High Resolution Simulation of Super Typhoon Ramonsoon(2014): Inner Core Structure, Convective Burst and Validation of a Simple Energetic Model

Super Typhoon Ramonsoon(1409) is one of the most powerful landfall tropical cyclones(TCs) of China since 1949 which brought gale, heavy rains and storm surge to South China and leads to a great loss of lives and property. In this study, Ramonsoon is simulated with Typhoon Regional Assimilation and Predication System(T-RAPS) of State Key Laboratory of Severe Weather in Chinese Academy of Meteorological Sciences. Three nests of two-way interaction are employed with horizontal resolutions of 18, 6 and 2km. A 120 hours simulation is performed with this cloud-resolving numerical model and the initial time is 0000 UTC 16 July. Compared with best track analysis of Shanghai Typhoon Institution(STI), the simulation is successful with maximum track error less than 90km and simulated peak intensity of 71.5m/s which is almost the same as best track data. Furthermore, model verification by geostationary satellite images and doppler weather radar reflectivity indicate TC size and structure as well as the environment circulations in simulation are reasonable. Analysis is focused on the rapid intensification(RI) process in South China Sea before Ramonsoon landfall at Hainan Island at about 0700 UTC 18 July. According to best track analysis, the TC intensifies for 27m/s during 24 hours before landfall and in this simulation, RI begins at about 1000 UTC 17, and model predicted RI rate is 1.2m/s per hour in the following 20 hours period. Namely, RI process can be reproduced well. Significant variations in its inner core structures are found before and after RI onset. Before RI phase, symmetrical vortex structures are well established with an increasing of inertial instability in eye region. Meanwhile convective bursts(CBs) are found vigorous within radius of maximum wind(RMW) at 8km altitude shortly before RI onset. At the initial stage of RI, warm core extends upward from 9km to 15km and the eye keeps warming during RI phase with enhancing compensating subsidence from 12 to17km within TC inner core. However, the intensification is interrupted by the increasing of vertical wind shear from 1500 to 2100 UTC 17. During this period, CBs shift outside beyond RMW to downshear left quadrant and the TC keeps steady-state which confirms that TC intensification process is sensitive to the locations of CBs. Therefore, the CBs probably would be the reason of Ramonsoon's RI. Based on the model outputs, a simple energetic model is verified. Since intensification rate depends on TC intensity according to this energetic model, using simulation outputs of SST, radical wind profile, as well as moisture and temperature distributions in surface layer, the intensity of maximum potential intensifying rate is estimated and the calculated result is highly consistent with our full-physical numerical model simulation of TC intensity. In addition, averaged net income of energy at air-sea surface within TC inner core is found to have a significant lag correlation with 6-hour TC intensity tendencies. In particular, within 3 times of RMW, the maximum lag correlation coefficient attains 0.62 and the lag time is 5.75 hours . It means energy budget at bottom within TC inner core varies in advance of TC intensity and may play a crucial role in RI process. However, the connection between stochastic CBs and entropy flux in bottom layer is still ambiguous which need to be investigated further.