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Thermally-Driven Circulations

Thermally-Driven Circulations. What two countries were the Apollo astronauts viewing? Do you see any intriguing cloud formations?. Thermally-Driven Circulations. Land-Sea Breezes Slope-Valley Flows Urban Heat Island Circulations. Land-Sea Breeze. Definition:

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Thermally-Driven Circulations

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  1. Thermally-Driven Circulations What two countries were the Apollo astronauts viewing? Do you see any intriguing cloud formations? M. D. Eastin

  2. Thermally-Driven Circulations Land-Sea Breezes Slope-Valley Flows Urban Heat Island Circulations M. D. Eastin

  3. Land-Sea Breeze • Definition: • Low-level coastal circulation that • undergoes a regular diurnal • oscillation in response to • mesoscale heating gradients • Why should we care? • Over 50% of the worlds • population lives in coastal • areas impacted by the • land-sea breeze • Important factor in triggering • or enhancing convection • Florida • Great Lakes • Air pollution transport • Aviation meteorology • Recreation M. D. Eastin

  4. Land-Sea Breeze • Physical Processes: • Produced by differential heating across • the land-water interface of the low-level • air when synoptic forcing is weak • Negligible circulation exists at sunrise • Sea Breeze: • During the day, intense heating of the • boundary layer over land produces a • surface meso-low and a meso-high aloft • The relative lack of boundary layer • heating over water produces a surface • meso-high and a meso-low aloft • Air flows down the pressure gradients, • resulting in near-surface onshore flow • and offshore flow aloft • Mass continuity requirements produce • onshore ascent (convection) and • offshore descent (clear air) M. D. Eastin

  5. Land-Sea Breeze • Basic Characteristics of the Sea Breeze: • Maximum onshore flow occurs in the mid-afternoon • Shallow (300-500 m) • Maximum surface winds 5-10 m/s • Penetrate onshore up to 100 km M. D. Eastin

  6. Land-Sea Breeze • Sea Breeze Front: • Often a sea-breeze front will develop • at the leading edge of the onshore flow • Behave much like a small but intense • cold front or gust front • ΔT of 5-10ºC • Change in wind speed and direction • Moisture increase • Enhanced convergence • Weak vertical motion (~1 m/s) M. D. Eastin

  7. Land-Sea Breeze • Physical Processes: • Land Breeze: • After sunset, radiational cooling of the • boundary layer over land produces a • surface meso-high and a meso-low aloft • The relative lack of boundary layer • cooling over water produces a surface • meso-low and a meso-high aloft • Again, air flows down the pressure • gradients and mass must be conserved, • resulting in near-surface offshore flow, • offshore ascent (convection), • onshore flow aloft, and • onshore descent (clear air) • Before sunrise, the adiabatic warming • associated with the onshore descent • removes the pressure gradients, and • the circulation is negligible M. D. Eastin

  8. Land-Sea Breeze • Basic Characteristics of the Land Breeze: • Maximum offshore flow occurs at midnight • Less intense than the sea breeze • Maximum surface winds 2-5 m/s Infrared satellite image of Land Breeze over Japan M. D. Eastin

  9. Land-Sea Breeze • Forecast Considerations: • Weak or strong synoptic forcing • Pre-existing cloud cover • Time of onset • Inland penetration distance • Magnitude of ΔT • Strength of opposing synoptic flow • Maximum temperature forecasts • Convective initiation M. D. Eastin

  10. Land-Lake Breeze • Basic Characteristics: • Similar process as the land-sea breeze • Can be important for the triggering and • enhancement of deep convection • Circulation is often fairly strong in the • winter/spring months when the water • is still very cold but the land is • beginning to warm • Lake-effect snow is often enhanced • via the land-lake breeze circulations M. D. Eastin

  11. Slope-Valley Flows • Definition: • Low-level, diurnal circulation that • responds to mesoscale, horizontal • gradients in surface heating/cooling • in regions of sloped terrain • Why should we care? • Play a large role in determining • local weather in mountainous • regions when major synoptic • systems are not present • Important factor in triggering • or enhancing long-lived • convective storms • Lee-side of Rockies (DCZ) • North Carolina • Air pollution • Influence frost/freeze forecasts M. D. Eastin

  12. Slope-Valley Flows • Slope Flow: • Flow up or down the slope of a valley wall • Caused by differential heating/cooling and density gradients between the air • immediately adjacent to a valley • wall and the “mid-valley” air at the • same elevation • Cool, dense air flowing down elevated • terrain at night (nocturnal drainage flow) • Warm, less dense air moving toward • higher elevations during the day • (daytime upslope flow) Example of Slope Flow in the Morning Cool Warm M. D. Eastin

  13. Slope-Valley Flows • Valley Flow: • Flow up or down the valley • Caused by along-valley horizontal • pressure gradients due to either • the slope flow or density variations • with air in the free atmosphere • Cool, dense air flowing “down-valley” • at night (nocturnal drainage flow) • Warm, less dense air moving • “up-valley” during the day • (daytime upslope flow) Example of Valley Flow in the Morning Cold Warm Valley Floor Plains M. D. Eastin

  14. Slope-Valley Flows Typical Diurnal Cycle in the Valley: A Sunrise Onset of upslope winds Weakening down-valley wind (valley cold, plains warm) B Mid-morning Well developed upslope winds No valley wind (valley and plains same T) C Noon Weakening upslope winds Developing up-valley wind (valley warm, plains cold) D Mid-afternoon No slope winds Well developed up-valley wind (valley warm, plains cold) A B C D From Defant (1951) M. D. Eastin

  15. Slope-Valley Flows Typical Diurnal Cycle in the Valley: E Evening Onset of downslope winds Weakening up-valley wind (valley warm, plains cold) F Early Night Well developed downslope winds No valley wind (valley and plains same T) G Midnight Weakening upslope winds Developing down-valley wind (valley cold, plains warm) H Late Night No slope winds Well developed down-valley wind (valley cold, plains warm) E F G H From Defant (1951) M. D. Eastin

  16. Slope-Valley Flows Typical Diurnal Cycle on the Plains: A – Sunrise C – Noon E – Evening G – Midnight DCZ Continental Divide Continental Divide From Toth and Johnson (1985) Great Plains Great Plains M. D. Eastin

  17. Urban Heat Island Circulations • Definition: • Low-level mesoscale circulation produced by diurnal thermal flux gradients between • urban and rural areas Why should we care? • Play role in triggering or enhancing • convection above or downwind • of major metropolitan areas • (e.g. Atlanta, Houston) • Air pollution (increased smog) • Influence winter precipitation forecasts • Despite efforts to remove the effects, • could be significantly biasing the • global climate record M. D. Eastin

  18. Urban Heat Island Circulations • Physical Processes: • Results from a combination of differences in the following thermal characteristics: • Larger urban heat capacity • Lower daytime urban evaporation • Lower urban albedo • Anthropogenic urban heat release Basic Characteristics: • Primarily a 2-10ºC nocturnal difference • ΔT increases as the urban population increases • Most prominent with light winds beneath • a strong synoptic high pressure • Shallow (up to 1-2 km AGL) From Oke (1982) M. D. Eastin

  19. Urban Heat Island Circulations • Mesoscale Circulation: • The localized heat island produces a mesoscale circulation with maximum ascent (w < 1.0 m/s) over the urban region with descent in rural areas • If ascent can lift near-surface air to its LFC, • deep convection could develop or be enhanced • Numerical simulations suggest a ~5-10% increase • in precipitation downwind of urban regions “Extra” Precipitation M. D. Eastin

  20. Thermally-Driven Circulations • Summary: • Land-Sea Breezes • Definition • Physical processes • Forecasting Considerations • Slope-Valley Flows • Definition and Structure • Physical Processes • Diurnal Cycle • Urban Heat Island Circulations • Definition • Physical Processes M. D. Eastin

  21. References Atkins, N.T., R. M. Wakimoto, and T. M. Weckworth, 1994: Observations of the sea-breeze front during CaPE. Part II: Dual-Doppler and aircraft analysis. Mon. Wea. Rev., 123, 944-968. Defant, F. 1951: Local winds. Compendium of Meteorology. T. J. Malone. Ed, Amer. Meteor. Soc, 655-675. Pielke, R. A., 1974: Three-dimensional numerical model of the sea breezes over South Florida. Mon. Wea. Rev., 101, 115-139. Pielke, R.A. and M. Segal, 1986: Mesoscale circulations forced by differential terrain heating. Mesoscale Meteorology and Forecasting, P. Ray, Ed., AMS, 516-548. Oke, T. R., 1982: The energetic basis of the urban heat island. Quart. Journal Roy. Meteor. Soc., 108, 1–24. Toth J.J., and R. H. Johnson, 1985: Summer surface flow characteristics over northeast Colorado. Mon. Wea. Rev., 113, 1458-1469. Wakimoto, R. M., and N. T. Atkins, 1994: Observations of the sea-breeze front during CaPE. Part I: Single-Doppler, satellite, and cloud photogrammetry analysis. Mon. Wea. Rev., 122, 1092-1114. M. D. Eastin

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