The GRAPES (Global/Regional Assimilation and PrEdiction system) model was used to study the rapid intensification of Super Typhoon Rammasun (1409) offshore China in this paper. On the basis of successful simulation of the intensity change and track

the model output was further analyzed to determine the large-scale environmental condition of Typhoon Rammasun and the mechanism for its rapid intensity change. The results show that the favorable background field with low-level convergence and high-level divergence occurred during the rapid intensification

abundant water vapor transport at lower-levels provided favorable energy for Typhoon

which was mainly transported by the southeastern boundary. Besides

the weak vertical wind shear lower than 6 m/s was in favor of gathering the latent heat of condensation and maintaining the “warm heart” structure

and there was a time lag of about 24 hours between the vertical wind shear reduced and Typhoon's increased sharply. Analyses on the structure showed that the dynamic and energetic fields were highly asymmetric during its strengthening stage but became structurally symmetric during its mature stage. Convection was stronger in the southeast of the typhoon than in the northwest during its strengthening stage. The range of eyewall was small in the low-level but extended in the high-level. Positive vorticity increased in the eye of the typhoon and expanded to the mid and upper-troposphere. Besides

the tangential-wind speeds increased with the increasing of typhoon intensity

especially during the process of rapid intensification. Furthermore

there was evidently warm-core structure in the lower and mid-upper level which highly concentrated to the center of Rammasun

and the positive temperature anomaly of the typhoon center exceeded 11 ℃

while the warm-core extending towards to lower and higher layer. All above happened during the rapid intensification were the main reasons of the intensification of Rammasun.