Mid-Mountain Clouds at Whistler During the Vancouver 2010 Winter Olympics and Paralympics

Pure Appl. Geophys. 171 (2014), 157–183 2012 Her Majesty the Queen in Right of Canada DOI 10.1007/s00024-012-0540-2 Pure and Applied Geophysics

1 2 2 2 3 4 RUPING MO, PAUL JOE, GEORGE A. ISAAC, ISMAIL GULTEPE, ROY RASMUSSEN, JASON MILBRANDT, 4 4 1 5 6 RON MCTAGGART-COWAN, JOCELYN MAILHOT, MELINDA BRUGMAN, TREVOR SMITH, and BILL SCOTT

Abstract—A comprehensive study of mid-mountain clouds and 1. Introduction their impacts on the Vancouver 2010 Winter Olympics and Pa- ralympics is presented. Mid-mountain clouds were frequently present on the Whistler alpine venue, as identified in an extensive Forecasting orographic cloud poses a great archive of webcam images over a 45-day period from February 5 to challenge in areas of complex terrain where meteo- March 21, 2010. These clouds posed serious forecast challenges rological conditions exhibit dramatic spatial and and had significant impacts on some Olympic and Paralympic alpine skiing competitions. Under fair weather conditions, a diurnal temporal variability (BANTA, 1990;WHITEMAN, 2000; upslope (anabatic) flow can work in concert with a diurnal tem- BARRY, 2008;HOUZE, 2012;GULTEPE et al. 2012). perature inversion aloft to produce a localized phenomenon known During the Vancouver 2010 Olympic and Paralympic as ‘‘Harvey’s Cloud’’ at Whistler. Two detailed case studies in this Winter Games (hereafter referred to as the 2010 paper suggest that mid-mountain clouds can also develop in the area as a result of a moist valley flow interacting with a downslope Olympics), mid-mountain clouds frequently impacted flow descending from the mountaintop. A southerly inflow through the alpine competitions in the Whistler area (see the Sea-to-Sky corridor can be channeled by the local topography Fig. 1). These clouds are locally called ‘‘Harvey’s into a westerly upslope flow toward Whistler Mountain, resulting in orographic clouds on the alpine venue. Under favorable circum- Cloud’’ with reference to a mountain race official and stances, these clouds are trapped to the mid-mountain zone by the weather observer named Harvey Fellowes (JOE et al. leeward subsidence of an elevated southerly flow. The presence of 2010;MAILHOT et al. 2010; also see ‘‘Appendix’’). the downslope subsidence was manifested by a distinguished dry layer observed on the top of the mid-mountain clouds in both cases. However, given the complex terrain in the area, the It is the subsidence-induced adiabatic warming that imposes a underlying mechanisms involved in their formation strong buoyant suppression to trap the mid-mountain cloud. On the and maintenance are not fully understood. This study other hand, the subsidence-induced dry layer has the potential to provides a comprehensive analysis of this low-level trigger evaporative instability to periodically breakup the mid- mountain cloud. orographic cloud, based on the intensive meteorological data available during the 2010 Olympics. The Key words: Mid-mountain clouds, alpine visibility, main goals are to document the mid-mountain cloud orographic flow, evaporative instability. phenomenon, highlight its impacts and the forecast challenges, and provide further insight into its thermal and dynamical origins. The Vancouver Organizing Committee for the 1 National Laboratory for Coastal and Mountain Meteorol- 2010 Olympics (VANOC) contracted with Environ- ogy, Environment Canada, 201-401 Burrard Street, Vancouver, BC V6C 3S5, Canada. E-mail: [email protected] ment Canada (EC) to provide weather services for the 2 Cloud Physics and Severe Weather Research Section, Games (DOYLE et al. 2006;JOE et al. 2010). One of Environment Canada, Toronto, ON, Canada. the greatest challenges faced by EC’s Olympic 3 National Center for Atmospheric Research, Boulder, CO, Forecast Team (OFT) was to accurately predict vis- USA. 4 Recherche en pre´vision nume´rique, Environment Canada, ibility for the downhill ski events on the Whistler Dorval, QC, Canada. alpine venue. Poor visibility due to persistent fog or 5 Pacific Storm Prediction Centre, Environment Canada, alpine cloud was one of the reasons that the Inter- Vancouver, BC, Canada. 6 Monitoring Operations Centre, Environment Canada, national Ski Federation abandoned Whistler as a Richmond, BC, Canada. World Cup venue several years earlier (KINGSTON, 158 R. Mo et al. Pure Appl. Geophys.

Figure 1 Topography in the vicinity of Vancouver. Left the greater domain. Right the Whistler area. The locations of the following automatic weather stations are also marked by their identiﬁers: Whistler Radar (VVO 557 m), Nesters (VOC 659 m), Timing Flats (VOT 805 m), Whistler Creekside (VOB 933 m), Whistler Mt. Mid Level (VOL 1,320 m), Whistler Mt. High Level (VOA 1,640 m), Whistler Mt. High Level Wind (VOH 1,643 m), Roundhouse Helipad (RND 1,856 m), and Whistler Peak (PEK 2,120 m). The Whistler alpine venue ranges from VOT to VOH. The topography data are from the SRTM 300 DEM dataset (FARR et al. 2007)

2010; also see ‘‘Appendix’’). For the 2010 Olympics, forms on the Whistler Mountain slopes under fair the visibility thresholds of alpine competitions were weather conditions. It appears to originate with a set between 200 and 500 m.1 However, with a sparse southerly inflow, and is likely capped by an inversion observational and climatological record in the aloft. This limited knowledge provided the forecast- Whistler area, operational meteorologists do not have ers with a heuristic technique to diagnose and the necessary tools to provide reliable visibility anticipate the cloud. To meet the Olympic forecast forecast with such a degree of precision. requirements, however, the venue meteorologists had The OFT members learned prior to the Olympics to rely on more accurate guidance. A better concep- that severe visibility reduction on the Whistler alpine tual model and a good observing system were also in venue could be expected with heavy precipitation, great demand. blowing snow, or alpine clouds. In particular, the To support mission-critical operations of the OFT, prevalence of Harvey’s Cloud leading to poor visi- EC worked with other partners to establish an bility was recognized during in situ forecasting Olympic Autostation Network (OAN) at a very high exercises in the winters leading up to the 2010 spatial–temporal resolution (DOYLE et al. 2006;JOE Olympics (H. FUNG,A.COLDWELLS,I.DUBE´ ,A. et al. 2010, 2012;GULTEPE et al. 2012). EC also GIGUE` RE, P.-A. BERGERON, and J. GOOSEN, personal developed an experimental high-resolution numerical communications). This mid-mountain cloud often weather prediction (NWP) system for the 2010 Olympics, with customized model outputs, including wind gust and visibility, available for venue fore- 1 During the planning phase of the 2010 Olympics, based on casting (MAILHOT et al. 2010, 2012). In addition, the discussions with sport officials at VANOC, initial visibility thresholds were set as low as 20 m. In the test events in 2009, it World Meteorological Organization conducted a was learned from the International Sport Federation officials that research and development project called the Science the critical visibilities were higher than previously indicated, and of Nowcasting Olympic Weather for Vancouver 2010 the officials needed to know when visibility was forecast to drop below 200 m on all or part of the course, as this could affect event (SNOW-V10), which organized a team of interna- scheduling (C. Doyle, personal communication). tional scientists, including several cloud physics Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 159 experts, to work side-by-side with the OFT on a directly south of the town of Whistler, between research support desk (ISAAC et al. 2012). The project Fitzsimmons Creek and the Cheakamus River, with a also assembled some advanced observational strate- summit elevation of 2,182 m. gies, NWP models, and visualization systems, from All alpine skiing events of the 2010 Olympics which the venue meteorologists benefited greatly for were held on Whistler Mountain at Whistler Creek- their alpine cloud and visibility forecasts. side. The men’s Olympic events took place on the A main goal of the SNOW-V10 project is to Dave Murray Downhill slope, starting at an elevation improve our understanding, and ability to forecast, of 1,684 m. The women’s Olympic events, and all low cloud and visibility in complex terrain (ISAAC Paralympic alpine skiing events, took place on the et al. 2012). This study focuses on the formation of Franz’s Run, starting at an elevation of 1,595 m. Both mid-mountain clouds in the Whistler area, based on courses end at Timing Flats (805 m). This alpine the OAN data and NWP model outputs available for venue is partially protected from southerly winds by the 2010 Olympics. It consists of an overall survey the Khyber Cliff that forms the north side of the and two detailed case studies. The results can be used Cheakamus River. It is prone to the southwest inflow to set forth a conceptual framework for the opera- through the valley and, to a lesser extent, to the tional forecast of mid-mountain clouds in the northerly outflow as well. Whistler area. Such a conceptual model is also expected to have a broader impact on the study of 2.2. The Olympic Autostation Network Data orographic clouds in complex terrain, and can be applied to predict mid-mountain clouds and the Data from a variety of weather observing systems associated weather in other areas with similar geo- installed in the Whistler area are used in this study. graphical features. These include data from eight OAN automatic The remainder of the paper is organized as fol- stations (Fig. 1), located at Nesters (VOC), Timing lows. Section 2 describes the topographical features, Flats (VOT), Whistler Creekside (VOB), Whistler OAN data, and EC’s NWP models. The definition Mt. Mid Level (VOL), Whistler Mt. High Level and observations of mid-mountain clouds on the (VOA), Whistler Mt. High Level Wind (VOH), Whistler alpine venue during the 2010 Olympics are Roundhouse Helipad (RND), and Whistler Mountain presented in Sect. 3. The two case studies are given in Peak (PEK). Most of these stations were equipped Sect. 4. A conceptual model for mid-mountain cloud with sensors to measure pressure, temperature, formation is constructed in Sect. 5. Conclusions are humidity, wind, precipitation, and visibility, at given in Sect. 6. A note on Harvey’s Cloud, based on 1-min intervals. Some of them were also equipped two interviews with Mr. Harvey Fellowes, is included with high-resolution digital webcams. Images from as an ‘‘Appendix’’. these webcams were archived at 10 min intervals, available from the Pacific Storm Prediction Centre in Vancouver. 2. Geography, OAN Data and Model Outputs An Olympic radiosonde station was also set up at Whistler (co-located with VOC) to launch upper air 2.1. The Whistler Alpine Venue observations for the 2010 Olympics. A Doppler radar (VVO) was installed near Whistler to provide areal Whistler was the co-host of the Vancouver 2010 and vertical coverage of precipitation and wind at Olympics. It is located approximately 125 km north 10-min intervals. In addition, a Vaisala 915 MHz of Vancouver (Fig. 1). Whistler Mountain2 is located wind profiler installed at the Squamish airport (WSK) supplied vertical wind profiles at 30-min intervals. 2 Whistler Mountain was originally named ‘‘London Moun- The official trainings of the Vancouver 2010 tain’’ by British naval officers who surveyed the area in the 1860s, Olympics started on February 5, with the actual allegedly because of heavy fog and cloud typically gathering Games beginning on February 12 and ending on around the mountain. [http://www.whistlermuseum.org/whistler history]. March 21. This study focuses on the period from 160 R. Mo et al. Pure Appl. Geophys.

February 5 to March 21, 2010, when OAN was Most model data used in this study were the LAM1k switched to an operational mode and fully supported outputs, available from the Canadian Meteorological by EC. Centre data archive at 1-h internals.

2.3. EC’s High-Resolution NWP Model Outputs 3. Mid-Mountain Clouds During the 2010 Olympics EC’s experimental mesoscale deterministic prediction system used for the 2010 Olympics consisted Reduced visibility during alpine ski competitions of a nested limited area model (LAM) configuration is a contributing factor in injury (MCBETH et al. of the Canadian Global Environmental Multiscale 2009). The early period of the 2010 Olympics was (GEM) forecast model with 15, 2.5, and 1-km marred by numerous occurrences of poor visibility at horizontal grid spacings over domains in the Van- the alpine venues that resulted in delays and cance- couver and Whistler areas, ultimately driven by EC’s lations (KINGSTON, 2010). Even with advance warning 15-km regional forecast model (REG15k). The nested of visibility reductions by the OFT meteorologists, LAM configurations are referred to as the LAM15k, these events had a significant impact on logistics, LAM2.5k, and LAM1k, respectively. They share a scheduling, and broadcasting at the affected venues. hybrid terrain-following, 58-level vertical coordinate, At the Whistler alpine venue, reduced visibility is with about 10 levels nesting within the first kilometer often related to the occurrence of Harvey’s Cloud, or above ground. These models were run twice daily, mid-mountain cloud in general. with the LAM15k being initialized from 1600 PST (Pacific Standard Time, PST = UTC - 0800) and 3.1. Definition of Mid-Mountain Cloud 0400 PST with initial and boundary conditions provided by the REG15k (starting at the same time). Mid-mountain clouds can be generally defined as The LAM2.5k runs were initialized at 2200 PST and orographic clouds with bases touching the mid- 0700 PST from the corresponding LAM15k forecasts, mountain slopes (BARRY, 2008;JOE et al. 2010; and then the LAM1k runs were initialized at 0300 MAILHOT et al. 2010). Clouds of this kind have been PST and 1200 PST from the 5-h forecasts of the observed in many areas across the globe (VARNEY, LAM2.5k. All of the three LAM integrations used the 1908;LANE-POOLE, 1925;YOSHINO, 1975;GIAMBELL- Milbrandt-Yau double-moment microphysics scheme UCA and NULLET, 1991;CHEN and FENG, 1995;COLVILE (MILBRANDT and YAU, 2005a, b) to perform calcula- et al. 1997;KUROSAKI and KIMURA, 2002;TAGO et al. tions of in-cloud microphysical and precipitation 2006;SCHOLL et al. 2007;COLBECK et al. 2008;NIU processes.3 Details of the GEM model can be found et al. 2010). Some of them can be regarded as in COˆ TE´ et al. (1998); the experimental high-resolu- upslope fog (TOTH et al. 2010). Note that a mid- tion prediction system for the 2010 Olympics were mountain cloud differs from a banner cloud, which described in MAILHOT et al.(2010, 2012). forms near the mountain peak (DOUGLAS, 1928; Some parameterizations were applied to derive WHITEMAN, 2000;SCHWEEN et al. 2007). diagnostic visibility outputs from these NWP models This study focuses on mid-mountain clouds (GULTEPE et al. 2009;BOUDALA and ISAAC, 2009). leading to poor visibility on the Whistler alpine venue. Based on the available OAN data, mid- mountain cloud is defined as a cloud that attaches to 3 The Milbrandt-Yau scheme is a versatile microphysics the alpine venue, with a base above the VOT station scheme, which has six separate hydrometeor categories for cloud and a top below the RND station. Images from an droplets, rain, ice, snow, graupel, and hail. A similar scheme with five hydrometeor categories (without hail) was developed by uphill-facing webcam at VOT (VOT-UH), a south- THOMPSON et al. (2004), and tuned for winter weather with a new west-facing webcam at VOL (VOL-SW), and a snow parameterization (THOMPSON et al. 2008). For the mid- downhill-facing webcam near VOH (referred to as mountain clouds examined in this study, there should be little if any VOH-DH) are used to identify the clouds; webcam impact on the choice of scheme, since all of these microphysics schemes use saturation adjustment to compute condensation. images from RND were not available for this study. If Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 161

Figure 2 A mosaic of mid-mountain clouds as seen from the downhill-facing webcam near VOH at the start of the Dave Murray Downhill slope the cloud top is too high to be seen from the VOH- two consecutive webcam images, it is called a mid- DH webcam, the cloud is not considered as a mountain cloud event. mid-mountain cloud. A few mid-mountain clouds captured by the VOH-DH webcam are shown in Fig. 2. 3.2. Observations of Mid-Mountain Clouds The webcam images were archived at 10-min intervals. Mid-mountain clouds with a lifespan less A total of 93 mid-mountain cloud events on the than 10 min are not taken into account in this study. Whistler alpine venue were identiﬁed during the When a mid-mountain cloud is detected on at least daylight hours of the 45-day period from February 5 162 R. Mo et al. Pure Appl. Geophys.

Table 1 Mid-mountain cloud events identiﬁed during the daylight hours between February 5 and March 21, 2010

Date Event durations (PST) Precip. at VOT?

Feb 5 1030–1200; 1240–1250; 1350–1410; 1640–1800 Yes; yes; no; no Feb 7, 8 1620–1830; 1020–1040 (Feb 8) No; no Feb 10 1030–1150; 1220–1250; 1440–1450; 1620–1630 No; yes; yes; yes Feb 10 1700–1830 Yes Feb 11 0750–0830; 0950–1030; 1540–1600; 1630–1730 No; yes; yes; yes Feb 12 0620–0950; 1100–1200; 1600–1620; 1720–1740 Yes; no; no; yes Feb 13 0630–0650; 1100–1150; 1230–1310 No; yes; yes Feb 14 0720–0800; 0820–1130; 1320–1400; 1420–1450 No; no; no; no Feb 14 1540–1600; 1640–1650 No; no Feb 16 0800–0810; 0940–1150; 1220–1350; 1410–1450 Yes; yes; yes; yes Feb 16 1520–1620; 1700–1810 No; no Feb 24 0900–0940; 1000–1030; 1100–1530; 1600–1620 Yes; yes; yes; yes Feb 24 1650–1850 No Feb 25 0610–0820; 0850–1150; 1330–1410; 1500–1540 No; no; no; yes Feb 25 1610–1620; 1710–1840 Yes; no Feb 27 1210–1230; 1250–1410; 1430–1450; 1600–1640 Yes; yes; yes; no Feb 28 0630–0950; 1030–1100; 1300–1310; 1520–1620 Yes; yes; no; no Feb 28 1700–1720 No Mar 2, 3 1340–1420; 1450–1520; 1140–1150 (Mar 3) Yes; yes; no Mar 4, 7 1730–1740; 1810–1850; 1750–1800 (Mar 7) No; no; yes Mar 10 0900–1000; 1020–1030; 1050–1130; 1230–1240 Yes; yes; yes; yes Mar 11 1300–1430; 1450–1500; 1550–1650; 1830–1840 No; no; yes; yes Mar 13 0630–0650; 0710–0730; 0820–0850; 0930–0950 Yes; no; no; no Mar 13 1020–1220; 1250–1300; 1340–1410; 1500–1840 Yes; yes; yes; yes Mar 15 1100–1110; 1220–1250; 1330–1340; 1410–1450 No; no; no; no Mar 16 0730–0800; 1210–1230 Yes; yes Mar 17 0520–0740; 0830–0940; 1000–1050 No; yes; no Mar 21 0620–0630; 0740–0930; 0950–1050; 1110–1130 Yes; yes; no; yes Mar 21 1200–1220; 1300–1320; 1740–1800; 1820–1920 Yes; no; yes; yes Total Total events = 93; total lifespan = 4,950 (min) Yes: 51; no: 42 The total survey time adds up to 35,450 min to March 21, 2010 (see Table 1). Some of their from 0630 PST to 1830 PST, the averaged frequency statistics are shown in Fig. 3. Roughly half of these of occurrence was 17 % (Fig. 3b). The relatively high events had a lifespan no longer than 30 min, and frequencies around 1100 PST in Fig. 3b suggest a 11 % of them lasted longer than 2 h (Fig. 3a). The weak late-morning tendency of mid-mountain cloud longest event lasted for 4.5 h (1100–1530 PST, Feb formation, which could be related to the morning 24, 2010). The average lifespan is 53 min. transition of the valley airflow. During the morning Most of the mid-mountain clouds had a generally transition, the thermally driven upslope (anabatic) flat top. Occasionally, the clouds were seen to swell wind strengthens and then flows horizontally toward vertically, implying a more energetic and unstable the valley center after reaching the temperature condition (e.g., Fig. 2f). Not all of these mid-moun- inversion aloft (ATKINSON, 1981;WHITEMAN, 1990, tain clouds formed under clear skies aloft. Some of them 2000;LI and ATKINSON, 1999;PRINCEVAC and FER- were accompanied by a large amount of higher level NANDO, 2008;GULTEPE and ZHOU, 2012). This clouds (e.g., Fig. 2a, c). About 58 % of the mid- transition circulation, which begins at about 0900 mountain cloud events produced measurable precipita- and ends at about 1100 local time (LI and ATKINSON, tion at the base of the Whistler alpine venue (Table 1). 1999), is favorable for stratus clouds to form on the The overall frequency of mid-mountain cloud mid-mountain slope. After the morning transition, the occurrence was 14 % (Table 1). For the 12-h period valley flow becomes fully coupled with the free Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 163

(a) robustly identified in Fig. 4. This is understandable, given that several other factors (e.g., heavy precipitation, blowing snow, etc.) could also produce severe visibility reduction on the mountain slope. Most mid-mountain clouds occurred when the boundary layer was stably stratified and a weak to moderate southerly inflow was pushing up through the Sea-to-Sky corridor. The local topography in the Whistler area channeled the southerly inflow into westerly upslope flow.4 It is understood that moist air was advected by the inflow from the coast, providing a favorable condition for the orographic cloud formation. No significant cloud was observed on the (b) Whistler alpine venue during February 17–22 and March 18–20, 2010, when northeasterly outflows prevailed in the Sea-to-Sky corridor.

4. Case Studies

Two case studies are presented in this section to address various aspects of mid-mountain cloud on the Whistler alpine venue. The selected cases represent two different flow configurations that are conductive to the generation of up-valley flow and mid-mountain Figure 3 a Lifespan-category percentages of mid-mountain cloud events cloud in the Whistler area. Both cases had a significant listed in Table 1. b Frequencies of occurrence of mid-mountain impact on the Olympic alpine skiing competitions clouds during the daylight hours between 0630 PST and 1830 PST (KINGSTON, 2010;HEIM, 2010). of the period from February 5 to March 21, 2010; the solid line represents the 12-h averaged frequency of occurrence (17 %), and The first case, which occurred on February 16, the dashed line represents the critical frequencies at the signifi- 2010, followed a cold-frontal passage as shown in cance levels of a = 0.10 (Pearson’s v2 test) Fig. 5. A negatively tilted upper-level trough of low pressure (Fig. 5a) and a surface front associated with a 988-hPa low in the Gulf of Alaska (Fig. 5b) laid atmosphere aloft, which is more favorable for con- along the British Columbia (BC) coastline at 0400 vective clouds to develop. PST and moved inland over the following 6 h. As a Mid-mountain clouds have a significant impact on surface anticyclone built off the Oregon coast, wes- the visibility at the Whistler alpine venue. Figure 4 terly flow developed across southwestern BC and an shows the diurnal variations of frequency of low up-valley pressure gradient established (Fig. 5d, f), visibilities along the alpine slope, based on the 1-min driving marine air into the Whistler area. The peak of observations of the Vaisala FD12P sensors at RND the precipitation at the Olympic venues was between (1,856 m), VOA (1,640 m), VOL (1,320 m), and 0400 PST and 1000 PST for this case. Mid-mountain VOT (805 m). The presence of mid-mountain clouds was strongly implied by the generally higher prob- ability of low visibility between 1 and 1.5 km, especially during the period from late morning to 4 Some venue meteorologists noticed that at VOB the upslope flow was from the west or northwest, and was highly early evening (also see ISAAC et al. 2012). The above- correlated with the foggy condition at VOL (Dube´, 2010, personal mentioned late-morning tendency could not be communication). 164 R. Mo et al. Pure Appl. Geophys.

(a)

(b)

(c)

(d)

Figure 4 Diurnal variations of occurrence frequency for visibilities below (a) 1,000 m, (b) 500 m, (c) 200 m, and (d) 100 m on the Whistler alpine venue, based on the visibility observations of the Vaisala FD12P sensors at RND, VOA, VOL, and VOT. The frequency is calculated for every 10-min bin, using the 1-min data of 45 days from February 5 to March 21, 2010 clouds formed behind the cold front and lingered (LLJ) and a precipitation-induced down-valley ﬂow. through the afternoon, in response to the synoptically Figure 6 shows a deep upper-level trough along the driven inﬂow. West Coast of North America at 0400 PST. The The second case occurred just 11 days later southerly LLJ was established downshear of a shal- (February 27, 2010). It was selected to demonstrate low mesolow that remained nearly stationary over the interaction between a southerly low-level jet southern Vancouver Island (Fig. 6b, d, f). Winds Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 165

Figure 5 GEM REG15k analyses (0-h forecasts) on February 16, 2010, valid at 0400 PST, 1000 PST, and 1600 PST, respectively. Left column: 500 hPa geopotential height (contour interval 6 dam, solid lines) and absolute vorticity (contour interval 4 9 10-5 s-1, dashed lines). Right column: Mean sea level pressure (contour interval 4 hPa, solid lines) and 1,000–500 hPa thickness (contour interval 6 dam, dashed lines). The Vancouver-Whistler area is marked by a black square around the system peaked near the ridge-top level in area created an outflowing cold surge, which lifted the Whistler area, and were channeled into an inflow the relatively warm inflowing air and led to the for much of the day. Evaporative cooling associated development of mid-mountain clouds near the with the widespread precipitation in the mountainous Olympic venues throughout the day. 166 R. Mo et al. Pure Appl. Geophys.

Figure 6 Same as Fig. 5, but for February 27, 2010

4.1. Mid-Mountain Clouds Due to Southerly Inflow: downhill training on the Whistler alpine venue February 16, 2010 (KINGSTON, 2010). As shown in Fig. 5, a cold front reached the coastline early in the morning. Snow fell On February 16, 2010, poor weather conditions across the Whistler area ahead of the front. A ridge of forced the Olympic organizers to postpone the men’s high pressure began to build offshore behind the super-combined event and to cancel the women’s Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 167 front, establishing a typical pattern for southerly (LILLY, 1968;RANDALL, 1980;DEARDORFF, 1980;KUO inflow to develop in the Sea-to-Sky corridor (LANGE, and SCHUBERT, 1988). There were, indeed, some 1998). The snow at Whistler was forecast to end in breaks of the dry layer, during which the orographic the morning, but fog patches on the alpine venue clouds became more energetic and propagated were expected to continue through early evening. upwards. On the other hand, the observed dry layer Figure 7 shows some webcam snapshots of the was likely an indicator of a subsidence aloft, which alpine conditions, and Fig. 8 shows the observations acted to trap the orographic clouds driven by the and model predictions of precipitation and visibility moist valley inflow from below. In Fig. 10b, a for the day at VOL, obtained from the SNOW-V10 temperature inversion occurred near the mountaintop real-time visualization system. At 0700 PST, snow during the afternoon and evening hours, consistent was evident through the VOH-DH webcam (Fig. 7a), with the subsidence theory. The strong static stability with visibility down to about 200 m (Fig. 8b). Mid- in the inversion made it very difficult for the mountain clouds began to develop behind the cold orographic cloud to penetrate from below. front by mid-morning (Fig. 7b). Although the cloud Note that, when a downslope flow descends from top was barely seen at 1000 PST, it became well- RND (1,856 m) to VOA (1,640 m), its mixing ratio defined in the afternoon (Fig. 7c, d). Within the should remain constant while RH decreased. At 1400 slope-clinging cloud, the visibility at VOL reduced to PST, the RH, temperature, and pressure readings were about 100 m (Figs. 7e, 8b). The cloud base was just 90.6 %, -3.4 C, and 790 hPa at RND, and 78.3 %, above VOT (Fig. 7f). Note that a layer of higher-level -1.1 C, and 835 hPa at VOA, respectively. These cloud existed above the mountains in the afternoon corresponded to a mixing ratio of 3.2985 g kg-1 at (Fig. 7c, d). RND and 3.2702 g kg-1 at VOA. They were close Figure 9 shows the soundings of the weather enough to strongly suggest that the reduced RH at VOA balloons released from Whistler Nesters (VOC) on was purely a result of the downslope flow. February 16, 2010. At 1000 PST, a thick layer of As shown in Fig. 8, the NWP models generally moist air was observed up to 400 hPa, with a near predicted lower visibilities in precipitation during 0 C isothermal layer below 850 hPa. Cold advection morning period (0400–0800 PST). The LAM1k in the mid-troposphere (e.g., between 500 and model also predicted poor visibilities near noon 400 hPa in the morning sounding) promoted synop- (1100–1300 PST). However, all models failed to tic-scale decent and led to an afternoon profile that predict the poor visibilities after 1240 PST, when was dominated by a subsidence inversion at 700 hPa. synoptic-scale precipitation had stopped behind the A near-saturated layer was observed below this layer, cold front. consistent with the upper-level clouds observed in Visibilities predicted by LAM1k for the Whistler Fig. 7c, d. At 1600 PST, the mid-mountain cloud area are shown Fig. 11. The patterns of poor visibility observed in Fig. 7d was at the 850 hPa level. strongly imply the presence of mid-mountain clouds However, Fig. 9 shows a dry layer at this level, in the vicinity of VOL during 1000–1200 PST. The suggesting that the mid-mountain cloud would not model visibilities near VOL improved significantly extend from the alpine slope to the valley center at after 1300 PST, inconsistent with the observations. VOC. This was confirmed by webcam images taken Forecast verification revealed that the model pre- at VOC (not shown). Moderate southwesterly inflow dicted a stronger ridge behind the cold front in the winds were observed in the afternoon. afternoon, leading to drier conditions in the valley. Relative humidity (RH) and temperature profiles Figure 12 shows the LAM1k model soundings at along the Whistler alpine venue on this day are VOC. As compared to its observed counterpart in shown in Fig. 10. A dry layer at the VOA level began Fig. 9, the model sounding at 1000 PST was too dry to develop after 1030 PST and dissipated in the late between 400 and 700 hPa. It could be possible that afternoon. This dry layer manifested itself as a cloud- the balloon released from VOC had drifted away topped boundary layer, and its presence could trigger from the area, so that the upper portion of the real evaporative instability to breakup the underling cloud sounding was not comparable with the model 168 R. Mo et al. Pure Appl. Geophys.

Figure 7 Webcam images captured along the Whistler alpine venue on February 16, 2010 sounding. Another possibility is that the drier layer (see Figs. 7, 9). However, the moist layer near the valley aloft in Fig. 12 was caused by an over-forecast of the bottom in Fig. 9 was absent in the model. As mentioned drying subsidence aloft. Below 700 hPa, the model earlier, the ridge behind the cold front was over- proﬁles at 1000 PST were similar to their observed forecasted in the afternoon, leading to drier conditions in counterparts. Therefore, the model still correctly pre- the valley. This incorrect moisture proﬁle in the model dicted the presence of mid-mountain clouds from mid- could be the main reason why simulated clouds eroded morning to near noon. At 1600 PST, the model correctly away too quickly. Note that both wind speed and predicted a moist layer near the 700-hPa level, which direction below 700 hPa were well predicted by the allowed a layer of clouds to form above the mountains LAM1k model. Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 169

Figure 8 SNOW-V10 precipitation rate and visibility charts for VOL on February 15–16, 2010. Precipitation was measured simultaneously by four sensors: FD12P, Parsivel, POSS, and Hot Plate. Visibility was derived from the measurements of FD12P and Parsivel (see GULTEPE et al. 2009, 2010, 2012). Model visibilities based on a parameterization scheme in GULTEPE et al. (2009) were denoted as LAM1k and LAM2.5k, and those based on a scheme in BOUDALA (2009) were denoted as Reg-BI and LAM1k-BI

To illustrate how channeled ﬂows contribute to low-level winds in the Whistler area. At 1100 PST, the mid-mountain cloud formation, Fig. 13 plots south to southwest inﬂow prevailed up to the some horizontal cross-sections of model (LAM1k) mountain top. Near the valley bottom, the moist 170 R. Mo et al. Pure Appl. Geophys.

Figure 9 The skew T-log p diagrams of the Whistler soundings launched on February 16, 2010 at 1000 PST and 1600 PST, respectively. One full wind barb represents 5 m s-1 (&10 kt) and a flag denotes 25 m s-1 (&50 kt) southwest inflow was channeled through the Sea-to- drawn in Fig. 13a). At 1100 PST, an upslope flow driven Sky corridor, and was further diverted into westerly by the channeled valley inflow was indicated by the or northwesterly toward Whistler Mountain by the weak positive (brown-colored) vertical velocity along local topography near VOL (Fig. 13d). It thus drove a the alpine slope (Fig. 14a near VOL). On the other hand, dynamical upslope flow to produce orographic clouds the southwesterly mountaintop wind forced an oro- on the Whistler alpine venue. graphic wave with a leeward subsidence over the Near the mountaintop, southwesterly winds Whistler alpine venue (i.e., the negative, or blue- moved over the Garibaldi Ranges and then crossed colored, vertical velocities above VOL in Fig. 14a). It Khyber Cliff of Whistler Mountain (Fig. 13a). This was the subsidence-induced adiabatic warming below mountaintop flow would descend onto the leeward the ridge level that acted to trap the orographic clouds to slope of Khyber Cliff (see Fig. 1 for geo-references). the mid-mountain zone. Figure 14bshowsawave- Depending on its nature and strength, the leeward induced drier layer above VOL, which however was not subsidence could either trap the orographic clouds to as dry as the dry layer observed in Fig. 10a. the mid-mountain zone or destroy them completely. The leeward subsidence above VOL became This is illustrated in Fig. 14, which shows the vertical much stronger in the early afternoon (1300 PST, cross-sections of the model atmosphere along a Fig. 14c). The resulting downslope flow would southwest-to-northeast line (i.e., along the green line disperse the mid-mountain cloud on the alpine venue. Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 171

(a) condition over the Whistler area in the overnight and morning hours. As shown in Fig. 15, precipitation at VOL began overnight around 0200 PST, peaked in the early morning around 0800 PST, and eased to light in the afternoon after 1330 PST. In the midst of heavy precipitation, visibility at VOL reduced to below 1,000 m around 0700 PST. As precipitation eased late in the morning, visibility further dropped to below 300 m. Mid-mountain clouds were observed through the (b) afternoon (Table 1; Fig. 16). Some of these clouds were partially obscured by the precipitation from the clouds aloft. Periodically, the mid-mountain clouds developed vertically and their tops became invisible to the VOH-DH webcam. In Fig. 17, a dry layer around the VOA level began to develop after 1000 PST. It was periodically broken in between, when mid-mountain clouds were observed to grow vertically up to the VOA level. Isolated, weak temperature Figure 10 inversions were observed along the slope between Height-time diagrams of relative humidity and temperature for the late morning and early afternoon (1000–1300 PST, period 1000–2200 PST, February 16, 2010, based on the 1-min data from seven OAN autostations (VOC, VOT, VOB, VOL, VOA, Fig. 17b). RND, and PEK) Scientists on the SNOW-V10 Research Support Desk noticed that some RHI (Range-Height-Indica- Note that the forecasted mountaintop winds were tor) displays of the Whistler Doppler radar often actually weaker in the afternoon than in the morning. showed a layer at the VOL level with winds toward The stronger afternoon leeward downslope subsi- the mountain, and the visibility reduction at VOL was dence in the model was likely due to the under- worse when this upslope flow was stronger (bear in forecast of the southwest inflow and moisture near the mind that the radar can only indicate the radial valley bottom in the afternoon, as can be seen by component of the flow). In Fig. 18, an upslope flow is comparing Fig. 12 with Fig. 9. It could also result indicated by the red layer (positive or away radial from over-forecasting the strength of orographic velocities) just above the 1-km level. It began to waves. develop in the morning and became stronger in the afternoon when the visibility at VOL reduced to below 300 m. Note that this upslope flow was not an 4.2. Mid-Mountain Clouds Due to Southerly anabatic wind, because the daytime heating on the Low-Level Jet: February 27, 2010 mountain slope was blocked by the precipitating mid- The men’s slalom competition was scheduled for mountain clouds. This radial velocity pattern also February 27, 2010 as the final Olympic alpine skiing occurred during the previous case on February 16, event. It was held at Whistler in challenging weather 2010, but only in the morning hours. conditions, which included wet snow, rain, and fog Northeast winds were observed near the valley on the mountain slope (HEIM, 2010). Many skiers bottom through the day (e.g., at VOC; not shown). were not able to finish the race because of missed They were detected by the Doppler radar as those gates or accidents in the intermittent fog. blue layers of toward-radar radial velocities in As shown in Fig. 6, the poor weather conditions Fig. 18. Part of these northeast winds could be were largely due to a deep upper-level trough of low understood as the down-valley flows caused by pressure along the coast, which created an unstable melting and evaporation of precipitation aloft, similar 172 R. Mo et al. Pure Appl. Geophys.

(a) (b)

Figure 11 LAM1k model predictions of visibility (combined effects of fog, rain, and snow) at the lowest model level (approximately 40 m AGL) on February 16, 2010 (initialized at 0300 PST February 16). The model topography contours are plotted with solid lines (contour interval: 200 m)

to the diabatic flows described in STEINER et al. (2003), velocity of hydrometeors, detectable by the Doppler ASENCIO and STEIN (2005), and THE´RIAULT et al. (2012). radar at higher elevation angles. Figure 19 shows the wind profiles observed at As shown in Fig. 15, the reduction of visibility at Squamish (WSK). South to southeast winds began to VOL after 1000 PST was well predicted by some of develop above 1,500 m at 0100 PST. A southerly jet the NWP models. The low-level flows predicted by formed around 0800 PST with a core of 17 m s-1 at the LAM1k model at 1000 PST and 1300 PST are the 2.5 km level, which is consistent with the shown in Fig. 20. In the morning (left column), evolution of the mesolow along the Washington northerly outflow dominated from valley bottom up coast (Fig. 6b, d). In Fig. 18, the red layer in the radar to 1,000 m along the Sea-to-Sky corridor (Fig. 20e). RHI display began to develop soon after the southerly At the 1,300-m level, southerly inflow appeared to the winds aloft extended down to the mid-mountain zone. south of the VVO radar, and southeast winds blew This red layer was still topped by a blue layer of out of the Cheakamus Lake then merged with the toward-radar radial velocity in the afternoon down-valley flow from Whistler (Fig. 20c). Southerly (Fig. 18c, d). The blue layer could be the remnants winds dominated at the mountaintop level, with a jet of down-valley flow caused by the diabatic cooling of stream blowing across Whistler Mountain (Fig. 20a). hydrometeor evaporation within the dry layer at the In the early afternoon (right column), the southerly jet VOA level (see Fig. 17a). It could also be the falling at the mountaintop level began to weaken (Fig. 20b), Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 173

Figure 12 Same as Fig. 9, but for a predicted sounding from the LAM1k model run initialized at 0300 PST, February 16, 2010 and the southerly winds had descended into the valley with the observations shown in Fig. 17b. At 1300 along the Sea-to-Sky corridor, at least down to the PST, the upslope flow, driven by the southwesterly 1,000-m level (Fig. 20d, f). These features are up-valley flow (Fig. 20d), became stronger. But it generally consistent with various observations shown was still capped by a weaker subsidence aloft. Rapid in Figs. 6, 18, and 19. increase in low-level temperature in the afternoon Figure 21 shows the battle over the mountain was well predicted by the model (Fig. 21d). slope, as illustrated by the LAM1k model, between the channeled upslope flow and the leeward subsidence from the southerly wind across the mountain 5. Discussion barrier. At 1000 PST, the leeward subsidence appar- ently dominated the upper portion of the slope (i.e., The two case studies in the previous section the blue-colored negative vertical velocities above provided some insight into the dynamical origin of Khyber Cliff in Fig. 21a). The weak upslope flow, the mid-mountain clouds on the Whistler alpine which was forced by the northerly down-valley flow venue. Given the complex terrain configuration in the (see Fig. 20c), penetrated slightly beyond VOL. area, the development of mid-mountain cloud can be Figure 21b indicates a near isothermal layer below understood in terms of the interaction of a channeled the mid-mountain level at 1000 PST, in agreement upslope flow with a downslope flow descending from 174 R. Mo et al. Pure Appl. Geophys.

(a) (a) (b) (b)

Figure 13 Horizontal cross-sections of the horizontal winds predicted by the LAM1k model run initialized at 0300 PST, valid at 1100 PST February 16, 2010, at a 2,200 m, b 1,600 m, c 1,300 m, and e 1,000 m. The green line with a 30o bearing in (a) is the cross-section line used to create Fig. 14 the mountaintop, as illustrated schematically in by this dry layer. In fact, the cloud-topped dry layer Fig. 22. The upslope flow is mainly driven by a makes the underlying atmosphere potentially unstable valley inflow through the Sea-to-Sky corridor, which through mixing processes at the cloud boundaries. also brings in the necessary moisture from the coast. This cloud-top entrainment (evaporative) instability The orographic uplift leads to moisture condensation could trigger embedded convection to breakup the and cloud formation on the Whistler alpine slopes. underlying cloud (STEVENS, 2010). In addition, the The orographic clouds are capped by a leeward leeward subsidence can also allow potential insta- subsidence as a result of a southerly mountaintop bility to build up by the accumulation of sensible and flow. The subsidence-induced adiabatic warming latent heating below, and eventually lead to the below the ridge level creates a stable layer in which destruction of the mid-mountain clouds through deep negative buoyancy overpowers the upslope flow convection (e.g., HOUZE, 2012). effect. A strong mountaintop flow could produce a A stably stratified valley flow approaching the Chinook windstorm to completely disperse the cloud mountain can be partially blocked and separated into on the leeward slope. On the other hand, a very weak an upslope component and a reversed surface flow mountaintop wind could not trap the leeward oro- (SMOLARKIEWICZ et al. 1988;RASMUSSEN et al.1989). graphic clouds to the mid-mountain zone. This flow reversal is also schematically illustrated in The dry layer observed on the top of the mid- Fig. 22. As mentioned earlier, the upslope component mountain cloud is caused by the downslope subsi- results in moisture condensation and cloud formation. dence. The growth of orographic cloud is not limited The reversed surface flow creates a drier layer near Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 175

(a) (b)

Figure 14 Vertical cross-sections of the LAM1k model outputs (horizontal wind, vertical velocity, temperature, and relative humidity) along the southwest-to-northeast green line shown in Fig. 13a. Left panel: the vector is composed of the along-section horizontal wind (m s-1), and the vertical velocity (m s-1) multiplied by a factor of 5; the real vertical velocity value is color shading. Right panel: the wind barb is meteorological wind in m s-1 (see Fig. 9 for wind barb convention), the temperature is in C(line contours, dashed lines for negative values, contour interval: 1 C), and the relative humidity is color shading the valley bottom, as can be identified in Figs. 10 and driven by the valley inflow. The southerly mountaintop 17. Otherwise the mountain cloud would be indis- wind provides a capping mechanism for the mid- tinguishable from the valley fog. mountain cloud through lee wave subsidence. The ele- The dynamical origin of the mid-mountain cloud vated nature of the mountaintop wind would prevent it is quite different from that of the banner cloud from dynamically inducing a strong leeward upwelling forming at the mountain peak level. The most from the valley bottom to the mountain top. accepted theory for banner cloud formation evokes an upwelling on the leeward side of the mountain, which is dynamically induced by the cross-barrier flow over 6. Concluding Remarks a sharp mountain peak (DOUGLAS, 1928;REINERT and WIRTH, 2009). For the mid-mountain cloud on the This study focuses on a specific phenomenon Whistler alpine venue, the upslope flow is mainly occurring on Whistler Mountain, locally known as 176 R. Mo et al. Pure Appl. Geophys.

Figure 15 Same as Fig. 8, except for 27 February 2010

Harvey’s Cloud. It is a mid-mountain cloud, concern during the Vancouver 2010 Winter Olympics belonging to the class of low-level orographic clouds. and Paralympics; several alpine ski training runs and Its presence can cause severe visibility reduction on competitions were rescheduled due to the presence of the mountain slopes, posing a serious threat to the mid-mountain clouds on the venue (KINGSTON, 2010; alpine skiing competitions. It was a primary forecast HEIM, 2010). Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 177

Figure 16 Cloud observations from the downhill-facing webcam near VOH at the start of the Dave Murray Downhill slope on 27 February 2010

Our analysis indicated that the averaged occurrence gives rise to an upslope flow and produces orographic frequency of mid-mountain cloud on the Whistler alpine clouds on the alpine slopes. On the other hand, a venue was between 14 and 17 % during the daylight southerly mountaintop wind across the southwestern hours through the selected 45-day period. Two detailed ridge of Whistler Mountain can produce a leeward case studies demonstrated that mid-mountain clouds can downslope subsidence to trap the orographic clouds to form as a result of a valley inflow being capped by a the mid-mountain zone. It is pointed out that a cloud- downslope flow aloft. When a southerly inflow through topped dry layer observed in this study has the potential the Sea-to-Sky corridor is channeled by the local to breakup the stratiform mid-mountain cloud through topography into westerly toward Whistler Mountain, it an evaporative instability. 178 R. Mo et al. Pure Appl. Geophys.

(a)

(b)

Figure 19 Figure 17 Winds measured by the Squamish wind proﬁler on February 26–27, Same as Fig. 10, except for the period 0400–1600 PST, February 2010. A long wind barb represents 5 m s-1 (&10 knots) 27, 2010

Figure 18 The RHI (Range-Height-Indicator) displays of the Whistler Doppler radar radial velocity along a 79 bearing line (VVO towards VOL) at (a) 0700 PST, (b) 1000 PST, (c) 1300 PST, and (d) 1330 PST 27 February 2010. The mapping format is RABT (Red Away, Blue Towards)

The thermal and dynamical origins of the mid- accurate and timely predictions in mountainous ter- mountain clouds identiﬁed in this study provide a rains remain challenging, and have to rely heavily on conceptual framework for the operational prediction a dense observational network and good guidance of poor visibilities on Whistler Mountain. However, from high-resolution NWP models. It has been Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 179

(a)(a) (b)(b)

(c)(c) (d)(d)

((e)e) ( (f)f)

Figure 20 Horizontal cross-sections of the horizontal winds predicted by the LAM1k model run initialized at 0300 PST, February 27, 2010 demonstrated that EC’s mesoscale deterministic pre- case study. The challenge for the operational mete- diction system for the 2010 Olympics was capable of orologists is to anticipate the model errors and adjust producing dynamically consistent guidance for their forecasts accordingly. It was shown that some operational forecasting over complex terrain. The user-friendly nowcasting tools from SNOW-V10 LAM1k model, in particular, could resolve the provided great help to operational forecasters for detailed topographical features in the Whistler area, real-time analysis, model validation, and short-range and generated realistic mesoscale structures within decision making. short time periods. Of course, NWP model guidance While the local topography in the Whistler area is could be misleading at times, as shown in our ﬁrst complex, it is not unique. Nor is the fact that the 180 R. Mo et al. Pure Appl. Geophys.

(a) (b)

Figure 21 Same as Fig. 14, except for a south-to-north cross-section line (i.e., the white dotted line in Fig. 22)

Coast Mountains border the oceanic moisture and latent heat and its role in triggering intense deep source. In fact, mid-mountain clouds are also often convection on the lee side of the mountains. observed in some other regions of British Columbia, Therefore, the conceptual framework for mid- e.g., the Columbia and Kootenay districts of the mountain cloud formation addressed in this study southern interior. In a recent review article, HOUZE can fill a gap in the theory of mountain meteorology, (2012) goes through a long list of mechanisms for and has the potential for providing better guidance precipitating clouds in complex terrains. Among in operational alpine weather forecast. In reality, them, the downslope capping of low-level moist operational meteorologists cannot easily identify flow is found in many regions around the world mid-mountain clouds from other types of orographic (e.g., near the Himalayas, the Gulf of Mexico and clouds from NWP model output fields. The most Arabian Sea). However, the formation of mid- possible is building a set of mountain cloud occur- mountain cloud under such capping has received rence rules, with mid-mountain cloud being one of little attention. Most studies of the downslope cap- them. Although further studies are required, the ping have focused on the accumulation of sensible feasibility of obtaining some rules for mid-mountain Vol. 171, (2014) Mid-Mountain Clouds Pure and Applied Geophysics 181

Figure 22 Schematic illustration of the main flow features leading to the mid-mountain cloud formation over the Whistler alpine venue. The right panel represents a bird view of a southwesterly valley inflow (black) engaging with a southerly mountaintop flow (grey) on the alpine venue. The left panel is a vertical cross-section view along the white dotted line in the right panel. The southerly valley inflow is channeled into westerly and upslope flow, leading to orographic cloud formation. The cloud is trapped by leeward descend of the southerly mountaintop wind. The topographic profile is based on the SRTM 300 DEM dataset (FARR et al. 2007). It has a higher resolution (*90 m) than the 1-km resolution profile of LAM1k model in Fig. 21 cloud formation is suggested by the preliminary Appendix results from this study. Harvey’s Cloud on Whistler Mountain Federation International de Ski (FIS) World Cup Acknowledgments Ski Downhill alpine racing on Whistler Mountain has had an infamous history. The first race was scheduled The following individuals are acknowledged for their in 1975 but was cancelled due to heavy fog. Three contributions to this study: Neil McLennan for races were cancelled in 1996, 1997 and 1998 due to providing software to retrieve and display the NWP heavy snow and fog. This led to no more scheduled model outputs; Ford Doherty for archiving the OAN FIS races until 2008, when the race course needed to data in the Pacific Storm Prediction Centre; Ivan be certified as meeting FIS standards. Heckman and Edwin Campos for their assistance Whistler Mountain Corporation employs snow with the SNOW-V10 data; Chris Doyle for providing forecasters to aid with their operations. These fore- background information on the visibility thresholds casters have personal and localized knowledge of the during the 2010 Olympics; Allan Coldwells and weather on a slope-by-slope basis. In discussions Henry Fung for sharing their preliminary studies of regarding the fog events that resulted in cancellation Harvey’s Cloud; Sabrina Wong for her help with of the alpine races, a phenomenon locally known as relevant references; Russell White for providing Harvey’s Cloud crept into the lexicon. It is used to valuable comments on an earlier version of this describe a mid-mountain cloud that seems to be manuscript. Beneficial discussions with Bertrand specific and peculiar to Whistler and is identified as Denis, Amin Erfani, Andre´ Gigue`re, Ivan Dube´, the cause of the race delays and cancellation. It was Philippe-Alain Bergeron, Jim Goosen, Alex Cannon, named after Mr. Harvey Fellowes, a long time Jennifer Hay, Monika Bailey, Faisal Boudala, and Whistler resident, who had a cabin for 14 years at Mark Barton contributed to improvements in the the 1,250 m level at a location called Raven’s Nest manuscript. We also thank two anonymous reviewers (near VOL in Fig. 1). Mr. Fellowes works for for their valuable comments on an earlier version of Whistler Mountain Corporation and participates as this paper. Some plots were generated using the an FIS alpine race course official. He is a wealth of NCAR Command Language software, freely avail- knowledge of the weather on the mountain that is able at http://dx.doi.org/10.5065/D6WD3XH5. important to FIS racing. 182 R. Mo et al. Pure Appl. Geophys.

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(Received November 30, 2011, revised April 24, 2012, accepted June 22, 2012, Published online August 7, 2012)