A High-resolution Simulation of Supertyphoon Rammasun (2014) —— Part I: Model Verification and Surface Energetics Analysis

A 72-h high-resolution simulation of Supertyphoon Rammasun (2014) is performed using the Advanced Research Weather Research and Forecasting model. The model covers an initial 18-h spin-up, the 36-h rapid intensification (RI) period in the northern South China Sea, and the 18-h period of weakening after landfall. The results show that the model reproduces the track, intensity, structure of the storm, and environmental circulations reasonably well. Analysis of the surface energetics under the storm indicates that the storm's intensification is closely related to the net energy gain rate (ε g), defined as the difference between the energy production (P D) due to surface entropy flux and the energy dissipation (D S) due to surface friction near the radius of maximum wind (RMW). Before and during the RI stage, the ε g is high, indicating sufficient energy supply for the storm to intensify. However, the ε g decreases rapidly as the storm quickly intensifies, because the D S increases more rapidly than the P D near the RMW. By the time the storm reaches its peak intensity, the D S is about 20% larger than the P D near the RMW, leading to a local energetics deficit under the eyewall. During the mature stage, the P D and D S can reach a balance within a radius of 86 km from the storm center (about 2.3 times the RMW). This implies that the local P D under the eyewall is not large enough to balance the D S, and the radially inward energy transport from outside the eyewall must play an important role in maintaining the storm's intensity, as well as its intensification.