized flow. The results indicate that the KRoNUT performs comparably or better than the plume models in explaining the vertical kinetic energy of the simulated circulations, establishing its utility as a representative model of the mean convective flow structure.

Second, leveraging the KRoNUT framework, the paper delves into a detailed process-level analysis of how advection and diffusion alone govern the evolution of both isolated and interacting convective circulations in the absence of buoyancy forcing. By analytically deriving and numerically visualizing the spatial cross-sections of individual tendency terms (e.g., -u ∂w/∂r, -w ∂w/∂z, viscous diffusion), the authors uncover several key insights. While vertical advection of vertical momentum is confirmed as the dominant forcing within the cloud core, the horizontal component of the circulation and its advection play a crucial role in shaping the evolution of the flow, particularly in the subsiding shell and the broader circulation. Furthermore, strong curvature in the flow field near the surface and the flanks of the updraft core leads to locally intense diffusive tendencies, the magnitude of which is highly sensitive to the scale (aspect ratio) of the circulation. This implies that short, wide circulations may evolve very differently under dry dynamics than tall, narrow ones. Finally, preliminary analysis of interacting KRoNUT pairs reveals a spectrum of behaviors, from cases where interaction stabilizes the geometric properties of the flows to those that lead to geometric clustering of circulation centers.

In conclusion, the study successfully demonstrates that the KRoNUT model serves as a powerful and simplified tool for disentangling the complex kinematics of convective circulations. It provides a novel perspective on the intrinsic dry dynamics driven by advection and diffusion, setting a foundational framework for future work that intends to incorporate the critical effects of latent heating and dynamic pressure.