Transcript October 8th: Cyclones and Forces

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Exams & Homeworks
Review of September 6th: Fronts
Continued
• Cold fronts are often sharper than warm fronts,
partially because it is easier for more dense air
to replace less dense air
• This is one reason that cold fronts tend to move
faster than warm fronts
• Sometimes a cold front “catches up” to the warm
front, forming an occluded front where they meet
• Oftentimes there is little change in temperature
from one side to another of the occluded front
• Stationary fronts do not move very much,
although they can often become warm or cold
fronts
Review Continued
• Extratropical cyclones generally first develop
along an intersection of two airmasses (like a
stationary front)
• As the cyclone develops, warm and cold fronts
form, and the cold front slowly approaches the
warm front
• Once an occluded from forms, the cyclone is
normally at its most intense, and will begin to
weaken afterward
• This is because it is no longer near a region of a
horizontal temperature gradient (which is why it
developed in the first place)
Review Continued
• Finding fronts on weather maps is very useful
• It is often useful to first find the area of lowest
pressure, since fronts typically originate from it
• In the case of most fronts (except occluded
fronts), there should be a large temperature
change across them
• All fronts should also have a fairly sharp wind
shift from one side to another
• Other factors, like precipitation, cloud cover, and
moisture gradients can indicate a front
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Fronts in Other Images
• Besides looking at surface observations,
fronts can often be seen in other ways
• If precipitation is associated with the front,
they can be seen on radar images
• The clouds associated with fronts can be
seen on satellite images also
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How Do Low Pressures Form?
• Keep in mind that pressure is basically just
a measure of the mass of air above the
surface (or some other level)
• So a surface pressure of 990 mb means
that the weight of the atmosphere above
that location is less than a spot with a
pressure of 1000 mb
• This means that to decrease surface
pressure, the mass of the atmosphere
above that location needs to decrease
Low Pressure Formation Continued
• In order to decrease the mass of the column of air
above the surface (i.e. to form a low pressure
system), you need a net divergence of air in that
column
• Conversely, to strengthen a high pressure system,
you need a net convergence of air in that column
• This is where features in the upper levels of the
atmosphere become important
• But first, we need an understanding of atmospheric
forces
Why Are Forces Important?
• When we speak of “forces,” we’re really
describing why the air in the atmosphere
moves the way it does
• This describes both how the air moves
horizontally (which direction it moves), and
why the air moves vertically
• For example, precipitation occurs because
something forces air upward until it
condenses
Horizontal Movement
• When a studying the horizontal movement
of air, three primary forces are often
discussed:
- The Pressure-gradient force (or PGF)
- The Coriolis force
- The Frictional force
The Pressure-Gradient Force
(PGF)
• Most simply, the word “gradient” just
means a “change” in the quantity being
observed (in this case, pressure)
• When the gradient is large, that means
that the quantity changes a lot over a
small distance
• Therefore, if we say that there is a “strong
pressure gradient” we are saying that
pressure changes quickly over a small
distance
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
Strong
Pressure
Gradient
Weak Pressure Gradient
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
The PGF Continued
• Remember that pressure effectively measures
how much air (or the force of that air) is above
you
• So a pressure difference in the horizontal means
that the force of the air above the two locations
is different
• This is not a balanced state
• Therefore, the atmosphere will attempt to correct
for this state by allowing air to flow from high
pressure to low pressure
• A larger pressure gradient will result in a
stronger wind
A Depiction
From: http://physics.uwstout.edu/WX/u6/U6_05.gif
The Coriolis Force
• The Coriolis force is a “fake” force which is a
result of the fact that the Earth rotates
• The Coriolis force causes moving objects to turn
to their right in the northern hemisphere and to
the left in the southern hemisphere
• However, if an object is at rest, the Coriolis force
does not apply
• (The Coriolis force is due to the fact that even
though the angular velocity of the Earth is the
same at all points, the tangential velocity is not.
The Coriolis force can be thought as preserving
an objects tangential velocity)
- When the PGF
and Coriolis force
are balanced, the
atmosphere is
said to be in
“geostrophic
balance”
- The resultant
wind is called the
“geostrophic
wind”
The Geostrophic Wind
• The geostrophic wind always blows
parallel to the isobars (lines of constant
pressure)
• A stronger PGF (when the isobars are
closer, i.e. the pressure gradient is
stronger) results in a stronger geostrophic
wind
What direction would you
expect the geostrophic
wind to blow in?
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
Almost … but why
the difference?
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
The Friction Force
• Close to the surface, geostrophic balance is not
a very good approximation
• This is because friction is quite strong near the
Earth’s surface
• The Earth’s surface is very rough (buildings,
trees, mountains, etc.), which induces friction
• Therefore, as you get farther away from the
Earth’s surface, the friction force decreases
• This means that geostrophic balance becomes
more realistic as you move away from the
Earth’s surface
• Friction always acts to oppose the wind (with a
strength proportional to the strength of the wind)
Friction Continued
• However, if friction acts to slow the wind,
then the Coriolis Force will weaken
• At the same time, the PGF remains the
same strength
• Therefore, geostrophic balance is no
longer in effect
• This causes the wind to blow slightly
across isobars, towards low pressure
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
The New Force Balance
From:
www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Ene
rgetics/Spiral_Winds/Spiral_Winds.html
Constant Pressure vs. Constant
Height Maps
• So far we’ve looked at Sea Level Pressure maps
(so pressure varies while the height is constant
everywhere - 0 meters)
• However, meteorologists often look at constant
pressure maps (so the height changes, rather
than the pressure)
• As we’ll learn more about later, you can think of
“high” heights as being analogous to high
pressures, and “low” heights as being analogous
to low pressures
• Similarly, the geostrophic wind will blow parallel
to lines of constant height, with lower heights to
the left of the direction of the wind
Heights and winds at 200 mb
Notice how much closer the winds are to geostrophic
balance at this level, compared with the surface
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml