To analyze the flow changes induced by the construction of JB Station, numerical simulations were performed for the 16 inflow directions in the JB-before and JB-after cases. In addition, detailed flow characteristics were described for the three inflow directions (westerly, north-northeasterly, and northerly) with strong katabatic winds and relatively high frequency occurrences in the wind rose analysis.
3.2.1. Wind speed changes at the surface at the station and at the height of the AWS
We analyzed the effects of the construction of JB Station on near-surface (z=1.25 m) airflow at the station and wind speed and direction at the AWS (z=5 m). Several changes in wind speed were observed at the AWS after the construction of the station (Fig. 5a). Of the westerly winds, westerly, west-southwesterly, west-northwesterly and northwesterly wind speeds at the AWS markedly decreased. The maximum decrease (81.4%) was simulated in the west-southwesterly flow. Conversely, easterly wind speeds changed minimally, with a maximum decrease of 7.2% in the easterly flow. This was likely because the AWS is located east of JB Station, and is downwind of the station only for westerly inflows (not shown). These results are consistent with observations (Fig. 4) at JB Station (JB-before and JB-after cases).
Wind speeds increased slightly after the construction of the station for five inflow directions: north-northwesterly (3.8%), northerly (0.4%), north-northeasterly (0.4%), southerly (2.1%), and south-southwesterly (5.0%). These increases initially occurred between the station buildings, due to channeling effects (Wang and Takle, 1996; Kim and Kim, 2009), which then enhanced the flows at the AWS. For the other inflow directions, the average surface wind speed around JB Station decreased by 22.9%. From the average wind speeds in the JB-before and JB-after cases, west-northwesterly (-36.9%) and westerly (-34.3%) winds showed particularly substantial reductions in wind speed. These reductions were less prominent for northeasterly (-15.3%) and southerly (-16.9%) winds (Fig. 5a). Wind direction was not notably affected (average change: 4.2°) (Fig. 5b).
3.2.2. Westerly winds (270°)
The terrain had many overall effects on westerly flows. In the valley located to the west of JB Station, air flowed down and up the west and east slopes of the valley, respectively, and downward flows appeared along the east slope of the valley (Fig. 6a). Flows around JB Station had horizontal wind speeds of 3.5 m s-1 to 4.0 m s-1, and wind speed decreased to the east of JB Station along the coast, although wind speeds were restored farther off the coast. In the JB-after case, the flows around JB Station were more complex due to flow distortions caused by the buildings (Fig. 6b). In the JB-before case, westerly winds flowed to the AWS unobstructed. However, the AWS is located to the east of JB Station, and flows were diverted by the buildings in the JB-after case, which were simulated as changes in wind direction at the AWS (Figs. 6a and b). Moreover, variations in wind speed around JB Station appeared between the JB-before and JB-after cases. On the east side of the buildings, wind speed mainly decreased due to secondary circulations, such as a recirculation zone; however, wind speed between buildings increased slightly due to channeling effects (Fig. 6c). In the JB-after case, the average near-surface (z=1.25 m) wind speed at JB Station decreased by 34.3% compared to the JB-before case, while wind speed at the AWS (z=5.0 m) decreased by 53.3%. The sudden reduction in wind speed between the two cases indicates that the AWS is located within the recirculation zone.
3.2.3. North-northeasterly winds (22.5°)
At JB Station, air flows from the ocean are weakened rapidly upon reaching land due to friction, and air flows up the slope on the east side of JB Station. Meanwhile, air flows down the eastern slope and up the northwestern slope of the valley located to the west of JB Station (Fig. 7a). In the JB-after case, the north-northeasterly inflows were distorted by the presence of the buildings (Fig. 7b). In contrast to the westerly winds, much smaller changes occurred in north-northeasterly wind speed (0.02 m s-1) and direction (2.8°) after the construction of the station, due to the upwind location of the AWS relative to the station. The average near-surface (z=1.25 m) wind speed decreased by 24.6% compared with the JB-before case. However, wind speeds to the west of the station increased because the flows were blocked by the maintenance building and intercepted by flows from the northwest direction (Fig. 7c).
3.2.4. Northerly winds (0°)
Northerly winds were predominantly downward flows that formed along the downhill slope, although upward flows appeared occasionally along the uphill slope near the station. Flows weakened rapidly near the southern coastline by the station (Fig. 8a). In the JB-after case, the flows were northeasterly to the east of the station and northwesterly to the west of the station, which converged downwind of the buildings (Fig. 8b). Wind speeds increased slightly in between the upper-atmosphere observatory and the main building, as well as the maintenance and main buildings, due to channeling effects (Fig. 8c). The average near-surface (z=1.25 m) wind speeds decreased by 17.3% compared with the JB-before case, while the wind speeds at the AWS (z=5.0 m) increased by 0.4%. Moreover, wind speeds increased to the southwest of the main building compared with the JB-before case, as the northwesterly flows were blocked by the maintenance building.
3.2.5.Effects of wind fences on the wind environment around JB Station
Strong katabatic winds are relatively common at JB Station, and our analysis confirmed that the construction of the station has increased wind speeds in some directions around the station. Since wind is the biggest threat to the crews at this Polar station, installation of wind fences may reduce damage to crews and facilities caused by strong winds. Therefore, we investigated the effects of wind fences with various construction parameters on wind speeds at JB Station and at the AWS. For the analysis, wind fences were positioned on the west and north sides of JB Station to target the strongest and most frequent westerly and northerly winds, based on AWS data analysis. We analyzed the effects of fence porosity (0%, 25%, 33%, 50%, 67% and 75%) and distance between the fences and JB Station (2H, 4H, 6H and 8H, where H is the height of the wind fences). Westerly winds were used as the inflow direction, as they are the most frequent and strongest winds at the station.
Figure 9 shows the surface wind vector field after the construction of the wind fences and the difference between the period after and before construction of the wind fences. Various wind-fence porosities (25%, 33%, 50%, 67% and 75%) were compared with the pre-wind-fence conditions, and the wind fences appeared to reduce wind speed without substantially changing wind direction (Figs. 9b, c, e and f). Lower wind-fence porosities were associated with larger reductions in wind speed in the area between the wind fences and the station, but increases in wind speed were simulated to the east of the station (Figs. 9 and 10). Reductions in wind speeds were significantly larger for the wind fences with a porosity of 0% than those with porosities of 25%, 33%, 50%, 67% and 75% in the windward direction of the fence compared with the conditions before the installation of the wind fence. However, this resulted in the formation of a recirculation area between the wind fences and the main building, which had a flow direction opposite to that of the inflow. In addition, the wind fence inhibited the increases in wind speeds observed in the JB-after case; however, strong winds occurred to the east of the station (Figs. 10a and d).
In the cases with distance to the station of 2H, wind fences with porosities of 0%, 25%, 33%, 50%, 67% and 75% decreased the average near-surface (z=1.25 m) wind speeds by 15.3%, 17.6%, 17.4%, 16.0%, 10.6% and 7.7%, respectively. As the distance to the station became larger, the average near-surface wind speeds decreased (Fig. 11a). For the fixed distance to the station, the efficiency of wind fences for wind-speed reduction was maximized at porosities of 25% (2H) or 33% (4H-8H). Wind-speed changes at the AWS (z=5.0 m) with distance and porosity showed a non-monotonic variation (Fig. 11b). The installation of wind fences with distance of 2H and porosities of 0%, 25%, 33%, 50%, 67% and 75%, decreased (changed) the wind speeds (directions) by 7.6% (3.5°), 5.8% (2.8°), 5.9% (3.1°), 8.5% (3.2°), 7.5% (2.6°) and 4.7% (2.2°), respectively (Fig. 11b). In the case of a distance of 4H, the wind-speed variation to porosities was similar to that in the cases of 2H. However, in the cases with distance of 8H, wind fences with porosities of 0%, 25%, 33% and 50% increased the wind speeds by about 6.9%, 7.1%, 4.4% and 1.9%, respectively; whereas, wind fences with porosities of 67% and 75% decreased the wind speed by about 2.3% and 1.5%, respectively. Wind direction at the AWS (z =5.0 m) was changed by about 1.3°, 0.5°, 0.7°, 1.0°, 1.2° and 0.9° by the installation of wind fences, with respective porosities of 0%, 25%, 33%, 50%, 67% and 75%. A previous study (Martin, 1995) reported that a reduction of wind speed by vertical wind fences on flat terrain was maximized for porosities of 40%-60%. However, in this study, the lower porosities (25%-33%), apart from the no-porosity cases, resulted in a larger decrease in wind speed around JB Station. This discrepancy was caused by the fact that wind fences were installed on a slightly inclined slope in this study. Analysis of the rates of change in near-surface wind speeds around JB Station showed that the maximum rates of change (increase or decrease) in wind speeds decreased monotonically as wind-fence porosity and distance to JB Station increased (Fig. 12).