Effects of Perturbation Type on Tropical Cyclone Size over Tropical North Western Pacific and Atlantic

In authors’ previous studies, the role of distinctive mean states in the western North Pacific (WNP) and North Atlantic (NA) in affecting tropical cyclone (TC) size was investigated. In this study, the effect of different synoptic scale perturbation types in the two basins on TC size is further investigated. Numerical model experiments with an initial synoptic wave train (SWT) and easterly wave (EW) environmental condition show that the former (latter) leads to the development of a larger (smaller) TC at an equilibrium state. The physical mechanism responsible for the difference is revealed. Compared with the EW, the surface wind speed of the SWT is larger. Therefore, the SWT generates more moisture in the outer region than EW through surface evaporation process. This favors the development of stronger convection in the outer region. The enhanced convection leads to greater diabatic heating, which lower the local sea level pressure (SLP), which changes surface radial pressure gradients in the inner-core and outer-core of the vortex. On the one hand, the falling of the SLP decreases the surface radial pressure gradient in the inner-core. As a result, the radial wind is weakened and then the radius of maximum wind (RMW) extends outwards after initial contraction. On the other hand, the lower SLP increases the radial pressure gradient in the outer region, strengthening the radial wind outside. The wind convergence favors stronger development of convection in the outer region. Through this positive feedback, inflow in the outer region is further strengthened. This accelerates local tangential wind and thus enlarges the TC size.