On the Rapid Weakening of Typhoon Trami (2018): Strong Sea Surface Temperature Cooling Associated with Slow Translation Speed

Abstract

This work investigates the rapid weakening (RW) processes of Typhoon Trami (2018) by examining sea surface temperature (SST) cooling based on air-sea coupled simulations during typhoon passage. The cold wake and Trami’s RW occurred as the storm was moving at a very slow translation speed. A marked structural change of Trami is found in a three-dimensional ocean-coupled model experiment during the RW stage, in which the convective clouds and convective bursts in the inner core of the simulated TC dramatically decrease, resulting in the loss of diabatic heating and leading to weakening of the TC. In the simulation, the enthalpy flux dramatically decreases in the inner core due to the SST cooling during the RW period, while a stable boundary layer (SBL) is formed in the TC’s inner-core region. The expanding SBL coverage stabilizes the atmosphere and suppresses convection in inner core, leading to weakening of the storm. A more stable atmosphere in the cold wake is also identified by the inner-core dropsonde data from the field program of Tropical cyclones-Pacific Asian Research Campaign for Improvement of Intensity estimations/forecasts. The strong SST cooling also changes the evolution of Trami’s eyewall replacement cycle (ERC) and limits the eyewall contraction after the ERC.

Numerical Study of Effects of Warm Ocean Eddies on Tropical Cyclones Intensity in Northwest Pacific

“…Figure 11 shows the spatial distributions of SST and zonal vertical cross-sections along 20.4° N at 0000 UTC 24 September (before the storm's arrival) and 0000 UTC 27 September (during the storm). The maximum SST cooling reached about 6 °C just behind the storm center, which is in good agreement with the observations and model results from previous studies [31,[33][34][35][36]. The slow propagation…”

“…Figure 11 shows the spatial distributions of SST and zonal vertical cross-sections along 20.4° N at 0000 UTC 24 September (before the storm's arrival) and 0000 UTC 27 September (during the storm). The maximum SST cooling reached about 6 °C just behind the storm center, which is in good agreement with the observations and model results from previous studies [31,[33][34][35][36] According to the JTWC best track data, Trami reached its peak intensity of 914 hPa at 1800 UTC 24 September and gradually weakened thereafter (Figure 2). This is consistent with our model simulation during Trami's weakening phase.…”

“…Ref. [33] studied the rapid weakening processes of Trami associated with its slow movement using air-sea coupled simulations, and Refs. [34,35] explored the impact of SST cooling and cold core eddies on the eyewall replacement cycle and upper ocean responses in Trami.…”

Numerical Simulation Analysis of Typhoon Moving Track on Sea Surface Cooling

“…The negative feedback of SSC to typhoon intensity requires that typhoon can have a long enough residence time on the cold wake. Therefore, past studies have shown that the translation speed of typhoons [17][18] and other related factors [19] together become important factors affecting the intensity of typhoons. In order to improve typhoon intensity forecasting skills, the numerical simulations are applied to explore the negative feedback mechanism of SSC to typhoon intensity [20] .…”