How vertical wind shear affects the intensification of Typhoon Jangmi (2008)

Vertical wind shear is one of the major influencing factors that inhibit tropical cyclone intensification. Large shears create asymmetries in cyclone vortex structure and its accompanying features (i.e., distribution of vertical velocity, cloud water, and rainfall). One of the most prominent theories describing the impact of shear on intensification states that in high shear environments the upper-level equivalent potential temperature maximum is vented from the cyclone center to the eyewall and the storm's minimum sea-level pressure subsequently rises through hydrostatic adjustment. Another prominent theory explains shear's impact by emphasizing the accompanying vortex tilt, which, through a thermal adjustment, causes warming near the TC center; this is thought to stabilize the cyclone.

These two theories are examined using a numerical simulation of Typhoon Jangmi (2008) with an advanced research version of the Weather and Research Forecasting Model at a high resolution (3km). The model simulated storm-averaged wind characteristics are determined and the vertical shear is diagnosed. The sensitive region of equivalent potential temperature impact on minimum sea-level pressure is found to be between 200-300hPa and 30-50km from Jangmi's center, although, notably, the simulated warm core is found to reside below 500hPa.

This study finds the venting hypothesis of Frank and Ritchie to be a more accurate way of explaining the shear-intensity relationship during Jangmi's evolution than the mid-level warming and stabilization theory of DeMaria. It is found that Jangmi's intensification period ends due to the large gradient that develops between equivalent potential temperature values in the eye and eyewall due to venting; this gradient reduces the lowering of MSLP and stabilizes the eyewall. Importantly, high levels of outward equivalent potential temperature fluxing descend from roughly 100hPa to slightly below 200hPa as the shear increase takes place around 57 hours; this is highly correlated with the end of Jangmi's intensification period. Despite large shear values, there does not appear to be any significant vortex tilt. A likely explanation for this is Jangmi's high penetration depth due to its size, strength, and high latitude at its peak intensity. Notwithstanding, the heating that does occur in the midlevels does not appear to lead to stabilization and weakening.