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

Hongxiong Xu,Yuqing Wang

doi:10.3389/feart.2021.707274

Abstract

In view of the increasing interest in the explicit simulation of fine-scale features in the tropical cyclone (TC) boundary layer (TCBL), the effects of horizontal grid spacing on a 7–10 h simulation of an idealized TC are examined using the Weather Research and Forecast (ARW-WRF) mesoscale model with one-way moving nests and the nonlinear backscatter with anisotropy (NBA) sub-grid-scale (SGS) scheme. In general, reducing the horizontal grid spacing from 2 km to 500 m tends to produce a stronger TC with lower minimum sea level pressure (MSLP), stronger surface winds, and smaller TC inner core size. However, large eddies cannot be resolved at these grid spacings. In contrast, reducing the horizontal grid spacing from 500 to 166 m and further to 55 m leads to a decrease in TC intensity and an increase in the inner-core TC size. Moreover, although the 166-m grid spacing starts to resolve large eddies in terms of TCBL horizontal rolls and tornado-scale vortex, the use of the finest grid spacing of 55 m tends to produce shorter wavelengths in the turbulent motion and stronger multi-scale turbulence interaction. It is concluded that a grid spacing of sub-100-meters is desirable to produce more detailed and fine-scale structure of TCBL horizontal rolls and tornado-scale vortices, while the relatively coarse sub-kilometer grid spacing (e.g., 500 m) is more cost-effective and feasible for research that is not interested in the turbulence processes and for real-time operational TC forecasting in the near future.

Highlights

Tropical cyclones (TCs) are among the strongest atmospheric vortices in nature, which often bring about heavy rainfall and damaging winds on their paths (Chen and Meng, 2001; Chen et al, 2019), and induce storm surge and other natural disasters in coastal regions when they make landfall (Duan et al, 2020)
As TC research moves toward large eddy simulation, it is crucial that horizontal grid spacing smaller than 1 km are well examined in order to be a good reference for the development of future operational TC forecast and research models
Our results agree with those of Green and Zhang (2015); we show that a grid spacing of 166 m can reasonably resolve the macro aspects of boundary layer rolls and tornado-scale vortices in the simulated TC, especially in the outer core region

Summary

Introduction

Tropical cyclones (TCs) are among the strongest atmospheric vortices in nature, which often bring about heavy rainfall and damaging winds on their paths (Chen and Meng, 2001; Chen et al, 2019), and induce storm surge and other natural disasters in coastal regions when they make landfall (Duan et al, 2020). The accurate forecast of TC track and intensity are important for preparedness for disaster prevention and reduction of damages potentially induced by a coming TC. How to improve the understanding and prediction skill of TC track and intensity has received great attention by both research and operational communities (Chen and Meng, 2001; Wang and Wu, 2004; Emanuel, 2018). The forecast of TC track has made remarkable progress in the last 3 decades or so, the research and forecast of TC intensity change are well behind and need to be improved. TC intensity change involves the interaction of multi-scale and nonlinear dynamical/physical processes that are. The TC planetary boundary layer (TCBL) turbulent processes belong to these crucial physical processes and can significantly influence the intensification and maintenance of a TC by turbulent surface flux and vertical mixing of heat, moisture and momentum (Hill and Lackmann, 2009; Gopalakrishnan et al, 2013; Zhang et al, 2017)

Results

Discussion

Conclusion