thermally-driven circulations

Mesoscale M. D. Eastin

Thermally-Driven Circulations

What two countrieswere the Apollo

astronauts viewing?

Do you see any intriguing cloud

formations?


Land-Sea Breezes

Slope-Valley Flows

Urban Heat Island Circulations



Land-Sea BreezeDefinition: 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


Land-Sea BreezePhysical 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)


Land-Sea BreezeBasic 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


Land-Sea BreezeSea 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)


Land-Sea BreezePhysical 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


Land-Sea BreezeBasic 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


Land-Sea BreezeForecast 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


Land-Lake BreezeBasic 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


Slope-Valley FlowsDefinition: 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


Slope-Valley FlowsSlope 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


Slope-Valley FlowsValley 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 FloorWarm Cold

Example of Valley Flow in the Morning

Plains


Slope-Valley FlowsTypical 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


Slope-Valley FlowsTypical 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


Slope-Valley FlowsTypical Diurnal Cycle on the Plains:

From Toth and Johnson (1985)C

ontin

enta

l Div

ide

Con

tinen

tal D

ivid

e

GreatPlains

GreatPlains

A – Sunrise C – Noon

E – Evening G – Midnight

DCZ


Urban Heat Island CirculationsDefinition: 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


Urban Heat Island CirculationsPhysical 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)


Urban Heat Island CirculationsMesoscale 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


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



ReferencesAtkins, 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.

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