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

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

3. MesoscaleM. D. Eastin 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

4. MesoscaleM. D. Eastin 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)

5. MesoscaleM. D. Eastin 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

6. MesoscaleM. D. Eastin 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)

7. MesoscaleM. D. Eastin 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

8. MesoscaleM. D. Eastin 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

9. MesoscaleM. D. Eastin 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

10. MesoscaleM. D. Eastin 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

11. MesoscaleM. D. Eastin 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

12. MesoscaleM. D. Eastin 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) Cool Warm Example of Slope Flow in the Morning

13. MesoscaleM. D. Eastin 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) Valley Floor Warm Cold Example of Valley Flow in the Morning Plains

14. MesoscaleM. D. Eastin 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) From Defant (1951) A C B D

15. MesoscaleM. D. Eastin 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) From Defant (1951) E G F H

16. MesoscaleM. D. Eastin Slope-Valley Flows Typical Diurnal Cycle on the Plains: From Toth and Johnson (1985) Continental Divide Great Plains Great Plains A – SunriseC – Noon E – EveningG – Midnight DCZ

17. MesoscaleM. D. Eastin 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

18. MesoscaleM. D. Eastin 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)

19. MesoscaleM. D. Eastin 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

20. MesoscaleM. D. Eastin 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 Thermally-Driven Circulations

21. MesoscaleM. D. Eastin 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.