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Edit Comment Close Premium member Presentation Transcript Local Scale CirculationsShawn JacobsFebruary 22, 2008: Local Scale Circulations Shawn Jacobs February 22, 2008 Outline: Outline The atmosphere Large scale to small scale Vertical structure Temperature Humidity Diurnal Variation Evolution of the boundary layer Definition and boundary layer discussion Boundary layer and local circulations Terrain Up slope/down slope Up valley/down valley Sea breezes Florida California Urban effects St. Louis Anthropogenic America’s heartland QuestionsThe Atmosphere: Large to Local Scale: The Atmosphere: Large to Local Scale Large scale circulations--planetary waves Global in size Rossby or “longwaves” Typically 4-7 circulating within each hemisphere In response to uneven heating of the earth’s surface 2000km + The Atmosphere: Large to Local Scale: The Atmosphere: Large to Local Scale Synoptic scale circulations Cyclones and hurricanes “Shortwaves” Form from baroclinic instability and horizontal/vertical motions 100’s km – 1000’s The Atmosphere: Large to Local Scale: The Atmosphere: Large to Local Scale Local Scale Circulations Typically small and localized Sea breezes Mountain/Valley flows Slope flows Anthropogenic effects Urban Heat Island Irrigation 10’s of meters to 100 km Vertical Structure: Temperature: Vertical Structure: Temperature T decreases with height through the troposphere T increases through the stratosphere T decreases in the mesosphere T increases through the thermosphereVertical Structure: Temperature and the Formation of the BL: Vertical Structure: Temperature and the Formation of the BL Boundary layer is where local circulations occur Exception—elevated local circulations Boundary layer—the layer of air from the earth’s surface usually up to about 1km where the wind is influenced by friction of the earth’s surface and objects on it. Boundary layer depth is determined by the temperature profile Typically large BL near the equator compared to the polesVertical Structure: Diurnal Change: Vertical Structure: Diurnal Change The boundary-layer experiences more rapid changes diurnally compared to the slower synoptic scale in the free atmosphere aloft Mixed layer The bottom 5% of the ML is known as the surface layer Vertical Structure: Diurnal Change: Vertical Structure: Diurnal Change Nocturnal boundary layer Residual layer Trapped heat, moisture, and pollutants!Vertical Structure: Humidity: Vertical Structure: Humidity Free atmosphere is relatively dry Evaporation from the surface during daytime adds moisture to the boundary layer Dew and frost formation lowers humidity in the lowest levels of the BLEvolution of the Boundary Layer: Entrainment: Evolution of the Boundary Layer: Entrainment Entrainment occurs whenever air from a nonturbulent region is drawn into an adjacent turbulent region. During day, mixed layer grows by entrainment and “burns off” inversion Thermals cease at night, recreating nocturnal inversion Special Effects of the BL:: Special Effects of the BL: Terrain Discontinuities such as coastlines Anthropogenic impacts such as cities and agricultureBasic Circulation : Basic Circulation Idealized situation Colder air north, warmer air south Colder air is more dense begins to “stack up” over an area—isobars bunch together Warmer air is less dense Isobars spread apart Pressure gradient formsSpecial Effects on the BL—TerrainUpslope /Down Slope: Special Effects on the BL—Terrain Upslope /Down Slope Complex localized flow can develop when introducing terrain into the equation Upslope during the day Down slope at night Special Effects on the BL—TerrainUpslope /Down Slope: Special Effects on the BL—Terrain Upslope /Down Slope Real life examples Wildland forest fires Fires run up hill, extreme fire behavior during up-slope runs Smoke carried down hill Down slope fires “creep” down hill Pollution issuesSpecial Effects on the BL—TerrainUp Valley/Down Valley: Special Effects on the BL—Terrain Up Valley/Down Valley Differential heating of the air mass in the valley relative to the plain PG forms H L Valley contains less volume than that over the plain Increase of energySpecial Effects on the BL:Terrain and the Froude number: Special Effects on the BL: Terrain and the Froude number Fr = V / (N*S) Where V is the wind speed N is the Brunt-Vaisala frequency The frequency at which a displaced air parcel will oscillate when displaced vertically within a statically stable environment S is the vertical or horizontal scale of the mountain Special Effects on the BL:Terrain and the Froude number: Special Effects on the BL: Terrain and the Froude number Froude number ~1 downslope windstorms Chinook windstorms are great at forcing Denver’s brown cloud eastward Residents can literally breathe a sigh of relief Froude number >1 flow up and over mountain—gravity waves Froude number <1 flow around the mountain—convergence?Special Effects on the BL:Terrain and the Froude number: Special Effects on the BL: Terrain and the Froude number Flow around a topographic barrier Convergence—upward lift Thunderstorms Northern Black Hills Southern Black HillsSpecial Effects on the BL—Terrain Influenced Circulations: Special Effects on the BL—Terrain Influenced Circulations Puget sound vorticity zone Denver Convergence Vorticity Zone induced terrain circulation Special Effects of the BL:The Sea Breeze: Special Effects of the BL: The Sea Breeze Temperature of the land heats more rapidly than over water Onshore flow develops Off shore at night Special Effects of the BL:The Sea Breeze: Special Effects of the BL: The Sea Breeze Cool sea breeze example Convergence off of the Sea Breeze boundary Wildland fires Special Effects of the BL:Local Winds and Water: Special Effects of the BL: Local Winds and Water Consider a large lake Wind blowing from the WNW at 10 mph Due to a decrease in friction as the air travels from the rough land to relatively smooth lake surface the relative wind speed increases, if the lake is large enough, the Coriolis force will act upon the wind NW wind flowing across the opposite side of the lake, once the air parcel reaches the opposite side, friction forces dominate once again and slow the winds, winds return to WNW but further south than without the lake Special Effects of the BL:Mausim: Special Effects of the BL: Mausim Mausim—Arabic for monsoon—seasons, or change of wind Winds blow one direction/season, blow the opposite direction the other Similar to a huge sea breeze, however not on a daily cycle Mausim of India, during winter air over the elevated terrain (Siberia) becomes much cooler than the Indian Ocean, high pressure forms—subsidence– down slope winds that are off-shore Summer—the continent becomes much warmer than the water—pattern reverses itself Humid air comes on shore, interacts with mountains, air rises, cools, and dumps tremendous amounts of precipitation Cherrapunji, India ~425 inches of precipitation yearly Pollution issuesSpecial Effects of the BL:Urban Effects: Special Effects of the BL: Urban Effects Cities are warmer than surrounding countryside Deeper BL Greater mixing heights Effects of the UHI can be experienced downwind of the metropolitan areaSpecial Effects of the BL: Inadvertent Induced Circulations: Special Effects of the BL: Inadvertent Induced Circulations “Inland sea-breeze” Intense irrigation over areas of the Western Great Plains Theory—differential surface heating would produce an inland circulation Yan and Anthes (1988), and Segal et al. (1988) both simulated these conditions Results are similar to modeled sea-breeze circulation Strips of land 100-200 km was all that was needed to produce these resultsQuestions: Questions Thanks! You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.