Transcript Document

Thermally-Driven Circulations
What two countries
were the Apollo
astronauts viewing?
Do you see any
intriguing cloud
formations?
Mesoscale
M. D. Eastin
Thermally-Driven Circulations
Land-Sea Breezes
Slope-Valley Flows
Urban Heat Island Circulations
Mesoscale
M. 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
Mesoscale
M. 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)
Mesoscale
M. 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
Mesoscale
M. 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)
Mesoscale
M. 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
Mesoscale
M. 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
Mesoscale
Infrared satellite image of Land Breeze over Japan
M. 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
Mesoscale
M. 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
Mesoscale
M. 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
Mesoscale
M. 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)
Example of Slope Flow in the Morning
Cool
Warm
Mesoscale
M. 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)
Example of Valley Flow in the Morning
Warm
Plains
Mesoscale
M. 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)
A
B
C
D
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)
Mesoscale
From Defant (1951)
M. 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)
E
F
G
H
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)
Mesoscale
From Defant (1951)
M. D. Eastin
Slope-Valley Flows
Typical Diurnal Cycle on the Plains:
E – Evening
G – Midnight
Mesoscale
Great
Plains
Continental Divide
C – Noon
Continental Divide
A – Sunrise
DCZ
Great
Plains
From Toth and
Johnson (1985)
M. 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
Mesoscale
M. 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)
Mesoscale
M. 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
“Extra”
Precipitation
• Numerical simulations suggest a ~5-10% increase
in precipitation downwind of urban regions
Mesoscale
M. D. Eastin
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
Mesoscale
M. 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.
Mesoscale
M. D. Eastin