Impacts of Stable Boundary Layer on Tropical Cyclone Structure and Intensity in Coupled WRF Model

It has been recognized that air-sea interaction plays an important role in Tropical Cyclones (TCs) intensity. However, exactly how the air-sea coupling processes affect the atmospheric boundary layer and, ultimately affect the TC structure and intensity remains unresolved. Recent observations from the Impact of Typhoon on Ocean in Pacific (ITOP) field campaign have shown that the storm-induced ocean cooling can lead to the development of persistent stable boundary layer in TCs, which may have a significant influence on the near surface inflow layer. The same feature is also been found in the numerical experiments of Typhoon Choiwan (2009) using high-resolution, coupled WRF-3DPWP (CWRF) model. To quantify the impact of air-sea coupling induced stable boundary layer and its associated thermodynamic and dynamic processes on storm structure, uncoupled WRF simulation is also conducted for comparison. A comprehensive tracer and trajectory analysis has been developed in the models to examine the dynamic and thermodynamic properties of the near surface flow. It is shown that convection is suppressed in the rainbands downstream of the area of stable boundary layer over the storm-induced cold wake. It is also found the near surface inflow air in the stable boundary layer in CWRF tends to penetrate further inward into the TC inner core and eyewall with higher theta-e value due to enhanced surface heat flux. In contrast, the air in unstable and/or neutral boundary layer in the uncoupled WRF model tends to go into rainbands. In addition to the thermodynamic impact, the cold wake-induced near surface pressure gradient can enhance the inflow angle and strength. Therefore, the stable boundary layer can have a directly impact TC structures on not only the outer rainband organization but also on inner core region, such as the size and strength of eyewall. More detailed analysis of mass and energy budgets are underway.