A simple model for mixing and cooling in cloud-wind interactions

TL;DR

This paper presents an entropy-based formalism to analyze mixing and cooling in cloud-wind interactions, simplifying complex system states and enabling better understanding of cloud evolution and growth.

Contribution

It introduces a novel entropy-based approach to quantify mixing effects and generalizes criteria for cloud growth considering various physical processes.

Findings

Mixing mainly affects the entropy distribution along the $K$ direction.

The shape of the cooling curve influences cloud condensation.

The formalism can be extended to include additional physics effects.

Abstract

We introduce a simple entropy-based formalism to characterize the role of mixing in pressure-balanced multiphase clouds, and demonstrate example applications using Enzo-E (magneto)hydrodynamic simulations. Under this formalism, the high-dimensional description of the system's state at a given time is simplified to the joint distribution of mass over pressure ($P$) and entropy ($K=P/\rho^\gamma$). As a result, this approach provides a way for (empirically and analytically) quantifying the impact of different initial conditions and sets of physics on the system evolution. We find that mixing predominantly alters the distribution along the $K$ direction and illustrate how the formalism can be used to model mixing and cooling for fluid elements originating in the cloud. We further confirm and generalize a previously suggested criterion for cloud growth in the presence of radiative cooling, and demonstrate that the shape of the cooling curve, particularly at the low temperature end, can play an important role in controlling condensation. Moreover, we discuss the capacity of our approach to generalize such a criterion to apply to additional sets of physics, and to build intuition for the impact of subtle higher order effects not directly addressed by the criterion.