Lecture 9 Chapter 7 Atmospheric Circulations Scales of atmospheric motions Eddies big and small Local wind systems Global winds Global wind patterns and the oceans Geostrophic wind Air initially at rest will accelerate until it flows parallel to the isobars at a steady speed with the pressure gradient force PGF balanced by the Coriolis force CF Surface wind Scales of Atmospheric Motions scales of motion microscale synoptic scale planetary scale Lots of important weather events occur on microscales like evaporation of liquid water molecules from the earth s surface Scales of Atmospheric Motions Scales of Atmospheric Motions Eddies Big and Small Eddies Big and Small eddy rotor wind shear turbulence Wind shear can sometimes be observed by watching the movement of clouds at different altitudes Wind Shear Can be formed from vertical changes in 1 Wind speed 2 Wind direction The formation of clear air turbulence CAT along a boundary of increasing wind speed shear The view of wind is from the side with a top layer of air moving over a layer below Figure 1 p 173 Turbulent eddies forming downwind of a mountain chain in a wind shear zone produce these waves called Kelvin Helmholtz waves The visible clouds that form are called billow clouds A dust devil The formation of a dust devil On a hot dry day the atmosphere next to the ground becomes unstable As the heated air rises wind blowing past an obstruction twists the rising air forming a rotating column or dust devil Air from the sides rushes into the rising column lifting sand dust leaves or any other loose material from the surface Fire whirl formation is similar to a dust devil Local Wind Systems Small scale winds that occur in localized areas Mountain valleys Beaches ocean fronts These are mesoscale circulations Are thermally driven Thermal Circulations isobars and density differences thermal circulations Thermal Circulations A thermal circulation produced by the heating and cooling of the atmosphere near the ground The H s and L s refer to atmospheric pressure The lines represent surfaces of constant pressure isobaric surfaces Sea and Land Breezes Sea and land breezes also occur near the shores of large lakes such as the Great Lakes Sea Breeze Mesoscale coastal winds developed during the daytime caused by heating of the land surface Pressure gradient forms due to temperature contrasts between the Heated land and cooler air over the ocean Land Breeze Pressure gradient is reversed at night and the winds blow from the land to the ocean Land and Sea breeze terminology An onshore wind blows from water to land whereas an offshore wind blows from land to water Converging sea breezes in the afternoon produce uplift that enhances thunderstorm development and rainfall Mountain and Valley Winds valley winds mountain winds The nighttime mountain breeze is sometimes called gravity winds or drainage winds because gravity causes the cold air to drain downhill Idealized Model of the Valley and Mountain Winds Types of Thermally Driven Winds found in Mountainous Regions 1 Plain Mountain Winds 2 Valley Winds 3 Slope Winds Thermally driven refers to the forcing due to temperature differences Thermally Driven Winds Found in Mountains Whiteman 2000 Cross section of a Mountain Valley Whiteman 2000 Valley Winds Daytime Air is warmer in the valley than over the plain Up Valley Winds Pressure is lower in the valley and higher over the plain at the same elevation The pressure gradient force is directed from the plain to the valley A up valley wind is produced that blows from the plain into the valley Down Valley Winds Nighttime Pressure gradient force reverses direction A down valley wind occurs Whiteman 2000 The Volume Effect of Valleys Whiteman 2000 Examples of Valley Shapes Whiteman 2000 Yosemite Valley Yosemite National Park Up Valley and Down Valley Surface Winds Measured in Yosemite National Park Winds blow up valley during day and down valley during night Date and Time Tethersonde Profiles in Yosemite Valley CA Vertical Structure of Winds and Temperature 700 600 PST 600 630 500 700 400 725 300 955 200 1035 100 0 282 284 286 288 290 292 294 Potential Temperature K 296 298 6 4 2 0 2 4 Up Valley Wind Component m s 6 1 Doppler SODAR at Tuolumne Meadows Yosemite Time Height SODAR Wind Profiles 12 August 2003 Vertical Structure of Down Valley Winds Yosemite National Park 12 Aug 2003 Nose of Down valley wind Nose is location of Wind speed maximum Upslope and Downslope Winds Sunlight heats mountain slopes during the day and the slopes cool by radiation at night Air in contact with the surface is heated cooled in response A difference in air density is produced between air next to the mountainside and air at the same altitude away from the mountain Density differences produce upslope day or downslope night winds Daily upslope downslope wind cycle is strongest in clear summer weather when prevailing winds are light Slope winds Whiteman 2000 Slopes winds are usually in the range of 1 4 m s 2 8 mph are weaker and more gentle than valley winds Peak wind speed occurs a few meters above the the slope surface Daytime upslope winds are typically stronger and deeper than nighttime downslope winds A transition period occurs between the upslope and downslope winds in evening and morning See Time Lapse Consequences of Upslope winds Upslope flow during the day leads to formation of clouds and precipitation along mountain ranges Special cases of slope flows Whiteman 2000 Clouds cause brief periods of shade on slopes and thus weaken the strength or even reverse the slopewinds Consequences of downslope flows Whiteman 2000 Downslope winds are often called drainage winds Downslope winds can produce a cold air pool in a valley or basin Some of these cold air pools can last several days to a week trapping pollutants in the valley basin Cold air pools are often associated with dense fog which is hazardous to aviation Down Slope Flow Evolution during Evening Special cases of slope flows Whiteman 2000 Snow is cold and downslope winds occur over snow Tree canopy is warm and heated and upslope winds form Temperature Inversion Review Height Cold Air Pool really just a temperature inversion that is confined within topography Stable temperature increases with height Temperature Cold Air Pool in Early Evening Height Synoptic Scale Winds Valley Winds Downslope or Drainage Winds Temperature Cold Air Pool Cold Air Pool Height Synoptic Scale Winds Stagnant Cold Air Pool Weak downslope winds