Download Mid-latitude Cyclones and Weather Forecasting

Mid-latitude Cyclones and
Weather Forecasting
AT351
Lab 9
March 26, 2008
Long Waves vs Short Waves
 Uneven heating causes troughs and ridges to
form around the globe
 An almost constant ring of trough/ridge patterns
exist, called longwaves
 Waves usually move west-east, but can
sometimes appear to move westward
 Called retrograde motion
 Within these longwaves,
longwaves, disturbances exist
called shortwaves
 Shortwaves deepen in longwave troughs
 Shortwaves weaken in longwave ridges
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Long Waves
Short Waves
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Mid-latitude (Extratropical) Cyclone
 A cyclone (area of low pressure) in the middle
(35°-70°) latitudes
 Important for global heat transport
 Help to redistribute energy between the tropics and
the poles
 Often associated with significant weather events
 Described by the Polar Front Theory
 Form on boundaries between warm and cold air
 Polar front is the boundary between polar cell and
Ferrel cell
 Cold Polar air meeting warm tropical air
Features of a Mid-latitude Cyclone
 Deep low pressure area with attached cold and warm
fronts
 Often an occlusion forms, the triple point lending to the
formation of severe weather
 Precipitation associated with the cold and warm fronts
organizes in typical “comma cloud”
cloud” structure
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Stages in Cyclone Development
Polar Front Theory
 Initially, there is a stationary front that acts as
the boundary separating cold, continental polar
air from warm, maritime tropic air
 Winds blow parallel to this front on either side
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Cyclogenesis
 A wave forms on the front due to a shortwave
disturbance
 The front develops a "kink" where the wave is
developing
 Precipitation will begin to develop along the front
Strengthening
 The cyclonic circulation around the low becomes more
defined
 The low pressure intensifies
 The cold front and warm front become more organized
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Mature Cyclone
 The cold front catches up with the warm front and an
occlusion forms
 The cyclone is at its strongest at this point
 Severe weather often develops near the “triple point”
point”
Dissipation
 The occlusion grows with time
 Eventually, the occlusion is so great that the supply of
warm, moist air is cut off
 When this happens, the system starts to dissipate
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Interaction with Upper Levels
 Previous model for cyclone development only includes
surface characteristics – but what happens higher up
can determine what happens below
 Remember this picture?
 Turns out that divergence aloft can help to remove mass
from a column, hence lowering the surface pressure
even more
Interaction with Upper Levels
 Downstream of an upper level trough, the air
tends to diverge
 If a surface low is located slightly downstream of
an upper level trough, the divergence will be
located above the low and help to intensify it
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Another Example – Jet Streaks
 A jet streak is an area within a jet stream
that has the highest wind speeds
 Typically jet streaks are thought of as
made up of four quadrants – separated by
left and right, entrance and exit
 The right entrance region and the left exit
region both contain divergence (aloft) and
so they promote the development of
surface low pressure systems
Mid-latitude Cyclones :
The Upper Level
 A 500 mb trough moves into place directly above a
surface stationary front
 If a shortwave trough moves into the main flow, the
flow pattern is disturbed
 As the 500 mb trough deepens, the associated upper
level divergence strengthens, helping to intensify the
surface low
 Stronger winds aloft force the upper level trough to move
eastward faster, and eventually it becomes located
above the surface low
 When the surface and upper level low are “stacked”
stacked”,
convergence at both levels starts to “fill”
fill” the low pressure
area, weakening the cyclone
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Weather Forecasting - Qualitative
 Most information for a qualitative forecast can be seen
on a simple weather map
 If you see a low pressure area that has been moving
eastward towards Colorado, what type of weather might
you expect?
 If a cold front is moving southward through Wyoming,
what would you expect the temperature to do?
 If it’
it’s going to be cloudy tomorrow, will it be warmer or
colder than it was today?
 Often the best forecast is persistence: if it’
it’s warm and
sunny today, and it was warm and sunny yesterday, the
odds are pretty good that it will be warm and sunny
tomorrow (unless you know something else)
Weather Forecasting - Quantitative
 In order to predict specific quantities in a
forecast (temperature, humidity, rainfall) we rely
on computer models
 Numerical weather prediction (NWP) uses a
system of equations that describes the behavior
of the atmosphere
 NWP model uses the current state of the
atmosphere as its initial condition and steps
through a small time step, recalculating every
number for each step until the forecast time is
reached
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MOS
 The output from a model can be shown in the
form of Model Output Statistics (MOS)
 MOS is a summary of the predicted condition of
the atmosphere at each forecast time
 MOS is often called “model guidance”
guidance”, because
forecasters will use the generated numbers as a
guide to make their forecast
 MOS isn’
isn’t perfect though, and forecasting takes
some intuition
Downfalls of MOS


There isn’
isn’t just one model that is used for NWP
Multiple models are used that have differences
in resolution and in the equations used and
assumptions made
 The models never agree on everything
 A good forecaster will look at multiple model
predictions and have a feel for which model
performs the best under certain circumstances
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Station Name – Artesia, NM
Date and Time the Model was run
Date and time the forecast is valid
Temperature
Dewpoint
Cloud Cover
Wind Direction
Wind Speed
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So why aren’t forecasts always
right?
 Observations aren’
aren’t good enough!
 A model is only as good as its initial conditions
 Even having an observation for every square meter of the planet would
leave out smaller details…
details…and we don’
don’t have even close to that many
observations
 Computers aren’
aren’t fast enough!
 In order to truly create a perfect forecast, one would have to use the
exact equations on a really, really small spatial scale
 In order to create a model that will create a 12 hour forecast in less than
12 hours, we must approximate certain parts of equations and run the
model on a grid with spacing of multiple kilometers
 Chaos reigns supreme!
 Ever heard of the butterfly effect? It’
It’s more than just a movie
 The smallest disturbance will eventually grow into a large difference –
this limits the range of forecasts to just a few days
 Even if the initial conditions and computing power were perfect, chaos
theory would limit us to a reasonable range of about 2 weeks
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