The Roles of Antecedent Soil Moisture and Land–Atmosphere Interactions in the Inland Intensification of Tropical Storm Bill (2015)

Abstract Extreme weather events cause significant societal impacts, in particular, when they behave unexpectedly. The “brown ocean (BO)” effect, describing the ability of the land surface via soil moisture to support tropical cyclone (TC) maintenance and intensification (TCMI) after landfall, remains poorly understood. Building upon our previous modeling framework utilizing the NASA Unified WRF (NU-WRF) system, this follow-on study explores the contributions of the dynamics of the soil moisture and advected water vapor to the TCMI of Tropical Storm (TS) Bill (2015) over the U.S. southern Great Plains (SGP). Impacts of various soil moisture conditions and surface enthalpy flux conditions on Bill’s inland intensification were investigated by comparing their land–atmosphere interaction components of energy fluxes along with a backward trajectory analysis and three-dimensional visualization of low-level atmospheric moisture. Results demonstrate that the high antecedent soil moisture across the central United States (Great Plains and Mississippi Valley) from prior rainfall was crucial for the TCMI of TS Bill over the SGP. Without ample latent heat flux over land, even a moisture-laden low-level jet from the ocean rapidly dried over land, preventing intensification and causing storm dissipation in our simulations. Backward trajectory analysis suggests that high soil moisture content can enhance humidity within the storm’s inflow, including the advection from the ocean, far inland, thus supporting the TCMIs. Ultimately, the inflow feeding the inland TC core is influenced by active land–air interactions within the boundary layer, where soil moisture and lower boundary conditions directly impact lower-tropospheric humidity, enabling or hindering the BO effect and subsequent TC intensification.