Friday, 3 September 2010: 9:30 AM
Alpine Ballroom A (Resort at Squaw Creek)
Forecasting cloud forming over complex mountain terrain is one of the greatest challenges faced by meteorologists today. Based on intensive observations and high-resolution numerical weather prediction (NWP) models available for the Vancouver 2010 Winter Olympics and Paralympics, this study investigates some poor visibilities due to cloud formation on the Whistler Mountain slopes. We first examine the case that occurred on 28 February 2010. The forecast guidance from most NWP models suggested visibilities in the Whistler area would improve during the afternoon as a ridge of high pressure built over southern British Columbia. It turned out, however, that the northern slope of the Whistler Mountain remained covered by alpine clouds, as correctly predicted by the 1-km meso-scale model of Environment Canada (GEM-LAM1k). Further analysis indicated that the formation of these alpine clouds could be related to lee-side convergence due to capping aloft and flow around the mountains, a typical scenario that occurs with weak southerly inflow to the Whistler area. Operational forecasting of low cloud onset and duration in this mountainous area is generally difficult, but could be possible with good guidance from high-resolution NWP models.
Another interesting case occurred when periods of rain were brought to the Sea-to-Sky corridor on 16 March 2010 by a pre-frontal southwesterly flow aloft. Vertical cross sections of the Whistler Radar show a low-level down-valley flow induced by the diabatic cooling effect of orographic precipitation. Below this down-valley drainage flow, an up-slope flow layer developed with winds directed toward the mountain in the morning. The visibility observed on the mountain slope was worse when these up-slope winds were stronger. The formation, frequency, and predictability of such a unique scenario producing poor visibilities in complex mountain terrain are examined through further analysis.
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