The Sensitivity of Tropical Cyclone Intensification Rate Based on Ensemble Method

In order to improve the predictability of tropical cyclone (TC) intensity, it is helpful to understand the most sensitive factors that contribute to the forecast. The ensemble-based method is adopted to find these sensitive factors. To estimate the sensitivity of intensification rate, the partial correlation between the 6 h intensity change (as a metric of intensification rate) and several physical variables is calculated when current intensity is held constant.

The results show that the equivalent potential temperature (hereafter EPT) located at 1-3 times the radius of maximum wind (hereafter RMW) below 2 km has the largest value of correlation, which is approximately 0.7. For dynamical variables, the results show that the strength of the secondary circulation is positively correlated with the intensification rate. This implies that the secondary circulation is not simply a response for the current TC intensity; in contrast, it may play some roles in affecting the intensification rate.

Further analyses are conducted to understand how EPT at 1-3 times RMW below 2 km affects the intensification rate. The results show that the larger EPT air can lead to larger intensification rate in the following 4-6 h. This high EPT air can cause larger convective instability and vertical velocity in the eyewall region. The larger vertical velocity air releases more latent heat and helps transport higher absolute angular momentum to the inner core region in mid-level troposphere near 3-8 km, which in turn increases the tangential wind and inertial stability. The larger latent heat release along with the increasing heating efficiency helps TC to develop the warm core, and to reduce the minimum central sea-level pressure.

A new perspective for the intensification rate increase due to high EPT is proposed here from both dynamical and thermo-dynamical aspects. Also, the results can serve as useful guidelines for targeted observation of TCs.