Multiple Equilibria and Soil Moisture-Precipitation Feedbacks in Idealized Convection-Permitting Simulations With an Open Hydrological Cycle

Abstract

Soil moisture-precipitation feedbacks are influenced by both small-scale land-atmosphere coupling and large-scale atmospheric circulations, and their sign has important implications for the stability of regional hydroclimate. However, the importance of both local and non-local processes makes it difficult to model soil moisture-precipitation feedbacks with high fidelity, limiting our ability to use models to understand controls on their sign. Here, we address this challenge by exploring a promising but seldom-used approach to studying soil moisture-precipitation feedbacks over tropical land: coupling small-domain convection-permitting simulations to a land-like surface and a parameterization of large-scale dynamics. The large-scale dynamics parameterization, based on the weak temperature gradient (WTG) approximation, is a key component that produces an open hydrological cycle with interactive moisture convergence. We first show that WTG-constrained simulations coupled to a freely-evaporating land surface support both precipitating and non-precipitating equilibria across a wide range of insolation. We then leverage this bistability to probe the influence of soil moisture feedbacks on dry spells by asking whether non-precipitating equilibria remain stable as the underlying surface dries out. We find that surface drying can trigger transitions from dry equilibria back to precipitating equilibria—a negative soil moisture-precipitation feedback—and attribute this transition to increasingly inefficient boundary layer ventilation by the parameterized large-scale circulation. In sensitivity experiments, alternative versions of the WTG scheme modify the parameter space where the negative feedback occurs, but none eliminate it entirely. Our results provide a foundation for leveraging the rich behavior of WTG-constrained simulations to probe controls on soil moisture-precipitation feedbacks over tropical land.