A direct aircraft observation of helical rolls in the tropical cyclone boundary layer

Jie Tang,Kaikwong Hon,Jun A Zhang,Pakwai Chan,Yuan Wang,Xiaotu Lei

doi:10.1038/s41598-021-97766-7

Jie Tang, Kaikwong Hon + Show 4 more

Open Access

https://doi.org/10.1038/s41598-021-97766-7

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Abstract

Helical rolls are known to play a significant role in modulating both the mean and turbulence structure of the atmospheric boundary layer in tropical cyclones. However, in-situ measurements of these rolls have been limited due to safety restrictions. This study presents analyses of data collected by an aircraft operated by the Hong Kong Observatory in Typhoon Kalmaegi (1415) and Typhoon Nida (1604). Examination of the flight-level data at ~ 600 m altitude confirmed the existence of sub-kilometer-scale rolls. These rolls were mostly observed in the outer-core region. Turbulent momentum fluxes were computed using the eddy correlation method. The averaged momentum flux of flight legs with rolls was found to be ~ 2.5 times that of legs without rolls at a similar wind speed range. This result suggests that rolls could significantly modulate turbulent transfer in the tropical cyclone boundary layer. This roll effect on turbulent fluxes should be considered in the planetary boundary layer parameterization schemes of numerical models simulating and forecasting tropical cyclones.

Highlights

Helical rolls are known to play a significant role in modulating both the mean and turbulence structure of the atmospheric boundary layer in tropical cyclones
All these roll legs are cross-wind legs that are located in the region within a radius range of 150 km to 250 km
The other spectral peak at the wavelength of 480 m which is half of the roll wavelength is substantial in these spectral and cospectral plots. This result indicates that sub-kilometer scale rolls with different wavelengths could co-exist in the tropical cyclone boundary layer (TCBL)

Summary

Introduction

Helical rolls are known to play a significant role in modulating both the mean and turbulence structure of the atmospheric boundary layer in tropical cyclones. Previous theoretical and observational studies[5,6,7] have documented the significant impact of rolls on transports of momentum, heat and moisture through the atmospheric boundary layer. Temperature, and pressure are measured with a sampling rate of 20 Hz. Table 1 shows the flight information including the time, mean aircraft altitude, maximum measured wind speed.

Results

Conclusion