6.3 Large Eddy Simulation of an Entire Tropical Cyclone

Tuesday, 14 January 2020: 2:00 PM
158 (Boston Convention and Exhibition Center)
Junsi Ito, Atmosphere and Ocean Research Institute, The Univ. of Tokyo, Kashiwa, Chiba, Japan; and H. Niino and T. Oizumi

Most numerical models of a tropical cyclone do not have resolution to simulate turbulent eddies explicitly, but their effects are parameterized. Such parameterizations must introduce significant uncertainties. Large Eddy Simulation (LES) which resolve the eddies can mitigate the uncertainties, while it requires considerable computational resources.

Taking advantage of the huge computational power of a massive parallel supercomputer (K-supercomputer, AICS, RIKEN), LES of entire tropical cyclones is realized. The numerical model used in this study is Japan Meteorological Agency’s Non-Hydrostatic Model (JMA-NHM). The computational domain covers 2000 km by 2000 km in the horizontal and 23 km in the vertical directions, and horizontal boundary conditions are doubly cyclic. The grid number is 20000 by 20000 in the horizontal directions, and 60 in the vertical direction where grid spacing increases with increasing height. Before starting the LES, a preliminary run with JMA-NHM with dx=2km is conducted. In this preliminary run, a tropical cyclone develops from a weak initial vortex to a mature stage after 120 hours integration. The gridpoint values of this mature stage are interpolated to prepare the initial condition for the LES. The time integration of the LES is then performed for 10 hours.

The simulated wall cloud around the eye consists of a number of cumulus clouds. Although the fine scale structures are resolved in the LES, the maximum of the near-surface wind changes little from that simulated based on coarse-resolution runs. The boundary layer height is smaller in the LES, and this may shrink the radius of the maximum wind.

We explore especially near-surface coherent structures in the TC boundary layer. Three kinds of coherent structures appeared inside the boundary layer: Type-A roll, which is caused by an inflection-point instability of the radial flow and prevails outside the radius of maximum wind. The second is a Type-B roll that also appears to be caused by an inflection-point instability but of both radial and tangential winds. Its roll axis is almost orthogonal to the Type-A roll. The third is a Type-C roll, which occurs inside the radius of maximum wind and only near the surface. It transports horizontal momentum flux in an up-gradient sense and causes the largest gusts.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner