Final Exam Flashcards
What are the forces involved in the surface wind model?
(be prepared to recognize them in a diagram)
in the Northern Hemisphere
Pressure Gradient Force:
- pushes air parcel from high to low pressure; perpendicular to isobar
Coriolis Force: (not at the equator)
- deflects air to the right in the NH
- CF vector is drawn perpendicular and to the right of the resultant wind in the NH
Turbulent Drag:
- ‘Friction’ vector is drawn in the opposite direction of the resultant wind
- By slowing down the wind, it reduces the strength of the Coriolis Force
What are the forces involved in the surface wind model?
(be prepared to recognize them in a diagram)
in the Southern Hemisphere
Pressure Gradient Force:
- Pushes air parcel from high to low pressure;
perpendicular to isobar
Coriolis Force: (not present at the equator)
- Deflects air to the left in the Southern
Hemisphere
- CF vector is drawn perpendicular and to the
left of the resultant wind in the SH
Turbulent Drag:
- ‘Friction’ vector is drawn in the opposite
direction of the resultant wind
- By slowing down the wind, it reduces the strength of the Coriolis Force
What is gradient wind?
Wind that follows curved isobars/isohypses above the planetary boundary layer
Only occurs if the two forces are unbalanced (curving)
What forces are involved in the gradient wind model?
Pressure Gradient Force
Coriolis Force
Centrifugal Force
How do gradient winds differ from geostrophic winds?
Geostrophic Wind: balance between the PGF and CF
Parallel to straight isobars/isohypses
Gradient Wind: imbalance between the PGF, CF, and the Centrifugal Force
Parallel to curved isobars/isohypses
What do the u and v components of a wind indicate?
magnitude and direction of the wind
u: East (+) -> West (-) wind speed
v: North (+) -> South (-) wind speed
In a Northern Hemisphere upper-level cyclone with gradient winds, which is larger:
the PGF or the CF?
Does this make the wind speed supergeostrophic or subgeostrophic?
PGF is larger than the CF
Subgeostrophic because the wind is slower than geostrophic
In a Northern Hemisphere upper-level anticyclone with gradient winds, which is larger:
PGF or the CF?
Does this make the wind speed supergeostrophic or subgeostrophic?
CF is larger than the PGF
Supergeostrophic because the wind speed is faster than geostrophic
If presented with a diagram of the forces/vectors in a surface wind model, be prepared
to label those forces/vectors
PGF: perpendicular to isobars (H->L)
CF: right of wind (NH) or left (SH)
Friction: opposes wind direction
What are the 8 atmospheric/geographic conditions that promote cyclogenesis in the
mid-latitudes of North America?
- East of Mountains
- East of Trough
- Small Wavelengths
- Highly Amplified Troughs
- Fast Winds
- Front Zones or Baroclinic Regions
- Cold Air moves over Warm Air, Wet Surface
- Further from the Equator
If presented with a tephigram showing the T curve, be prepared to determine whether a particular layer of air is stable, unstable, or conditionally unstable.
(Looking at the slope of the environment lapse rate in that layer compared to the dry adiabatic and moist adiabatic lapse rates in that layer)
stable: ELR < MALR < DALR
unstable: DALR < MALR < ELR
conditionally unstable: MALR < ELR < DALR
What is the difference between the dry and moist adiabatic lapse rates?
Dry: 9.8°C/km
Moist: lower than dry, varies depending on moisture and temperature
Why does the moist adiabatic lapse rate not have a single value?
The moist adiabatic lapse rate depends on the temperature and humidity
More condensation releases more latent heat, altering the cooling rate
What is the cause of the adiabatic lapse rate?
Change in air parcels, expanding and cooling or compressing and warming, as they rise or sink without heat exchange
What does it mean to say that an atmosphere (or layer of the atmosphere) is ‘absolutely
stable’ or ‘absolutely unstable’?
Stable: ELR<MALR
- air parcel cooler than environment -> sinks
Unstable: ELR > DALR
- air parcel warmer than environment -> rises
What does it mean to say that an atmosphere (or layer of the atmosphere) is
‘conditionally stable’?
depends on whether the air parcel is saturated
unsaturated: stable
saturated: unstable
What does it mean to say that a layer of air is being ‘stabilized’ or ‘destabilized’?
stabilized: ELR = lower value (does not support rising air)
destabilized: ELR = higher value (supports rising air)
How does horizontal divergence of a column of air result in stabilization (e.g. when a column of air moves from a rough terrain to a smoother terrain)?
horizontal divergence of a column of air stabilizes because it causes convergence (sinking) above, warming the upper atmosphere and lowering the ELR- making it harder for air to rise
What is a subsidence inversion?
Sinking air causes the upper part of a layer to warm more than the lower part -> inversion -> stability
Why does subsidence of a layer of air in the upper atmosphere result in stabilization?
compression -> top warms more than the bottom -> less vertical mixing -> stability
What kind of dewpoint depression in the upper atmosphere is indicative of subsidence?
large dewpoint depression
- producing a strong subsidence inversion that can trap moisture and pollution in the lower atmosphere
How does horizontal convergence of a column of air result in destabilization (e.g. when a column of air moves from a smooth terrain to a rougher terrain)?
horizontal convergence of a column of air destabilizes because it causes divergence (stretching) above, cooling the upper atmosphere and increasing the ELR, making it easier for air to rise
Does heating/cooling of air near the surface generally result in stabilization or
destabilization?
Why?
At the Surface
Heating: destabilization
Cooling: stabilization
Does heating/cooling of air aloft generally result in stabilization or destabilization?
Why?
Aloft
Heating: stabilization
Cooling: destabilization
What are the basic ways that the atmosphere can be destabilized to produce
thermals/convection?
Air aloft cools due to:
- winds bringing in colder air (cold advection)
- clouds (or air) emitted infrared radiation to space
Surface air warms due to:
- daytime solar heating of the surface
- influx of warm air brought in by wind
- air moving over a warm surface
Why are lifting condensation levels higher from the ground in dry environments?
less moisture: initial dew point is lower, takes longer for a rising air parcel to cool to its dew point, the parcel must rise higher to reach saturation
How does the lifting of an unsaturated layer of air near to a higher altitude result in
destabilization?
Becomes thicker:
- the bottom of the layer cools less than the top of the layer
- steepens the slope of the lapse rate, increasing instability
may convert from stable to conditionally unstable
What are the four basic ways that uplift can be created to produce clouds?
- convection
- lifting along topography
- convergence of air
- lifting along weather fronts
What are the six ‘triggers’ for thunderstorm development?
- random turbulence
- heating of the surface
- local topographic convergence
- large scale orographic lifting
- upper level divergence
- frontal uplift
What are the characteristics of ordinary or ‘air mass’ thunderstorms?
- form in regions with limited vertical wind shear
- often form along shallow zones where surface winds converge
stages: cumulus > mature > dissipating
example: terrain, sea-breeze/lake-breeze fronts, outflow from another storm
Why don’t air mass thunderstorms last long or produce severe weather?
weak shear > updraft and downdraft collide > cuts off energy
What is the difference between a cold-core low and a warm-core low? Be prepared to
recognize or sketch a vertical cross-section of each, in which isobaric layers are
indicated.
cold-core low: strengthens with height (isobars tighter aloft)
warm-core low: weakens with height (isobars tighter at surface)
What is the difference between a cold-core high and a warm-core high? Be prepared to
recognize or sketch a vertical cross-section of each, in which isobaric layers are
indicated.
cold-core high: weakens with height
warm-core high: strengthens with height
How do multi-cell and supercell thunderstorms differ from air mass thunderstorms?
Both last longer and are more organized than air mass
multi-cell:
- multiple updraft/downdraft regions
supercell:
- strong rotation, large CAPE, severe
What is CAPE and why are high values of CAPE often associated with severe weather?
Convective Available Potential Energy
- measures the difference between the parcel of air temperature and the environmental temperature
- warmer air parcel will be buoyant and rise
- leading to the formation of thunderstorms
What conditions will produce large amounts of CAPE?
larger difference between the air parcel temperature and the environmental temperature
- warm, moist surface
- cold air aloft
- high humidity
- strong vertical wind shear
What are the conditions necessary for the genesis of tropical cyclones, and where/when
do they occur?
- Warm ocean waters
- Atmospheric instability
- Coriolis force
- Low vertical wind shear
- Pre-existing disturbance
- High humidity in the Mid-Troposphere
- Adequate time over warm water
- Outflow in the Upper Atmosphere
What are the distinguishing characteristics of the structure of a tropical cyclone?
Eye: calm hurricane center (30-50km wide), air sinks, subsidence warms and dries the air (warm-core low pressure)
Eyewall: ring of cumulonimbus clouds with powerful winds, ascending air 10-20 miles from the center (extreme convection), caused by the strong rotation of air around the cyclone
Rainbands: bands of thunderstorms spiraling out from the eyewall stretching hundreds of kms, gusty winds, tornadoes, localized flooding, convergence & rising of air
In what ways are extratropical cyclones and tropical cyclones similar/dissimilar?
extratropical: warm-core, form over warm water, symmetric
tropical: cold-core, form along fronts, asymmetric
both: rotate cyclonically and produce storms
What are the characteristics of deterministic forecasts? What are its advantages and
disadvantages?
input data > numeric model > single output
advantages:
- high detail – fine spatial and temporal resolution
- physically grounded – based on real-world atmospheric processes
- operationally reliable – widely used by national weather agencies
disadvantages:
- sensitive to errors of initial conditions – small inaccuracies can grow quickly (chaos theory)
- single solution only – doesn’t express uncertainty (missing multiple outcomes)
- expensive – requires powerful computer resources
What is ensemble forecasting and how and why is it done? How are probability
forecasts created using ensemble forecasting?
run the same model multiple times with small tweaks to initial conditions > analyze the spread > use statistics
advantages:
- estimates uncertainty
- better for extremes
- useful of decision-making
- supports longer-range forecasts
disadvantages:
- computationally intensive
- harder to interpret
- lower resolution of local features
- slower to produce
Why is it so important to produce many ‘runs’ of a model (such as a global or regional model) to assess the kind of weather that might occur in the coming days (especially regarding precipitation and storms)?
weather is chaotic and small differences can lead to large variation in results
- improve accuracy
- provides probabilities
- spots dangerous possibilities early
Why is the location, timing, and amount of precipitation so much harder to forecast
than temperature conditions?
- small-scale processes and complex interaction of variables
- highly sensitive to initial conditions
- spatial variability
- convection is chaotic
What are the six different forecasting techniques?
- persistence
- analogue
- trend
- model
- ensemble
- probabilistic
Why is it so important to recognize ‘chaos theory’ in meteorology?
