Exploring Causal Relationship between Environment and Drizzle Properties using Machine Learning

Cloud and precipitation properties are controlled by both local and large-scale forcings. Current weather and climate models represent clouds and precipitation through parameterizations that are based on theoretical relationships between environment, clouds, and precipitation. However, these relationships vary considerably among different weather and cloud conditions, thereby leading to inaccurate simulation of cloud and precipitation properties. In this study, we use observations from a site in the Eastern North Atlantic Ocean (28W, 39.5N) to establish a potential causal relationship between large-scale environment, cloud, and precipitation properties. We estimate the structure of a directed acyclic graph (DAG) with the NOTEARS algorithm (Non-combinatorial Optimization via Trace Exponential and Augmented lagRangian for Structure learning) (Zheng et al., 2018 \cite{Zheng2018DAGsLearning}) with a multi-layer perceptron (MLP) neural network classification architecture. We classify liquid water path (LWP), rain rate, and rain drop diameter in two classes based on lower and upper quantiles to identify the governing mechanisms responsible for the two tails of the distribution. We also invoke Random Forest classification to compare our causal model results with conventional decision tree-based approaches. We hypothesize the dominant role of cloud LWP and net radiative cooling in controlling the cloud and precipitation properties. In this way, this study demonstrates the application of a causal machine learning method to identify which environmental properties potentially control cloud and precipitation development. These results will be extremely valuable to both observational and numerical modeling communities as they could help improve the current parameterizations in the weather and climate models.