Download Miscellaneous Flows

Miscellaneous Flows
AT 540
Scales of Motion
•
Microscale
– Meters, minutes
– Turbulent eddies
•
Mesoscale
– Km’s to 100s of km’s,
10s of minutes to hours
– Thunderstorms, MCSs
•
Synoptic scale
– 100s km’s to 1000s of
km’s, days to week
– High/low pressure
systems
•
Global scale
– Range entire planet,
weeks
– longwaves
[After Orlanski (1975)]
Eddies
• Eddies are generally defined as a small volume of fluid
that behaves differently from the larger flow from which it
exists
• Formed by mechanical disturbance or convection
• For example, any time strong shear exists, eddies will
develop – near the surface, around buildings, etc.
Sea and Land Breezes
•
Sea and land breezes are
– Mesoscale coastal winds
– Thermal circulations driven by
differential heating/cooling of
adjacent land and water surfaces
– Most prevalent when/where solar
heating is strong
•
Sea breeze development
– Land is heated creating “bulging”
pressure surfaces
– Heated column produces “H”
aloft over land
– Air aloft flows outward from land
to ocean
– Upper flow creates surface “H”
over water and “L” over land
– Surface flow responds with flow
toward land at low levels
– Large scale ascent over land
destabilizes column - enhances
cloud development thunderstorms
•
Sea breezes
– Cool coastal communities
– Bring more humid air
• Haze
• Fog
– Often produce summer
thunderstorms inland from the
coast
Sea and Land Breezes - II
•
Converging Gulf of
Mexico and Atlantic sea
breezes produce uplift
and thunderstorm
development in Florida
– Florida has largest
number of days (90)
per year in the U.S.
with thunderstorms
•
Land breezes form at
night due to stronger
radiative cooling of the
land surface leading to
sinking and offshore flow
of this cooler air mass
with return flow aloft
Sea and Land Breezes - III
Land Breeze
Sea Breeze
Lake Effect Snows
• Mesoscale snow storms that form on the
downwind side of a lake
• Common in the late fall and early winter near the
Great Lakes
• Relatively warm lake water is a source of
sensible and latent heat to the cP air
• This destabilizes the air mass and can result in
enhanced cloud and precipitation development
• Enhanced convergence over land and longer
fetch over water can increase potential for heavy
snowfall
Lake Effect Snows
Lake Effect Snows
• Oswego, NY, which is south of Lake Ontario on the
eastern end, received significant lake effect snowfall in
January 1966
• 5-day snowfall total of 101 inches!
Mountain/Valley
winds
•
•
•
•
•
Sunlight heats mountain
slopes during the day and
they cool by radiation at night
Air in contact with surface is
heated/cooled in response
A difference in air density is
produced between air next to
the mountainside and air at
the same altitude away from
the mountain
Density difference produces
upslope (day) or downslope
(night) flow
Daily upslope/downslope wind
cycle is strongest in clear
summer weather when
prevailing winds are light
Consequences of Mountain/Valley winds
• Upslope flow during the day leads to formation of clouds
and precipitation along mountain ranges
• Upslope flow along the Front Range transports pollutants
from the urban corridor into the high country
Downslope Winds
• Chinook – lee trough event
– Also called a föhn
– Warm, dry downslope winds
– Primarily a winter event
• Bora – lee cyclogenesis event
– Cold downslope winds
– Can occur in fall, winter or spring
• Santa Ana – southern CA
– Warm, dry E/NE wind – adiabatic compression
– fires
Chinook Downslope Winds
•
•
•
Main source of heating
is compression during
downslope flow
Latent heat release
from condensation
during upwind ascent
also contributes
Removal of
precipitation upwind
results in moist,
adiabatic ascent and
dry adiabatic ascent
Chinook Synoptic Conditions
• Strong surface pressure gradient (<20 mb) with
high pressure in the west
• WNW flow in 700-300 mb layer with mountaintop speeds exceeding 10-15 mph
• NVA in the midtroposphere
• Jet axis N of surface high wind area
• Stable layer near mountain-top level upstream
• Absence of a “cold pool” along Front Range
Bora Synoptic Conditions
• Strong surface pressure gradient (<20 mb)
with high pressure in the west
• WNW to N flow in 700-300 mb layer with
mountain-top speeds exceeding 10-15
mph
• Strong CAA from surface to 500 mb
• NVA in the midtroposphere
• Jet axis S of surface high wind area
Santa Ana Fires
October 28, 2003 11Z
Surface Streamlines
Weakening Santa Ana winds
October 28, 2003 2145Z-2230Z
Visible Satellite Loop
Southern California fires
Topographic Blocking of Cold Air
• When cold air masses impinge on mountain barriers,
their forward motion will be impeded unless they have
sufficient kinetic energy to go over the barrier
• If the flow is blocked, it will decrease in speed and move
toward lower pressure
• The Froude number can be used as a measure of
whether blocking will occur.
kinetic energy
2
F ∝
potential energy
• Small Froude numbers (i.e., <1) indicate at least partial
blocking; thus, weaker winds and higher topography
and/or colder air masses will be more likely blocked
Backdoor Cold Front
• Backdoor cold front is one that approaches from the east
or northeast – atypical movement of colder air
• Topographic cold air blocking or damming is the cause
• This is a common occurrence east of the Rockies and
Appalachians during the cool season
Backdoor Cold Front
Example
Split Front
• A split front (or cold front aloft) is a leading edge of low θw
air aloft that advances ahead of the surface cold front
• This can occur when topography blocks cold air from
advancing at the surface
• Can have pronounced rainbands well ahead of the
surface front due to the instability created by cold air aloft
w
Cyclonic Upslope Storms
• Strong, persistent upslope can generate significant
precipitation (e.g., Colorado)
March 2003 Colorado Snowstorm
• Snow fell continuously along the northern Front Range
from the night of March 17th through March 19th
• 32” at CSU campus weather station – largest total (1890)
• 31.8” at Stapleton in Denver – 2nd largest total (1872)
March 2003 Colorado Snowstorm
March 2003 Colorado Snowstorm
March 2003 Colorado Snowstorm
March 2003 Colorado Snowstorm
March 2003 Colorado Snowstorm
March 2003 Colorado Snowstorm
March 2003 Colorado Snowstorm
March 2003 Colorado Snowstorm
Reflectivity
CHILL Radar - Greeley, CO
March 2003 Colorado Snowstorm
Radial Velocity
CHILL Radar - Greeley, CO
March 2003 Colorado Snowstorm
Radial Velocity
CHILL Radar - Greeley, CO
Polar Lows
• Polar lows are cyclones that occur poleward of the polar
front
• Their diameters generally range from 500 to 1000 km –
smaller than extratropical cyclones
• They typically form in the winter along a cold front (arctic
front) where cold air from ice-covered surfaces moves into
an area of warmer air that is resting over relatively warm
water – baroclinic characteristic
• Polar lows also gain energy from sensible and latent
transfer from the water – hurricane characteristic
• They may also show other hurricane characteristics:
central “eye”, strong winds, similar horizontal size, and
dissipate over land
• Can be enhanced by upper-level divergence
Polar Lows
mature
developing
Polar Low
North Atlantic
26-27 February 1987
decaying
Monsoon
•
Monsoon winds are
– Seasonal
– Common in eastern and
southern Asia
– Similar to huge land/sea
breeze systems
•
During winter strong cooling
produces a shallow high
pressure area over Siberia
– Subsidence, clockwise
circulation and flow out from
the high provide fair weather
for southern and eastern Asia
•
During summer, air over the
continent heats and rises,
drawing moist air in from the
oceans
– Convergence and topography
produce lifting and heavy rain
formation
Cherrapunji received 30 feet of rain in July 1861!
Mexican Monsoon
• The Mexican Monsoon is a reversal of the winds over
the Southwestern U.S.
• Onset is around July 15 and persists for a couple of
months – significant increase in precipitation in NW
Mexico
From Douglas et al. (1993)