Understanding Moisture Flow in the Brown Ocean Effect of Tropical Storm Bill (2015)

The complexities of the "Brown Ocean" (BO) effect, which hypothesizes tropical cyclones maintain and intensify over land due to soil moisture, remain poorly understood. This study investigates the role of soil moisture dynamics and advected water vapor in the inland intensification of Tropical Storm Bill (2015) over the U.S. Southern Great Plains. Using the NASA Unified WRF system (NU-WRF), we examine the impacts of various soil moisture conditions and surface enthalpy fluxes on Bill's inland intensification. Our results show that high antecedent soil moisture from prior rainfall was crucial for Bill's intensification over land. Backward trajectory analysis reveals that high soil moisture enhances humidity within the storm's inflow, including advection from the ocean, far inland. Without sufficient latent heat flux from wet soil surfaces, even a moist low-level jet from the ocean rapidly dries out over land, inhibiting intensification. Our findings also highlight the importance of land-atmosphere coupling and interactions in the boundary layer, where soil moisture and lower boundary conditions directly impact lower tropospheric humidity, enabling or hindering the BO effect and subsequent tropical cyclone intensification. Overall, this study provides further evidence that the BO effect operates on a spectrum, and is driven by complex interdisciplinary processes.