Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Winds of the Sierra NevadaSpecial cases from the norm• Washoe Zephyr- lee side winds• High Wind Events in Owens ValleyWinds of the Sierra Nevada(top) Owens Valley terrain and the locations of surface weather stations and (bottom) vertical cross section of the topography through Independence(Zhong et al. 2008)Owens Valley terrain and the locations of surface weather stationsComposite wind roses for each of the surface stations using all available data for the station. The local valley axis at each site is indicated by shading(Zhong et al. 2008)DayNightFIG. 3. Composite wind roses for Lone Pine and Keeler for (top) daytime and (bottom) nighttime using all available data for the station(Zhong et al. 2008)FIG. 5. Composite wind roses for wind speeds greater than 7 m/s for each of the six stations. The two numbers below each station plot indicate the total frequency of wind speeds exceeding 7 and 10 m/s, respectively, determined from all available data at eachstation.(Zhong et al. 2008)The seasonal frequency of high wind events, based on period of record from 1988 to 1991, when observations at five of the six stations were available. A high speed event was defined as an event when three or more of the five stations experienced hourly average winds equal to or exceeding 7 m s1. December–February: DJF; March–May: MAM; June–August: JJA; September–November: SON. (Zhong et al. 2008)FIG. 8. The percentage of hourly mean winds equaling or exceeding 7 m/s for each hour of the day determined using all available hourly data.(Zhong et al. 2008)FIG. 9. The percentage of high winds with speeds equal to or exceeding 7 m/s as a function of hour of the day and wind direction based on all available hourly data.(Zhong et al. 2008)1. Forced channeling causes winds inside the valley to be driven along the valley axis in the direction of the along-valley component of the above-valley wind (i.e., in this special case where the upper wind is along the valley axis, this is perpendicular to the pressure gradient). 2. Pressure driven channeling causes the wind to flow along the valley axis from the high pressure end toward the low pressure end (i.e., along the pressure gradient). This illustrates the differing results inside the valley of the forced and pressure-driven channeling mechanisms.Channel effects of Valleys and Terrain(Whiteman 2000, Zhong et al. 2008)Channel effects of Valleys and TerrainWhiteman 2000Forced channelingPressure driven channelingIllustration of flow channeling in a north–south valley (indicated by the two parallel dashed lines) where the isobars above the valley are labeled. The geostrophic wind (VG) flows parallel to the isobars with low pressure on the left, and the wind inside the valley is indicated by V0. (a) Forced channeling causes winds inside the valley to be driven along the valley axis in the direction of the along-valley component of the above-valley wind (i.e., in this special case where the upper wind is along the valley axis, this is perpendicular to the pressure gradient). (b) Pressure drivenchanneling causes the wind to flow along the valley axis from the high pressure end toward the low pressure end (i.e., along the pressure gradient). This illustrates the differing resultsinside the valley of the forced and pressure-driven channeling mechanisms.(Zhong et al. 2008)Forced channelingPressure driven channelingJoint frequency distribution of the geostrophic and valley wind directions for valley wind speeds greater than 7 m/s. The geostrophic wind direction is determined using the 625-hPa wind at the nearest NARR grid point. The contours are the joint wind direction frequencies (%). The lines illustrate the relationship between the surface and geostrophic wind directions as expected from the forced (solid line) and pressure-driven channeling (dashed line) theories.(Zhong et al. 2008)2/112/1222121)(111NiiiNiiNiivuNvNuNVVCWind Constancy(Stewart et al. 2002)The wind constancy is defined as the ratio of the vector mean and the arithmetic mean wind speeds for each hour of the day.Wind Constancy(Zhong et al. 2008)Washoe ZephyrTwo Hypotheses for development: 1. Washoe Zephyr is caused primarily by downward mixing of strong westerly momentum aloft as the growing afternoon convective boundary layer entrains the westerly flow aloft 2. the wind system is thermally driven by a regional-scale pressure gradient developed between the lower pressure over the heated, elevated terrain in the Great Basin on the eastern side of the Sierra Nevada and the higher pressure on the western side.(Clements 1999)Vertical profiles of down-canyon winds in eastern Sierra(Clements 1999)Frequency of diurnal down-canyon windsFrequency distributions of winds from the southwest to northwest quadrant.(Zhong et al. 2008b)FIG. 4. Frequency distributions of westerly winds 5 m/s at Galena for each hour of the day for all seasons and for 2003–05.(Zhong et al. 2008b)FIG. 5. (top) Difference of sea level pressure between Sacramento and Reno, (second from top) 700-mb wind speed and direction from Reno soundings at 0000 UTC on each day, and surface westerly downslope wind for each hour of the day for (third from top) Reno and (bottom) Lee Vining for summer of 2003.(Zhong et al. 2008b)(Zhong et al. 2008b)700 mb wind speed vs. Surface windsNumerical Simulations using RAMS at 1 km horizontal grid spacing(a),(b) RAMS simulated wind speed and (c),(d) potential temperature on an east–west cross sectionthrough the center of the domain at (left) 1200 and (right) 2000 LST.FIG. 8. Time series of wind (top) speed and (bottom) direction simulated using different ambient wind speeds at a grid point near Reno and the observed speed and direction at Reno for 5 Jul 2005.There is no clear relationship between the synoptic wind speed and surface westerly wind speed and quite often the surface wind exceeds the wind speed aloft.Washoe Zephyrthe hypothesis that the Washoe Zephyr is caused primarily by downward mixing of strong westerly momentum aloft is unlikely to be correct. Instead, the strong correlation between the pressure difference on the two sides of the Sierra Nevada and the development of the downslope surface