1. Introduction

2. Materials and Methods

2.4. Monitering anemometers

Five anemometers were installed at a height of 4.0 to 8.0 m in consideration of the field conditions (Figure 4). In general, measuring equipment should be more than 1m away from the mast to minimize disturbance, but local governments suggested installing supporters as short as possible to prevent damages caused by strong winds. We utilized the anemometers (ARCO-SERIAL model) (Figure 5) according to ‘Standard Specification for Automatic Meteorological Observation Equipment’ of KMA [15]. Their related specifications are shown in Table 1. L-1, L-2, and L-4, located on the side of the LCT residential complex, are expected to be affected by downward winds and separated winds. L-3 is expected to be affected by canyon winds when southerly winds blew and separated winds from the Landmark Tower and B Tower when southeast winds blew. L-5 was expected to receive the uninterrupted sea breeze and the canyon winds blowing from the northeast.

Figure 4. Field measurement points around the LCT residential complex. (a) Locations of the installation points. (b)–(f) Views of points L-1 to L-5, respectively.

Figure 4. Field measurement points around the LCT residential complex. (a) Locations of the installation points. (b)–(f) Views of points L-1 to L-5, respectively.

The anemometer recorded four data reading per second [15]. The last 240 data readings collected for one minute were averaged to calculate the 1-min mean wind directions and wind speeds.

Figure 5. Anemometer configuration.

Figure 5. Anemometer configuration.

Table 1. Specifications of anemometers.

Table 1. Specifications of anemometers.

Wind Speed	Range	0~ 70 m/s
	Accuracy	± 2%
	Resolution	0.1 m/s
Wind direction	Range	0 ~ 360˚
	Accuracy	± 1˚
	Resolution	1˚

2.5. Haeundae meteorological observatory

The Haeundae Meteorological Observatory Auto Weathering System (AWS) in Haeundae, operated by KMA, is located 1.8 km northwest of the research site (Figure 6). This nearest reference station (AWS) was selected as a control point to compare and analyze the effects of the skyscraper wind. The LCT residential complex is directly exposed to the sea breeze, whereas AWS is located relatively inland, suggesting that the wind speed is expected to be relatively low.

AWS provides 1 min-mean data of wind speed and wind direction with the shortest period of time series data and daily maximum gust as statistical data . Using the data for the research period, we presented the windrose diagram with 1-min mean data in Figure 7. The main wind directions were north-northeast (38%) and northeast (24%), while the max 1 min-mean wind speed was 12.3 m/s (NNE), lower than the maximum wind speed of the typhoon.

Figure 6. Locations of the study site & AWS.

Figure 6. Locations of the study site & AWS.

Figure 7. Windrose at AWS.

Figure 7. Windrose at AWS.

3. Results

3.1. Mean and maximum wind speeds and directions

Figure 8 shows a time series comparison of the 1-minute average wind speed ($U$) at AWS (), the reference station, and at the five points() (L-1 to L-5). In addition, the wind speed and wind direction when the wind speed was the highest at each point were shown in more detail. The statistical distribution of wind at each point was visualized using the windrose diagram in Figure 9. Data were averaged after missing data, outliers, and data exceeding the measurable wind speed range (>70 m/s) were removed. As the center of the typhoon passes through the research site, the data shows the characteristics of a steep descent after reaching the peak wind speed. Table 2 shows the wind speed and wind direction for the max peak gusts, the max 1 min-mean wind speeds, and the max 10 min-mean wind speeds (the most frequently used) at each point. The relatively strong winds were observed at L-1, L-2 and L-3.

Table 2. Mean wind speeds and peak gusts at each point (L-1 to L-5 and AWS).

Table 2. Mean wind speeds and peak gusts at each point (L-1 to L-5 and AWS).

Point	Max Peak Gust (m/s)	Wind direction		Max1 min-mean wind speed (m/s)	Wind direction		Max10 min-mean wind speed (m/s)	Wind direction
L-1	44.19	10.1°	N	26.52	10.0°	N	22.57	33.3°	NNE
L-2	40.80	226.3°	SW	28.48	228.3°	SW	25.05	233.1°	SW
L-3	55.53	147.8°	SSE	38.39	147.2°	SSE	29.49	152.3°	SSE
L-4	45.68	238.8°	WSW	18.31	235.3°	SW	13.75	238.9°	WSW
L-5	21.03	56.0°	NE	11.35	74.7°	ENE	9.1	212.8°	SSW
AWS	15.20	81.6°	E	12.30	7.7°	N	10.11	10.5°	N

Figure 8. Comparison of 1 min-mean wind speeds over time (LCT residential complex vs AWS) for (a)–(e) L-1 to L-5 () vs AWS ().

Figure 8. Comparison of 1 min-mean wind speeds over time (LCT residential complex vs AWS) for (a)–(e) L-1 to L-5 () vs AWS ().

Figure 9. Windrose at each point (a)–(e) L-1 to L-5, respectively.

Figure 9. Windrose at each point (a)–(e) L-1 to L-5, respectively.

L-1 was installed at the eastern corner of the LCT residential complex, with its wind direction concentrated in northeast (24%), east-northeast (21%), and west-southwest (10%). Due to the spatiotemporal characteristics of L-1, this pattern appeared to be caused by separated winds that blew parallel to the side of the LCT residential complex (Figure 10). Max peak gust and max 1 min-mean wind speed were 26.52 m/s (N) and 44.19 m/s (N), respectively, which were 2.9 times and 2.2 times stronger than the wind speed at AWS.

Due to the same spatiotemporal characteristics as L-1, L-2 was concentrated in a specific wind direction by the separated wind which led to a stronger wind speed in the main direction (Figure 9 and Figure 10). The max peak gust and max 1 min-mean wind speed at L-2 were 40.80 m/s (SW) and 28.48 m/s (SW), respectively, whice were 2.7 and 2.3 times stronger than the wind speed of AWS.

Figure 10. Main wind direction for L-1(left) and L-2(right).

Figure 10. Main wind direction for L-1(left) and L-2(right).

L-3 was installed at the intersection on the north side of the LCT residential complex. The canyon wind passing between three buildings located in the south showed that the southerly wind was significantly stronger than the northerly wind, with a significant effect on the increase in wind speed (Figure 9 and Figure 11). At this point, the strongest wind was observed in this typhoon event, and the max peak gust and max 1 min-mean wind speed were 55.53 m/s (SSE) and 38.39 m/s (SSE), respectively, which were 3.7 and 3.1 times stronger than the wind speed at AWS.

Figure 11. Main wind direction for L-3(left) and L-4(right)

Figure 11. Main wind direction for L-3(left) and L-4(right)

On the other hand, lower wind speeds were observed at L-4 and L-5 than at L-1, L-2 and L-3. The main wind direction at L-4 was southeast. The reduction in the wind speed was attributed to the effects of trees located on the windward side of the area, where the skyscraper winds were expected. After the center of the typhoon passed, strong winds were observed from the southwest winds blowing along the narrow road. Since L-5, in the southern part of the LCT residential complex, was located on the opposite side of L-3, strong northerly winds were expected. However, the lowest wind speed was observed at L-5, similar to that of AWS, which was not affected by skyscraper wind. In Toronto, Canada, a set-back structure shape is recommended to reduce skyscraper winds caused by high-rise buildings [16]. On the south side of the LCT residential complex, buildings have a set-back shape, thus the skyscraper wind was alleviated. This in turn warrants an in-depth research on the set-back structure shape that mitigates the skyscraper wind.

Figure 12. Buildings with a set-back structure shape on the south side of the LCT residential complex.

Figure 12. Buildings with a set-back structure shape on the south side of the LCT residential complex.

3.2. Beaufort Wind Scale

In Section 3.1., we analyzed the relative strength of the wind speed based on the comparison of the wind speeds at the five points with that observed at AWS. Here, we also analyzed the strength of wind speed in absolute values through the Beaufort Wind Scale, devised by the World Meteorological Organization (WMO) in Table 3. The strength of the wind speed was divided into 13 grades.

At AWS, winds of grades 0-6 blew. The most frequent wind speed grade was moderate breeze (grade 4) (27.09%), while the maximum wind speed was strong breeze (grade 6) (0.82%). At L-1, L-2 and L-3, grade 10 (storm), 10 (storm) and 12 (Hurricane) appeared, respectively. The range of grade 10 or higher was strong enough to be described as ‘uprooting trees and causing great damage to buildings’. At L-1, L-2 and L-3, the wind speed frequency was above grade 6 (10.8 m/s), higher than that at AWS. The frequency of wind speed in the range of grade 4 and 5 (5.5 ~ 10.8 m/s) was higher at AWS than that at the five points. . In other words; the skyscraper wind grew more pronounced when the mass flux rose above a certain wind speed. At L-4, the maximum wind speed grade was grade 8 (gale, 0.1%), while the most frequent wind speed grade was 1 (light air). This in turn indicated that the overall wind speed was low, while strong winds blew temporarily. L-5 generally showed a low Beaufort number.

Figure 13. Frequency of Beaufort numbers at each point for LCT (a)–(e) L-1 to L-5 (), respectively, vs AWS ().

Figure 13. Frequency of Beaufort numbers at each point for LCT (a)–(e) L-1 to L-5 (), respectively, vs AWS ().

3.3. Turbulence intensity

We also analyzed the wind fluctuation characteristics observed at the five points. Overall, wind showed a spatiotemporally irregular distribution. In the atmospheric boundary layer, the air does not flow uniformly due to the roughness of the ground surface or the frictional effect of structures, and the fluctuation characteristics, such as the formation of vortices, increase. In particular, as the study site was a single high-rise building surrounded by low-rise buildings, the airflow was very unstable [17]. Turbulence intensity ($I$), which represents the wind fluctuation characteristics, is defined as the ratio of standard deviation ($\sigma$) of the wind speed to the mean wind speed ($U$) as follows:

$$I=\frac{\sigma }{U}$$

$$\sigma =\sqrt{\frac{1}{T}{\int }_{t_0-T}^{t_0} u^2\left(t\right)dt }$$

Figure 14 shows the relationship between mean wind speed and turbulence intensity at each point. In line with previous studies, the turbulence intensity fell as the mean wind speed rose, and converged above a certain wind speed [18,19].

When the wind speed was low, the frictional force acted more strongly than the inertial wind force, rendering the airflow unstable, and thus, the turbulence intensity rose up to 0.95. However, when the wind speed was above 10 m/s, the range of turbulence intensity converged to 0.1–0.2 as the inertial wind force was increased by the high wind speed. The strong turbulence intensity even at the high wind speed was observed when the center of the typhoon, a windless zone, passed through the study site.

Figure 14. Variations of turbulence intensity with mean speed for (a)–(e) L-1 to L-5, respectively.

Figure 14. Variations of turbulence intensity with mean speed for (a)–(e) L-1 to L-5, respectively.

4. Summary and conclusions

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