Quantification of the Land Surface and Brown Ocean Influence on Tropical Cyclone Intensification over Land: A Case Study of TC Kelvin (2018)

The ‘Brown Ocean (BO)’ effect, describing the ability of the land surface via soil moisture to support the maintenance and intensification tropical cyclones (TCs) after landfall, has taken on increased notoriety in recent years. In this study, an investigation of TC Kelvin in February 2018 over northeast Australia was conducted to understand the mechanisms of the BO effect, and in the process to develop a comprehensive analysis framework for landfalling TCs.

The NASA’s Land Information System (LIS) coupled to the NASA Unified WRF (NU-WRF) system was employed as the numerical model framework for 13 preliminary simulations and 11 main sensitivity simulation experiments targeting the BO effect. Impacts of soil moisture conditions and surface enthalpy flux conditions on TC Kelvin were investigated by evaluating simulated track and intensity, midlevel atmospheric thermodynamic properties, vertical wind shear, total precipitable water (TPW), and surface moisture flux closely. The results suggest that although there were recognized differentiations among the sensitivity simulations by the soil moisture conditions, the intensification of TC Kelvin over land was more strongly related to advected atmospheric moisture and the diurnal cycle of solar radiation (i.e., radiative cooling) than to overall soil moisture conditions or surface fluxes. As a result the microphysics and radiations schemes of the NU-WRF were more influential than the LIS and land surface model specifications. The analysis framework employed here for TC Kelvin can serve as a foundation to specifically quantify the factors governing the BO effect, and demonstrates that the BO is not a binary influence (i.e. all or nothing), but instead operates in a continuum from largely to minimally influential such that it could be incorporated to help improve prediction of inland effects for all landfalling TCs.