Key Underlying Dynamics of Ocean-Land-Air Interactions during the Landfall of TS Bill (2015)

Predicting the fate of TCs after landfall is getting attention more and more as unexpected intensification of landfalling TCs and severe damage due to the heavy rainfall by the remnants of the landfalling TCs have been witnessed more frequently recently. It is needed to comprehend the dynamic interactions between ocean air masses and land air masses when TCs ramp up to the land; how they evolve after landfall and how their maintenance or change affect the intensity of them and their fate. Also it is highly relevant that what other mechanisms are playing important roles in the determination of the TC maintenance and intensifications (TCMIs). For example, preexisting soil moisture condition and/or diurnal cycle of PBL height.

For the purpose, we analyzed the characteristic air flow patterns during the landfall of TS Bill (2015) using the NASA Unified Weather Research and Forecasting (NU-WRF) model and HYSPLIT trajectory model within the perspectives of “the Brown Ocean Effect”, the impact of the soil moisture conditions on the TC intensity change post-landfall. Our HYSPLIT backward trajectory analysis revealed that the inner core of the tropical cyclone vortex that had been formed over the ocean could survive after landfall because the vortical flow in the inner core could keep its contact with the mainstream of the warm and moist air in the surface-level atmosphere (e.g., planetary boundary layer height) from the tropical ocean for an extended period of time after landfall. The combination of the wet soil condition and the characteristic vortical flow chain enabled the TCMIs of the Bill. These detail descriptions of inflow air dynamics of landfalling TC will advance our understanding of them with potentially increased ability to differentiate and predict the TCMIs and plain decay of the storm.