Inflow and Outflow through the Sea-to-Sky Corridor in February 2010: Lessons Learned from SNOW-V10 *

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1 Inflow and Outflow through the Sea-to-Sky Corridor in February 2010: Lessons Learned from SNOW-V10 * Ruping Mo National Laboratory for Coastal and Mountain Meteorology, Environment Canada, Vancouver, BC, Canada Johnson Zhong, Cindy Yu, and Ken Kwok Pacific Storm Prediction Centre, Environment Canada, Vancouver, BC, Canada Corresponding author s address: Ruping Mo National Laboratory for Coastal and Mountain Meteorology Environment Canada Burrard Street Vancouver, BC V6C 3S5 Canada ruping.mo@ec.gc.ca Technical Report National Laboratory for Coastal and Mountain Meteorology 1 September 2010 * This report is based on a poster, presented at the 14 th Conference on Mountain Meteorology, American Meteorological Society (30 August 3 September 2010, Lake Tahoe Vicinity, California, USA); see Fig. 11 at the end of the report.

1. Introduction The XXI Olympic Winter Games, held 12-28 February 2010 in Metro Vancouver and Whistler of British Columbia (BC), Canada, required detailed and timely weather

Inflow and outflow through the corridor interact strongly with the complex mountain terrain, and can have significant impacts on visibility, precipitation, and temperature in

2 1. Introduction The XXI Olympic Winter Games, held February 2010 in Metro Vancouver and Whistler of British Columbia (BC), Canada, required detailed and timely weather forecasts along the Sea-to-Sky Corridor (hereafter S2SC, see Fig. 1). Inflow and outflow through the corridor interact strongly with the complex mountain terrain, and can have significant impacts on visibility, precipitation, and temperature in the area. The SNOW- V10 (Science of Nowcasting Olympic Weather for Vancouver 2010) project provided a unique opportunity for improving our understanding and ability to forecast these valley flows. Figure 1: Physiographic map showing the Sea-to-Sky Corridor in the BC Inner South Coast. 1

3 In this study, we analyzed the flow structures and their evolution in the S2SC through February 2010 using data available through SNOW-V10. Focusing on two atypical episodes, we tried to highlight the forecast challenges and explore key physical mechanisms behind the phenomena, and develop some conceptual models for future operational forecasts. 2. Geography and climatology The S2SC, as shown in Fig. 1, is a major valley that joins Metro Vancouver, up through Squamish, Whistler, and Pemberton. It is flooded by the sea in the south (Howe Sound) and cuts across the western Coast Mountains in the north. Winds through this complex terrain are primarily northerly or southerly. In February, about 75% of all reported winds at Pam Rocks (WAS) are from the northeast, north or northwest (Fig. 2). The strongest northerly winds occur with winter arctic outflow events, when cold air builds over the BC Interior under a ridge of high pressure. Figure 2: Wind rose plot at Pam Rocks (WAS) showing the frequency of wind direction and speed in February, based on 10-year hourly reports ( ). 2

4 Southerly winds occur in the S2SC when the pressure is higher on the coast than in the interior. This pattern usually develops after the passage of a front or when a ridge of high pressure builds offshore. Figure 2 shows that in February about 10% of reported winds at Pam Rocks are from the south. 3. Inflow and outflow in February 2010 Figure 3 shows that, through February 2010, about 55% of the surface winds observed at Pam Rocks were from the northeast, 35% from the north, and less than 5% from the south. This is slightly different from the climatology shown in Figure 2. On the other hand, winds aloft are primarily from the south, as detected by the wind profiler at Squamish (Fig. 4). Figure 3: Wind rose plot at Pam Rocks (WAS) showing the frequency of wind direction and speed in February

5 Figure 4: Monthly mean wind profile of February 2010 at Squamish Airport (WSK, elevation: 57 metres). 4. A case study of the inflow on 12 February 2010 Although the southerly inflow winds are not the dominant flows through the S2SC in February (Figures 2 & 3), their occasional arrivals were usually accompanied by warm, moist marine air, resulting in heavy precipitation and poor visibility in the valley. Therefore, accurately predicting the onset of inflow in the S2SC is critical to the Olympic operation. A southerly inflow was forecast to develop in the S2SC early in the morning of 12 February It arrived at least three hours earlier, and forced the forecasters at the Pacific Storm Prediction Centre (PSPC) to amend the corresponding forecasts. Was this event predictable based on the available operational model guidance and forecasters rules of thumb? What were the important weather processes occurring during its onset? We tried to answer these questions through this case study. 4

6 Table 1 shows that the northerly outflow shifted suddenly to southerly inflow at Pam Rocks around 0700 UTC of 12 February (2300 PST of the 11th). The along-valley pressure difference (WSK-YVR) was zero at 0700 UTC, suggesting that the southerly inflow was not due to pressure-driven channelling. The surface analysis at 0600 UTC (Fig. 5) indicates an approaching cold front over Vancouver Island. The inflow was forecast to arrive late overnight (around 1200 UTC), as the along-valley pressure gradient was expected to develop behind the front. Table 1: Winds observed at Pam Rocks on 12 February 2010, together with some relevant indices of pressure difference (WSK: Squamish, YVR: Vancouver, and YQQ: Comox). Time (UTC) Wind Direction Wind Spd Wind Gust Pressure Diff Pressure Diff (knots) (knots) (WSK YVR ) (YQQ YVR ) hpa 3.5 hpa hpa 3.5 hpa hpa 3.7 hpa hpa 4.7 hpa hpa 5.4 hpa 0900 Unknown hpa 5.5 hpa 1000 Unknown hpa 4.2 hpa Figure 6 shows that weak southerly winds were forecast by the GEM-Regional model of Canada (15-km resolution) to develop at Pam Rocks around UTC. Stronger inflow winds were forecast by higher resolution model to arrive at 1000 UTC (Fig. 7), which was still three hours behind reality. The marine forecast for Howe Sound was amended at 0930 UTC to reflect the observed inflow winds at Pam Rocks. The reasoning behind the amendment was that the strong pressure gradient along the Strait of Georgia (YQQ-YVR in Table 1) could lead to forced channelling (due to Ekman transport) in southern sections of the S2SC. 5

2010. Figure 6: Winds at Pam Rocks predicted by

7 Figure 5: Surface analysis of the Pacific Storm Prediction Centre, valid at 0600 UTC, 12 February Figure 6: Winds at Pam Rocks predicted by GEM-Regional model, based on initialization at 1200 UTC 11 February. 6

8 Figure 7: Surface winds predicted by GEM-LAM-1km model, based on initialization at 1200 UTC 11 February. Southerly winds of 20 to 25 knots were forecast to reach Pam Rocks at 1000 UTC, 12 February However, the wind profile at Squamish (Fig. 8) strongly suggests that downward momentum transport could be the main mechanism for the inflow winds observed at Pam Rocks at 0700 UTC. The fact that mixed down at Squamish occurred at 0900 UTC implies some disturbances propagating up the valley from the south. These disturbances were observed as convective radar echoes across the Strait of Georgia in Fig. 9. They were not, however, predicted by the operational NWP models, which accordingly failed to provide correct guidance to the forecasters. Further analysis indicated that the synoptic flow pattern around 0600 UTC on 12 February 2010 provided a favourable environment for mesoscale gravity waves to develop over the BC South Coast. A wave-cisk process could occur in combination with other mechanisms, such as shear instability and wave ducting, leading to the vertical turbulent mixing and the development of southerly inflow around 0700 UTC in the southern sections of the S2SC. 7

9 Figure 8: 24-hour wind profile at Squamish Airport on 12 February Figure 9: The cross sections (19 ) of radar reflectivity from Mt. Sicker on Vancouver Island to the S2SC indicating convective activities across the Strait of Georgia on 12 February

5. Outflow winds during 17 20 February 2010 A ridge of high pressure developing over the BC Interior during 17-20 February 2010 maintained a prolonged outflow pattern with generally dry and warm

10 5. Outflow winds during February 2010 A ridge of high pressure developing over the BC Interior during February 2010 maintained a prolonged outflow pattern with generally dry and warm conditions for the S2SC. Because there was no arctic cold air building under the ridge, only moderate outflow winds were observed near the valley bottom, with some weak afternoon sea breezes developing under the sunny skies. The Squamish wind profiler shows a layer of maximum outflow between 900 to 1500 meters with an interesting semi-diurnal cycle (Fig. 10). This should not be confused with a diurnal low level jet (LLJ) that is barely identifiable in the profile above 2 km. Figure 10: Wind profile at Squamish Airport during February

11 The LLJ aloft, which was also observed near the top of the nocturnal inversion in the Whistler rawinsondes, fit well with the conceptual model for nocturnal inertial oscillation. As the lower-level outflow jet-nose on the 18th is concerned, the maximum between UTC was caused by the vertical turbulent mixing due to afternoon sea breezes, and the maximum between UTC was associated with the diurnal oscillation of pressure gradient. A weak maximum near 1800 UTC of the 19th, which was also observed on the 17th and 20th (not shown), could result from the late-morning breaking down of the LLJ aloft. 6. Concluding remarks Diurnal thermal forcing, pressure-driven channelling, forced channelling, and downward momentum transport are four main forcing mechanisms of valley winds. Operational forecasts of inflow and outflow through the S2SC are often based on along-valley pressure difference and diurnal variation. Occasionally, forced channelling effect should be taken into account. We showed that momentum transport via some mesoscale systems can play an important role under favourable conditions. This report is based on a poster (see Fig. 11), presented at the 14 th Conference on Mountain Meteorology, American Meteorological Society (30 August 3 September 2010, Lake Tahoe Vicinity, California, USA); further information is available online at: 10

12 Figure 11: Poster presented at the 14 th Conference on Mountain Meteorology, American Meteorological Society (30 August 3 September 2010, Lake Tahoe Vicinity, California, USA). 11

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