Tracing the Rain Formation Pathways in Numerical Simulations of Deep Convection

Quantifying the microphysical process contributions to surface precipitation in numerical simulations can be challenging. This is due to the fact that many microphysical processes contribute to the formation and depletion of rain drops and there is almost always a spatial/temporal mismatch between where/when rain is formed and where/when it strikes the surface. In this work, we develop a tracing method that tracks the sources and sinks of raindrop mass and number as they are advected by the Weather Research and Forecasting model. Applying the method to an idealized squall line confirms that convective precipitation is dominated by warm rain processes (autoconversion and accretion) while stratiform precipitation is dominated by the melting of rimed and unrimed ice crystals. Sensitivity experiments in which the prescribed cloud drop number concentration is increased confirm the conventional wisdom that weakened autoconversion increases the fraction of raindrops originating from cold rain processes. The method also reveals that when applied to deep convection the Khairoutdinov and Kogan autoconversion scheme produces an excessive number of raindrops which are subsequently clipped in P3 microphysics to keep the rain size distribution within prescribed limits. This problem can mostly be mitigated by increasing the assumed radius for raindrops created by autoconversion. Plain Language Summary Weather and climate models simulate precipitation by accounting for a multitude of processes that form and remove rain drops. Due to spatial and temporal lags between the processes and the consequent surface precipitation, it can be challenging to quantitatively link the former to the latter. This paper remedies the challenge with a tool that tracks processes contributions throughout the rain lifecycle. The tool's utility is demonstrated through an idealized squall line which produces precipitation through various microphysical pathways. Additionally, perturbations to model parameters and their consequent impact on the squall line are interpreted through the lens of the new tool. Key Points A tool to trace the process contributions to surface precipitation is developed The tool confirms dominance of warm processes in convective rain and cold processes in stratiform rain The tool reveals that the autoconversion formulation produces an excessive amount of rain drops that the microphysics scheme clips