The Alignment and Intensification of Tropical Cyclone under a Sheared Environment

There is a general consensus that a vertically sheared environment is unfavorable for tropical cyclone (TC) intensification. However, due to the complicated role of asymmetric convection, the relationship between the intensity change and the strength of vertical shear becomes obscured. The controversy arises from the debate on the role of asymmetric convection especially when the shear is moderate. Of particular interest is how TCs intensify in a sheared environment with dramatic asymmetric convection. Relevant studies have been based on observations with discontinuous records or idealized numerical studies. In the current work, numerical simulation of a real case is carried out to explore the role of asymmetric convection on TC intensity.

The intensification of Typhoon Nanmadol (2004) is studied with the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), developed by the Naval Research Laboratory (NRL). The storm track, intensity and convection structure are well simulated by this model. Consistent with the previous observation and numerical studies, the intense convection appear to the downshear left side of the storm due to the stable forcing of the large-scale vertical shear. A cyclonic potential vorticity (PV) anomaly is generated by diabatic convection at the downshear left side. The PV anomaly then rotates anticlockwise and finally merges into TC center. As such, the vertical tilt reduces rapidly, minimum sea level pressure (MSLP) of TC drops rapidly and strong convection outbreaks in downshear left side close to the TC center. A new cyclonic PV anomaly is generated diabatically, which later merges again into a main PV center. As a result, the TC intensifies rapidly and the vertical tilt decreases further. It is also found that the intensification process is affected by the interaction between the inner-core convection and outer rain band.