Transcript AOS 100: Weather and Climate

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
Exam Breakdown:
• Average: 85.73%
• Median: 87.5%
Range
Total
100%
3
95-99%
22
90-94.9%
85-89.9%
52
55
80-84.9%
41
75-79.9%
70-74.9%
18
2
65-69.9%
9
60-64.9%
3
<60%
7
Review of September 8th: Fronts and
Forces
• Besides surface observations, it is also possible
to detect cyclones and fronts in satellite or radar
images
• In order for cyclones to intensify, you must
decrease the surface pressure
• The only way to accomplish this is to remove
mass from the column of air above the cyclone
• This means you need a net divergence of air in
the column to intensify cyclones, and a net
convergence to intensify anticyclones
Review Continued
• First, we need to understand some fundamental
atmospheric forces: the Pressure Gradient Force
(PGF), the Coriolis Force, and Friction
• The PGF will direct air from high pressure
towards low pressure
• A stronger PGF will cause a stronger wind
• However, the Coriolis Force will act to turn that air
to the right in the northern hemisphere, until it
becomes parallel to the isobars
Review
Continued
- 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”
L
L
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• Therefore, geostrophic balance is no
longer in effect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
A Summary
Now Let’s Revisit Divergence …
Upper Levels
Similarly to lower levels, at upper levels of the atmosphere,
there is often a series of high pressures (high heights) and
low pressures (low heights)
Upper Levels
Ridge
Trough
Convergence
Ridge
Divergence
Why Do These Patterns Occur?
• These patterns of convergence and
divergence have to do with vorticity
advection
• If there is positive vorticity advection,
divergence occurs
• If there is negative vorticity advection,
convergence occurs
• Let’s explain vorticity …
Vorticity
Vorticity is simply a measure
of how much the air rotates
on a horizontal surface
Positive vorticity is a
counterclockwise (i.e.
cyclonic) rotation
Negative vorticity is a
clockwise (i.e. anticyclonic)
rotation
Therefore, troughs contain
positive vorticity, and ridges
contain negative vorticity
Vorticity
Vorticity is simply a measure
of how much the air rotates
on a horizontal surface
Positive vorticity is a
counterclockwise (i.e.
cyclonic) rotation
Negative vorticity is a
clockwise (i.e. anticyclonic)
rotation
Therefore, troughs contain
positive vorticity, and ridges
contain negative vorticity
Trough
Ridge
Let’s Revisit …
Vorticity < 0
Vorticity < 0
Vorticity > 0
Convergence
Divergence
Let’s Revisit …
Vorticity < 0
Vorticity < 0
Vorticity > 0
Negative Vorticity Advection
Positive Vorticity Advection
Diagnosing Vorticity Advection
• To determine vorticity advection, first find
the locations of maximum (positive)
vorticity and minimum (negative) vorticity
• Then, determine what direction the wind
flow is
• Areas of negative vorticity advection (NVA)
will be just downstream of vorticity minima,
and areas of positive vorticity advection
(PVA) will be just downstream of vorticity
maxima
The Connection
Based on what we learned earlier, areas of convergence
at upper levels favor surface anticyclones, and areas of
divergence at upper levels favor surface cyclones
The Connection
Given what we learned earlier, areas of convergence at
upper levels favor surface anticyclones, and areas of
divergence at upper levels favor surface cyclones
A 3-D Look
The Connection
As the locations of upper level troughs and ridges
change, we might expect the positions of surface
cyclones and anticyclones to change
The Big Picture
Cyclone Growth And Decay
• Based on what we’ve learned, the position
of the surface cyclone in relation to the
upper level structure is key to
development
• A cyclone will grow if it is below an area of
PVA, and weaken if below an area of NVA
or neutral vorticity advection
• Commonly, a cyclone will intensify until it
becomes situated in an unfavorable
location in relation to the upper levels
An
Example:
Time 1
Above: Upper Level
Height and Wind Speed
Right: Surface Pressure
An
Example:
Time 1
Above: Upper Level
Height and Wind Speed
Right: Surface Pressure
Time 2
Above: Upper Level
Height and Wind Speed
Right: Surface Pressure
Time 2
Above: Upper Level
Height and Wind Speed
Right: Surface Pressure
Time 3
Above: Upper Level
Height and Wind Speed
Right: Surface Pressure
Time 3
Above: Upper Level
Height and Wind Speed
Right: Surface Pressure
Summary of Event
• At time 1, the upper levels and lower
levels are perfectly set up for the surface
cyclone to intensify
• At time 2, the upper trough is almost
above the surface cyclone, so the
intensification slows
• By time 3, the upper trough is exactly over
the surface cyclone, so the intensification
has halted