Air flow in a mountainous environment
• When wind flows over a hill or ridge,
it is set into oscillation.
Mountain wave (also called lee waves or
standing waves downwind)
Analagous to ripples in a stream downstream
of a rock, except that winds are often stronger
downwind from a mountain
What conditions cause dangerous mountain
waves for aviation?
1. Air flow roughly perpendicular to
mountain range
2. Increasing wind velocity with altitude
3. Strong wind speeds at the mountain top
4. A stable air mass layer aloft or inversion below
LIDAR definition: http://www.ghcc.msfc.nasa.gov/sparcle/sparcle_tutorial.html
1. Unstable Atmosphere
Mixing is facilitated
Reduced threat
of breaking
mountain waves
2. Stable Atmosphere
Suppression of
vertical motion
As a result, waves
cannot break
3. Stable Layer Aloft
In updraft portions, air cools adiabatically
Lenticular clouds form in updrafts at the
lifting condensation level
These clouds are stationary as the wind
blows through them!
Froude Number
Fr = /2W,
where  is the
wavelength of the
mountain wave
and W is the
width of a hill
The Chinook or Foehn
Upstream:
Cooler and moist
Downstream:
Warmer and dry
2
1
Cyclonic
wind shear
Wave-like
kink in front
Stationary
Front
3
Overunning
Warm air displacement
Potential to kinetic energy
(cold air falls, warm rises)
4
Convergence
Latent heat energy
Warm sector
shrinks
Most intense
stage
5
6
Supply of warm
air far from centre
Triple point
A secondary low often
forms here
Warm
DIMINISHES IN INTENSITY
(the low ‘fills’)
Convergence
aloft promotes
the surface
high
Divergence
aloft promotes
the surface
low
Surface
divergence
Surface
convergence
MIGRATE THROUGH
THE LONGWAVE TROUGHS
1
Condensation may
release even more
heat energy for the
storm
2
Differential temperature
advection intensifies the
wave
No temp advection
3
Less upper level
divergence
Thunderstorms
Conditions required:
1.
Conditional instability
2.
Trigger Mechanism
(eg. front, sea-breeze front, mountains,
localized zones of excess surface heating,
dry ground)
1.
Heating within boundary layer
Air trapped here due to stable layer aloft
increasing heat/moisture within BL
2.
External trigger mechanism forces parcel to
rise to the lifted condensation level (LCL)
Clouds form and temperature follows MALR
3.
Parcel may reach level of free convection
(LFC). Parcel accelerates under own buoyancy.
Warmer than surroundings - explosive updrafts
4.
Saturated parcel will continue to rise to LOC
CAPE
Convective available potential energy (J/kg)
The Severe Storm Environment
1. High surface dew point
2. Cold air aloft (increases conditional instability)
3. Statically-stable layer capping the boundary
layer
4. Strong winds aloft (aids tornado development)
5. Wind shear in low levels (allows for
long-lasting storms)
6. Dry air at mid-levels (increases downdraft
velocities)
See: http://www.cira.colostate.edu/ramm/newgoes/vis15min.htm
Downbursts
Strong downdrafts of cold air during thunderstorms
May be quite damaging
Linked to presence
of dry layer
Evaporative cooling
of rainfall lowers
temperature,
facilitating
downdraft
http://severewx.atmos.uiuc.edu/21/online.21.1.html
See: http://rsd.gsfc.nasa.gov/rsd/movies/preview.html
Tornado Development
1. Pre-storm conditions:
Horizontal shaft of rotating air at altitude of
wind shift (generally S winds near surface
and W winds aloft)
2. If capping is breached and violent convection
occurs, the rotating column is tilted toward
the vertical
What is a ‘supercell’ ?
Defined by mid-level rotation (mesocyclone)
Highest vorticity near updraft core
Supercells form under the following conditions:
High CAPE, capping layer, cold air aloft, large
wind shear
http://www-das.uwyo.edu
Tornadogenesis
1. Mesocyclone 5-20 km wide develops
2. Vortex stretching: Lower portion of
mesocyclone narrows in strong updrafts
3. Wind speed increases here due to conservation
of angular momentum
4. Narrow funnel develops: visible due to adiabatic
cooling associated with pressure droppage
The Fujita Scale
F-0: Light damage. Winds up to 116 km/h
F-1: Moderate damage. Winds 116 to 180 km/h
F-2: Considerable damage. Winds 180 to 253 km/h
F-3: Severe damage. Winds 253 to 332 km/h
F-4: Devastating damage. Winds 332 to 418 km/h
F-5: Incredible damage. Winds above 418 km/h
F-0 and F-1 tornadoes are considered "weak"
F-2 and F-3 are "strong"
F-4 and F-5 are "violent"
Tornado Development
1. Pre-storm conditions:
Horizontal shaft of rotating air at altitude of
wind shift (generally S winds near surface
and W winds aloft)
2. If capping is breached and violent convection
occurs, the rotating column is tilted toward
the vertical
What is a ‘supercell’ ?
Defined by mid-level rotation (mesocyclone)
Highest vorticity near updraft core
Supercells form under the following conditions:
High CAPE, capping layer, cold air aloft, large
wind shear
See: http://www.weatherpix.com/educate.htm
http://www-das.uwyo.edu
Tornadogenesis
1. Mesocyclone 5-20 km wide develops
2. Vortex stretching: Lower portion of
mesocyclone narrows in strong updrafts
3. Wind speed increases here due to conservation
of angular momentum
4. Narrow funnel develops: visible due to adiabatic
cooling associated with pressure droppage
The Fujita Scale
F-0: Light damage. Winds up to 116 km/h
F-1: Moderate damage. Winds 116 to 180 km/h
F-2: Considerable damage. Winds 180 to 253 km/h
F-3: Severe damage. Winds 253 to 332 km/h
F-4: Devastating damage. Winds 332 to 418 km/h
F-5: Incredible damage. Winds above 418 km/h
F-0 and F-1 tornadoes are considered "weak"
F-2 and F-3 are "strong"
F-4 and F-5 are "violent"
Aurora, Nebraska
June 22, 2003
Largest
Hailstone
Ever
Measured
18 cm across
48 cm circumference
Hailstone Formation
Layered structure due to WET GROWTH and
DRY GROWTH
Wet Growth
•
Temperature too warm for rapid freezing of
raindrops onto frozen hailstone embryo
•
Air can escape before freezing occurs
Results in a clear layer
Dry Growth
•Cold, temperatures
•Water droplets freeze very quickly on contact
with hailstone embryo
•Air cannot escape, so bubbles are frozen into
matrix
•Opaque layer
•Most rapid hailstone formation occurs with
temperatures of –25°C to –10°C
•Hailstones may be created during a single
pass through an updraft
•The largest hailstones are produced in supercell
storms, due to the strength of the updraft
What updrafts are required to hold a
Updraft
speed
required to suspend hailstone
hailstone
up?

Hail
Diameter
Updraft Speed
Terminal Velocity
-1
56 mi hr
-1
3 cm
25 m s
8 cm
55 m s -1
125 mi hr -1
10 cm
83 m s -1
185 mi hr -1
Lightning
Source of lightning: the cumulonimbus cloud
Collisions between supercooled cloud particles and
graupel (and hail) causes cloud to become charged
Most of the base of the cumulonimbus cloud
becomes negatively charged – the rest becomes
positively charged (positive electric dipole)
Net transfer of positive ions from warmer object to
colder object (hailstone gets negatively charged &
fall toward bottom - ice crystals get + charge)
Many theories exist: open area of research
Annual Lightning Strikes per km2
Four types
of cloudground
lightning
Most
common
•Intracloud Discharges
•Cloud to Ground Discharges
- death and destruction of property
- disruption of power and communication
- ignition of forest fires
- Lightning is an excellent source of soil
nitrogen!
Cloud-ground lightning
90% induced by negatively charged leaders
10% induced by positively charged leaders
Sometimes, there are ground to cloud leaders
Negative cloud-ground lightning
Leaders branch toward the ground at about
200 km/s, with a current of 100-1000 Amperes
The return stroke produces the bright flash
•Potential difference between lower portion of
negatively-charged leader and ground
~10,000,000+ V
•As the leader nears the ground, the electric
potential breaks the threshold breakdown
strength of air
•An upward-moving discharge is emitted from
the Earth to meet with the leader
The return stroke lasts about 100 microseconds,
and carries a charge of 30 kiloAmperes
Produces the main flash
The temperature along the channel heats to
30,000+ K, creating an expanding high pressure
channel
Produces thunder shockwaves
HURRICANE DEVELOPMENT
What conditions are required?
•Water temperatures > 26.5°C
•Convergence of surface winds
•Upper air divergence
•Organized mass of thunderstorms
•Coriolis force (none at equator)
Spring: Too cold,
ITCZ too far south
The First South Atlantic Hurricane
Tropical Wave
What causes surface winds to converge?
•
Low pressure over ITCZ
•
East side of tropical wave
(in North Atlantic, these waves are
often remnants of convective storms
from continental Africa)
•
Front from middle latitudes
Trade wind inversions can prevent storm development
even if surface conditions are ideal !
Upper level conditions:
1. Low shear
(shear disrupts convection and disperses
heat and moisture; El Niño causes shear over
North Atlantic, enhances activity over Pacific)
2. Upper level divergence
Strong, upper level divergence
Hurricane Mitch
Organized Convection Theory
1. Thunderstorms form over tropical wave,
near ITCZ, or at remnant of midlatitude
front (latent heat confined to limited area)
2. If air is much cooler aloft, strong
convection occurs
3. Upper air rapidly warms due to
condensation
4. Air pressure rises aloft, enhancing
divergence
5. Surface pressure drops
6. Surface air spins counter-clockwise and
moves more quickly near the centre
(conservation of angular momentum)
7. Rough seas enhance convection due to
friction and increase surface area
Positive feedback; strength only limited by latent and
sensible heat
SW
Storm Position
NE
Storm Position
Source: IPCC
Source: IPCC
1. Reduced Biodiversity
(rapid change)
2. Sea level rise and coastal flooding
(melting ice and thermal expansion)
3. Expansion of tropical disease range
4. Soil Moisture Decreases and
Desertification ?
5. Increased frequency of heat illness
(problem for the elderly)
6. Increased frequency of severe events?
7. Engineering problem of thermokarst
(transportation and housing)
8. Affect on outdoor winter recreation
and winter tourism
1. Increasing NPP?
2. Increased food production?: CO2
fertilization, range & growing season
(depends on soil moisture/depth/nutrients)
3. Increased water-use efficiency
4. Increased nutrient-use efficiency?
5. High latitude warming
(positive and negative)
Source: IPCC
Source: IPCC
Ice core data
Temperature,
CO2 and CH4
are all in
phase
Are the gas
concentrations
a cause or an
effect of
warming or
both ?
Source: IPCC
HADCM3 Model Prediction
Global Circulation Model Projection:
Non-uniform spatial distribution of
global surface temperature increase
Free Air Carbon Dioxide Enrichment (FACE)
FACE Results:
NPP increases
(eg. 40% in cotton; 25%
for Sweetgum for 550
ppm vs. 370 ppm)
Carbon sink increase limited for forests: Increase in wood
production is short-lived; C goes mainly to fine roots and
leaves; affected by soil fertility
No effect on LAI
Stomatal conductance decreases (increased water-use
efficiency)
Lower leaf nitrogen concentration: need less or have less?
Carbon Sinks
But what are we doing to our sinks ?
http://news.bbc.co.uk/2/hi/americas/3609887.stm
Annual Atmospheric Increase
3.3(±0.2) PgC (billion metric tonnes)
Why ?
Emissions from fossil fuels
Changes in land use
Oceanic uptake
Missing carbon sink
+5.5(±0.5)
+1.6(±0.7)
- 2.0(±0.8)
- 1.8(±1.2)
Possible source: Underestimation of terrestrial uptake
Mid-latitude forest regrowth ?
Will the missing sink last ?
Source: Woods Hole Observatory
Source: IPCC
Meanwhile, we are detecting
stratospheric cooling !
Why ?
Ozone depletion
Tropospheric [CO2] increases
Interannual climatic variability at
the global scale
Caused by changing atmospheric and
oceanic circulation in the tropical
Pacific Ocean
Top La Nina December 1998; Middle Normal December
1993; Bottom El Nino Dec 1997
See http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml
Q. Is there a relationship between the
frequency and/or strength of El Nino
Southern Oscillation and climate
change ?
A. We don’t know.
However, effects might be exacerbated in a
warmer climate (higher sea levels would
enhance flooding, precipitation heavier
during enhancement, evaporation greater
during drought phases)
El Nino-related flooding in N. California
Mainly due to shifting winds