Stable boundary layer and its impact on tropical cyclone structure in a coupled atmosphere-ocean model

Abstract

The atmospheric boundary layer (BL) in tropical cyclones (TCs) connects deep convection within rainbands and the eyewall to the air-sea interface. Although the importance of the BL in TCs has been widely recognized in recent studies, how physical processes affect TC structure and intensity are still not well understood. This study focuses on a particular physical mechanism through which a TC-induced upper-ocean cooling within the core circulation of the TC can affect the BL and TC structure. A coupled atmosphere- ocean model forecast of Typhoon Choi-Wan (2009) is used to better understand the physical processes of air- sea interaction in TCs. A persistent stable boundary layer (SBL) is found to form over the cold wake within the TC's right-rear quadrant, which influences TC structure by suppressing convection in rainbands downstream of the cold wake and enhancing the BL inflow into the inner core by increasing inflow angles over strong SST and pressure gradients. Tracer and trajectory analyses show that the air in the SBL stays in the BL longer and gains extra energy from surface heat and moisture fluxes. The enhanced inflow helps transport air in the SBL into the eyewall. In contrast, in the absence of a TC-induced cold wake and an SBL in an uncoupled atmosphere model forecast, the air in the right-rear quadrant within the BL tends to rise into local rainbands. The SBL formed over the cold wake in the coupled model seems to be a key feature that enhances the transport of high energy air into the TC inner core and may increase the storm efficiency. © 2014 American Meteorological Society.