Sensitivity of the Size of a TC to Sea Surface Temperatures in Its Outer Region

We investigated the sensitivity of the size of a tropical cyclone (TC) to warming or cooling sea surface temperatures (SST) in its outer region by simulating the SST beyond a radius of 200 km from the TC center. Sensitivity experiments showed that an increased SST outside the core region of the TC had a negative effect on its size. Warming in the outer region contributed to the local enhancement of the latent heat flux from sea surface, which promoted the development of small-scale convection and warmed the lower and midtroposphere. This warming altered the local pressure gradient force in the upper and lower troposphere in such a way that it weakened the secondary circulation of the TC and led to suppression of the spiral rainbands outside the eyewall. Further analysis showed that the outward-propagating rainband structure favored an increase in the size of the TC. The diabatic heat released by the rainbands induced an inflow at lower levels, facilitating expansion of the TC. The greater the distance of the rainbands from the center of the TC, given the same amplitude of diabatic heating, the stronger the forced inflow, resulting in a faster increase in the size of the TC.