Using Machine Learning to Predict Cloud Turbulent Entrainment-Mixing Processes

Abstract

Different turbulent entrainment-mixing mechanisms between clouds and environment are essential to cloud-related processes; however, accurate representation of entrainment-mixing in weather/climate models still poses a challenge. This study exploits the use of machine learning (ML) to address this challenge. Four ML (Light Gradient Boosting Machine [LGB], eXtreme Gradient Boosting, Random Forest, and Support Vector Regression) are examined and compared. It is found that LGB performs best, and thus is selected to understand the impact of entrainment-mixing on microphysics using simulation data from Explicit Mixing Parcel Model. Compared with traditional parameterizations, the trained LGB provides more accurate microphysical properties (number concentration and cloud droplet spectral dispersion). The partial dependences of predicted microphysics on features exhibit a strong alignment with physical mechanisms and expectations, as determined by the interpreting method, thus overcoming the limitations of the “black box” scheme. The underlying mechanisms are that the smaller number concentration and larger spectral dispersion correspond to more inhomogeneous entrainment-mixing. Specifically, number concentration after entrainment-mixing is positively correlated with adiabatic number concentration and liquid water content affected by entrainment-mixing, and inversely correlated with adiabatic volume mean radius. Spectral dispersion after entrainment-mixing is negatively correlated with liquid water content affected by entrainment-mixing, turbulent dissipation rate and relative humidity of entrained air. Sensitivity analysis further suggests that number concentration is mainly determined by cloud microphysical properties whereas spectral dispersion is influenced by both cloud microphysical properties and environmental variables. The results indicate that the LGB scheme has the potential to enhance the representation of entrainment-mixing in weather/climate models.