Simulation of Extreme Updrafts in the Tropical Cyclone Eyewall

Sensitivity of Fine-Scale Structure in Tropical Cyclone Boundary Layer to Model Horizontal Resolution at Sub-Kilometer Grid Spacing

“…Although the updraft distribution in the domain with the 2-km grid spacing did not show large differences, the strongest updrafts (near 9 m s −1 ; Figure 5A) are considerably less frequent than those simulated in the domain with the 500-m grid spacing (greater than 10% updrafts reached 12 m s −1 at 2-6 km altitude; Figure 5B). Note that only the 55-m and 166-m domains (Figures 5C,D) show another two low frequency of strong updrafts in the boundary layer at 300-400 m height and in the upper troposphere at about 11 km height, respectively, with maximum values increasing with decreasing of grid spacing, consistent with the LES simulations in Wu et al (2019) and Zheng et al (2020). The large difference in vertical motion in the boundary layer is mainly attributed to the resolved large eddies, corresponding the PBL rolls and tornadoscale vortices in the eyewall, which often result in extreme updrafts and increase both updrafts and downdrafts between (>3 and <−3 m s −1 ).…”

“…In the simulation, d01-d03, d04, and d05 was initiated at 27-h, 2-h, and 1-h prior to the initiation of the d06, which was integrated for 7hours. This period was selected because the simulated TC was in its rapid intensification stage, which was distinguished from the mature stage in previous studies (Rotunno et al, 2009;Green and Zhang, 2015;Zheng et al, 2020), so that we can examine the effects of horizontal grid spacing on the intensification of the model TC.…”

“…The development of computing power and advanced numerical technique have greatly contributed to the efforts of very high-resolution simulations to study the small-scale features in the TCBL. More studies on TCs (Zhu, 2008;Rotunno et al, 2009;Green and Zhang, 2015;Wu et al, 2018;Zhu et al, 2019;Zheng et al, 2020) have run models on sub-kilometer or even sub-100-meters resolutions, by which turbulence can be reasonably resolved. Rotunno et al (2009) suggested that a model cannot resolve turbulence until reducing the grid spacing to sub-100-meters.…”

Fine-Scale Structures in the Mid-Level Eyewall of Super Typhoon Rammasun (2014) Simulated With the WRF-LES Framework

“…Relatively little is known about the fine-scale structures in the mid-level eyewall of TCs although the extreme updrafts beyond the TC boundary layer were observed (Guimond et al, 2010;Heymsfield et al, 2010). Recently, Zheng et al (2020) examined two numerical experiments conducted with the Advanced Weather Research and Forecast (WRF) model, in which the large-eddy simulation (LES) technique is used with the finest grid spacing of 37 and 111 m. They found that the simulated extreme updraft in the TC eyewall exhibited relatively high frequencies in the lower, middle and upper troposphere, suggesting different types of fine-scale structures associated with the extreme updrafts. Following Zheng et al (2020), the numerical simulation of Typhoon Rammasun (2014) was conducted with the WRF-LES framework In this study.…”

High-Wind Drag Coefficient Based on the Tropical Cyclone Simulated With the WRF-LES Framework

“…It is generally believed that the drag coefficient in high-wind condition does not increase linearly with surface wind speed although the mechanisms responsible for the reduction of the drag coefficient have not been fully understood (Donelan, 2018). However, relatively few studies have been conducted to examine the drag coefficient in the numerical simulation, while the fine-scale (less than 1,000 m) features have been explicitly simulated over the past decade (Zhu, 2008;Rotunno et al, 2009;Zhu, 2013;Green and Zhang, 2015;Wu et al, 2018, Wu et al, 2019Jiang et al, 2020;Zheng et al, 2020;Zhou et al, 2020).…”