Aircraft Observations of Turbulence Characteristics in the Tropical Cyclone Boundary Layer
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The Hong Kong Observatory conducted six flights in the atmospheric boundary layer of five tropical cyclones: tropical storm Jebi (1309), typhoon Kalmaegi (1415), severe tropical storm Linfa (1510), typhoon Mujigae (1522), and severe typhoon Nida (1604). Three-dimensional wind data with a 20-Hz sampling rate are available for a height range of 500–670 m, with the mean wind speed from these low-level flights ranging from 10 to 62 m s−1. The turbulent momentum flux and turbulence kinetic energy (e) are measured using the eddy-correlation method, while horizontal scales of turbulent eddies, vertical eddy diffusivity (K), and the vertical turbulent mixing length scale are estimated indirectly. The dependence of the momentum flux, e, K, and the vertical mixing length on wind speed and height are compared with previous studies. Both the momentum flux and turbulent kinetic energy increase with the wind speed, although the rate of increase is smaller for higher wind speeds. It is also found that K increases with wind speed according to a power law up to 40 m s−1 before levelling off, while the vertical mixing length is nearly constant at 100 m. The results serve as a reference for evaluating and improving the turbulent parametrizion in tropical-cyclone models, while the observed large turbulent mixing near the top of the inflow layer of the eyewall region should not be neglected in numerical models.
KeywordsAtmospheric turbulence Boundary layer Tropical cyclones Vertical eddy diffusivity
We are grateful to the scientists and crew members who participated in the field work that helped collect the aircraft data used in this study. Zhongkuo Zhao was sponsored by the National Major Fundamental Research Program of China (Grant No. 2018YFC1507401) and the National Natural Science Foundation (Grant Nos. 41875021, 41830533, 41675019, 41675021). Jun Zhang was supported by Grant NA14NWS4680028 and NSF Grants AGS1822128 and ASG1654831.
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