Xiaogang Huang , Wen Yang, Xiaoping Cheng
Institute of Meteorology and Oceanography, National University of Defense Technology, Nanjing, Jiangsu, China
To understand the influence of tropical cyclone (TC) outer precipitation distribution on the secondary eyewall formation (SEF) and eyewall replacement cycle (ERC) in Sinlaku (0813), six experiments are conducted by using the Weather Research and Forecasting (WRF) model with different cumulus schemes. The results show that all the six schemes successfully simulate the ERC. The TC outer precipitation amount is almost the same, while the distribution characteristic varies, which leads to the difference on the intensity and radial location of the secondary eyewall. To quantitatively describe this distribution feature, the concentration index (CI) is defined. When CI is small, which indicates a compact and active precipitation region, the air-sea temperature difference will increase, causing more transfer of heat fluxes from the underlying ocean.
Through analyzing the physical processes above, we got a conceptualized schematic of TC ERC and SEF with different outer precipitation distribution(Figure 1). It is evident that the radius and intensity of the inner eyewall are basically the same. While the outer precipitation distribution is different. With a similar rainfall amount, the intensive degree and intensity of it vary. When the rainbands are compact and active, the transfer of the heat flux will increase induced by a greater air-sea temperature difference. Obtaining more energy from the underlying ocean promotes the generation and development of vortical hot towers (VHTs) in the outer region of TC, which locate solely at first and then merge with each other with a clockwise moving tendency. As a result, an axisymmetric vorticity ring forms through the straining process. The more the number of the VHTs, the stronger the vorticity ring, followed by an enhanced secondary maximum of tangential wind and outer eyewall.
With an active secondary eyewall, the updrafts are greater, leading to a strong upper-level compensating subsidence within the moat, which suppresses the lower-level convection. Therefore, the moat is dominated by the downward motion, causing a wider width and a larger radial location of SEF.
The results obtained suggest that observing the characteristic of the outer precipitation distribution may provide a clue for the prediction of the secondary eyewall features, which emphasizes the important role of it in ERC period. The more compact and active the precipitation region, the smaller the CI, causing a stronger secondary eyewall intensity and location.
Based on the analysis of different TC outer precipitation distribution characteristics, this paper researches on how this difference affects the formation and maintenance of the secondary eyewall by changing the transfer of surface heat flux. It should be pointed that the above conclusions are based only on the atmosphere model. The sea surface temperature (SST) is not constant in the realistic environment, which is influenced by the TC and some other factors. So the follow-up work is needed based on the coupled ocean-atmosphere system.
Key words: air-sea temperature difference, transfer of surface heat flux, eyewall replacement cycle, outer precipitation distribution
This work is supported by the National Natural Science Foundation of China Grant 41675058 and the 973 project (2015CB452802) of the Ministry of Science and Technology of China.