Observational study of wind characteristics, wind speed and turbulence profiles during Super Typhoon Mangkhut

doi:10.1016/j.jweia.2020.104362

Journal of Wind Engineering and Industrial Aerodynamics

Volume 206, November 2020, 104362

https://doi.org/10.1016/j.jweia.2020.104362 Get rights and content

Highlights

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Analyze wind records from 356-m-high meteorological tower during a super typhoon.
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Present and discuss typhoon wind and turbulence characteristics and profiles.
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Examine influences of surface roughness and wind speed on wind profiles.
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Provide references for typhoon-resistant design of high-rise structures.

Abstract

On the basis of the wind records from 356-m-high Shenzhen Meteorological Gradient Tower (SZMGT) which is the tallest of its kind in Asia, this paper presents an observational study of wind characteristics during Super Typhoon Mangkhut and aims to enhance the understanding of the boundary layer wind structure of strong typhoons over land. First of all, the raw records are selected for subsequent analysis through quality control concerning flow distortion, thermal stability, stationarity, and spikes. Then wind speed profiles, as well as their associations with upstream terrain conditions, are presented and discussed. Moreover, parameters for describing atmospheric turbulence, involving gust and peak factors, turbulence intensities, turbulence integral length scales, and turbulence ratios, are investigated considering different surface roughness regimes. Furthermore, Gaussianity, power spectral density, and coherence of wind velocities are investigated. The outcome of this study is expected to facilitate the wind-resistant design of high-rise structures in typhoon-prone regions and provide references for typhoon boundary layer simulation.

Introduction

With the rapid development of construction materials and technologies, the past few decades have witnessed the erection of numerous high-rise buildings with heights over 200 m. As such tall structures are inherently vulnerable to wind actions, accurate estimates of wind loading are of paramount importance for their safety and serviceability. Considering that the wind loading on a structure is related to the wind velocity in a quadratic manner, a small error in the determination of wind velocity may be enlarged and threaten the structural integrity. Davenport wind loading chain (Davenport, 1961), which is adopted by many design codes and standards (e.g., ASCE 7-16, AIJ-RLB-2015, GB 50009-2012), formulates wind loadings and dynamic responses of structures based on the statistical characteristics of mean winds and atmospheric turbulence parameters which are quantitively expressed by wind speeds and directions, gust and peak factors, turbulence intensities and integral length scales, power spectra and coherences, etc. A thorough understanding of strong wind characteristics in the atmospheric boundary layer is therefore a prerequisite for the evaluation of design wind loadings of high-rise structures.

The design wind speed and turbulence profiles however vary between different codes and standards, partially because of various nature of extreme winds in different parts of the world (Simiu and Yeo, 2019). Extratropical gales that occur in the middle latitudes have been studied for around a half century and generally well understood (Holmes, 2015). In tropical regions, however, the extreme winds and wind actions on structures are mostly associated with tropical cyclones (or typhoons/hurricanes), which are vortex structures characterized by gusty winds, extensive sizes, and convective circulation and less well understood (Li et al., 2019).

Field observation is regarded as the most reliable way to investigate the wind characteristics, wind speed and turbulence profiles of tropical cyclones. Therefore, a few observational studies have been reported in recent years, the vast majority of which are based on (1) anemometers installed at/on meteorological stations, buildings, bridges, or cables (e,g., Cao et al. (2009), Shu et al. (2015), Li et al. (2016), Li et al. (2019), Zhao et al. (2019), and Wang et al. (2020)); (2) anemometers installed on towers (e.g., Hui et al. (2009a, 2009b) based on a 50-m-high tower; Li et al. (2015b) based on towers with heights ranging from 10 to 100 m; Song et al. (2016) based on towers with heights ranging from 70 to 112 m; Wang et al. (2017) based on a 40-m-high tower; Lin et al. (2018) based on a 50-m-high tower; Li et al. (2018) based on a 100-m-high tower; Fang et al. (2019) based on towers with heights ranging from 50 to 120 m); or (3) remote sensing instruments such as Doppler wind lidars and sodars (e.g., Tse et al. (2013) and Tsai et al. (2019)). It is noteworthy that anemometers at/on meteorological stations, bridges or cables are mostly limited within near-surface levels, while those on buildings usually suffer from flow distortion caused by the buildings. For remote sensing techniques, whether lidars and sodars can effectively measure wind speed and turbulence in adverse weather conditions during tropical cyclones is still controversial (Zhang et al., 2018; Liao et al., 2020). On the contrary, meteorological towers equipped with anemometers are favorable as they can offer promising multi-level wind records with high fidelity.

Nevertheless, due to high construction cost, meteorological towers with height above 120 m are rather rare, and almost all the previous tower-based studies of the wind structure of tropical cyclones utilized records from towers lower than 120 m. As a result, observations of the wind characteristics of tropical cyclones at comparable heights with high-rise buildings (over 200 m) have been rarely reported, and the wind speed and turbulence profiles of tropical cyclones for structural design of tall buildings and boundary layer simulation of tropical cyclones are still not firmly established. To address this issue, more high-quality, multi-level wind observations during tropical cyclones are urgently required. In this paper, on the basis of observational records from 356-m-high Shenzhen Meteorological Gradient Tower (SZMGT), which is the tallest of its kind in Asia, a comprehensive statistical study of wind characteristics during Super Typhoon Mangkhut is conducted. Vertical profiles of wind speeds, gust factors, turbulence intensities, and turbulence integral length scales as well as their associations with upwind terrain conditions are presented and discussed. Gaussianity, power spectral density, and coherence of wind velocities are also investigated. The outcome of this study is expected to contribute to the understanding of the wind structures of strong tropical cyclones, facilitate the wind-resistant design of high-rise buildings in tropical cyclones-prone regions, and provide references for boundary layer modelling of tropical cyclones in wind tunnel tests or numerical simulation by Computational Fluid Dynamics (CFD). In particular, the high-resolution, multi-level wind records obtained from 356-m-high SZMGT may benefit the wind demand characterization which is one of the essential steps for Performance-Based Wind Design (PBWD; ASCE, 2019). PBWD has the potential to overcome the drawbacks (generally too conservative design) of using current codes and standards for high-rise building design in tropical cyclone prone regions. The remainder of this paper is organized as follows. Section 2 introduces Super Typhoon Mangkhut, the observational site, and the observational records utilized in this paper. Section 3 describes the data quality control procedures. Sections 4 Mean wind characteristics and profiles, 5 Turbulence characteristics and profiles discuss the mean wind and turbulence characteristics of the typhoon, respectively. Section 6 summarizes the main findings and gives concluding remarks.

Section snippets

Typhoon Mangkhut

Typhoon Mangkhut (JMA: 1822, JTWC: WP262018) was the 22nd tropical cyclone developed over the western North Pacific in the year of 2018. Mangkhut formed as a tropical depression at 12°N, 170°E on September 7, 2018 and gradually intensified afterward. It developed into a super typhoon on 11 September and moved westward quickly. On the morning of 15 September, Mangkhut made its first landfall over Luzon, Philippines with a 1-min sustained wind speed (the highest mean wind speed over a 1-min span)

Data quality control

Before the quality control process, the raw three orthogonal wind components (u_x, u_y, u_z) measured by CSAT3 were firstly separated into 10-min (or 1-h) segments, then transformed into the micrometeorological coordinates, where u (longitudinal wind speed) is aligned with the horizontal 10-min (or 1-h) vector mean wind speed, v (lateral wind speed) is parallel with the horizontal plane and perpendicular to u, and w (vertical wind speed) is oriented normal to the surface (Foken, 2008). The terrain

Evolution of mean wind speed and direction

The evolution of 10-min mean wind speed and direction measured by WMT703 is interpreted with contour plots in Fig. 7. The white-colored patches in the figure were caused by the lost measurements, and the shaded area represents the period when the wind measurements suffered from flow distortion caused by the tower structure (01:00–11:30/16). As shown in Fig. 7(a), the wind speed generally increased with height within 350 m above ground level, and low-level jets were not observed. The strongest

Turbulence characteristics

Parameters for describing turbulence characteristics of winds generally involve turbulence intensities, turbulence integral length scales, gust factors, and peak factors. Turbulence intensity (I) represents the ratio of wind speed fluctuation to the mean. Turbulence intensity affects the gust effect factor (G for rigid buildings or G_f for flexible buildings) in ASCE 7, which directly influences the calculation of design wind pressures. Compared with rigid buildings, errors in codified and

Conclusions

This paper presented an observational study of wind characteristics, wind and turbulence profiles during Super Typhoon Mangkhut based on the records from the 356-m-high Shenzhen Meteorological Gradient Tower. The analyzed results were compared with previous studies, existing models and structural design codes. The outcome of this study is expected to contribute to the understanding of typhoon wind structures, facilitate the wind-resistant design of high-rise buildings in typhoon-prone regions,

CRediT authorship contribution statement

J.Y. He: Conceptualization, Writing - original draft, Methodology. Y.C. He: Methodology, Investigation. Q.S. Li: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. P.W. Chan: Investigation, Resources, Data curation. L. Zhang: Investigation, Resources. H.L. Yang: Investigation, Resources. L. Li: Investigation, Resources.

Declaration of competing interest

The authors declare that they have not known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The work described in this paper was fully supported by grants from the National Natural Science Foundation of China (Project No: 51978593) and from the Research Grants Council of Hong Kong (Project No: CityU 11207519).

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Citation Excerpt :
It made landfall around 12:00 on 16th September when the storm exerted its greatest impact on the studied site. The maximum 10-min mean wind speed at the top of Zhuhai Center (324 m) was estimated as 46 m/s, in accordance with the field measurements from meteorological stations located around the studied site [25,26]. Meanwhile, easterly winds dominated during this period around the studied site, which corresponded to an open exposure (Fig. 1).
Wind resistance is of great importance for the safety and applicability of high-rise buildings, and wind tunnel tests are generally exploited to facilitate associated wind-resistant design. However, owing to the adoption of downscaled models, Reynolds-number (Re) effects on wind tunnel testing results may not be neglected especially for structures with circular or quasi-circular sections. Although great efforts have been made to explore the Re-effects, the range of Re concerned in most studies is too limited to cover that associated with the reality. This article investigates the Re-effects on various wind effects toward a super-tall building which is featured by a spatially varying curved cross-section, based on a comprehensive usage of wind tunnel test and field measurement. The dependence of Strouhal number, pressure, layer force, structural response and equivalent static wind load upon Re from 2.5 × 10⁴ to 3.8 × 10⁷ is analyzed and compared with their counterparts for triangle sections and circular cylinders. Results suggest typical wind effects of the building differ distinctly from those for a circular cylinder. Both the global geometric features of the cross section and its local details (in particular the blunt leading edges) may play a significant role in forming the flow structures around the building and therefore in resulting in corresponding wind effects. Basically, at portions where the cross section varies insignificantly with height, results are much similar to those for a triangle cross-section and are less sensitive to Re; while at portions where the cross section varies significantly with height, some results become dependent greatly upon Re. Overall, the structural responses of this building first increase and then tend to level off with the increase of Re, and wind tunnel testing results basically agree with field measurements when Re exceeds about 1.3 × 10⁵.

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