Download Lesson 04

SO254 – Upper-air charts
Upper-air circulation
tied to the 3-cell model
• We saw in Lesson 2 that
differences in insolation (more
in the tropics, less in the polar
regions), combined with the
Earth’s rotation, drives complex
circulation patterns
• Three circulation cells develop:
Hadley, Farrell, and Polar
• Figure at the right shows
average surface wind patterns
• What do the upper-level wind
patterns look like?
Stull 2016
Upper-air circulation
tied to the 3-cell model
• In the upper troposphere, the three
circulation cells are also seen:
• Hadley cell: tropical easterly winds,
noted by HHH
• Hadley/Farrell interaction: midlatitude westerly winds
• Subtropical jet (noted by HH)
• Farrell/Polar cell interaction: Polar jet
• The polar jet (and really, the
subtropical jet and the tropical
easterly winds, despite the lousy
figure) is wavy
• What does that waviness mean for the
weather at a particular place?
• How can we identify the waviness?
Stull 2016
Upper-air circulation tied to radiative
imbalance
• Incoming radiative flux at top of the
atmosphere (Einsol) varies by latitude
• Radiative flux that makes it into the
atmosphere (not reflected) also varies by
latitude (Ein)
• Outward radiative flux (Eout, dashed
line) also varies by latitude, but not as
much as Ein or Einsol
• Difference between outgoing and
incoming (Enet) is positive in the tropics
and negative poleward of about 33°N
and 33 ° S
• This difference in radiation drives global
circulation
• A difference in radiation between tropics
and poles, along with the rotation of the
Earth, is basically the reason why we have
weather
Stull 2016
Upper-air analysis
• You learned in the last lesson that the
thickness of a layer of air depends on several
things:
• The pressure of the top and bottom of the layer
• The mean temperature of the layer
• This relationship between thickness (Z2-Z1),
mean temperature, and pressure is called the
hypsometric equation
𝑅𝑇
𝑝1
𝑍2 − 𝑍1 =
ln
𝑔0
𝑝2
• Quick example: calculate the thickness of the
air layer between 1000 mb and 500 mb if the
mean temperature of that layer is freezing
(273.15 K)
Stull 2016
More on thickness
• One type of upper-air weather
chart is one that shows horizontal
variation in thickness
• See example at right
• A thickness chart is useful because
it can combine information from
two pressure levels (in the case of
the figure at right, 1000 mb and 500
mb), as well as the temperature
between those levels
• But, thickness charts have their
limitations, because thicknesses
have limitations
Which of these profiles, all of which
have mean temperatures of 273.15K,
might support snow?
500 mb
700 mb
GFS model forecast of precip type and intensity (color), sea level pressure (solid black lines), and 1000-500
mb thickness (dashed lines), valid at 18Z (1 pm EST) 30 Jan 2017. Source: pivotalweather.com
850 mb
1000 mb
0°C
0°C
0°C
0°C
0°C
Developing a concept of the upper-level chart
Height of the 500 mb surface (in meters):
Elevation above sea level of the 500 mb
surface
If there are no horizontal temperature
variations, 500 mb surface will be mostly flat
**A FLAT SURFACE IS UNREALISTIC**
(Where on the Earth might a flat 500-mb
surface actually be realistic?)
Developing a concept of the upper-level chart
• How do we know the height of the 500-mb pressure level?
• Radiosondes!!
https://www.youtube.com/watch?v=AoUxq4mTv5M
What is in the instrument?
Source: Plymouth State Univ
Value of radiosondes to 24-h weather forecasts (and
compared to other data types).
Source: NASA
Where are radiosondes typically launched?
Source: ECMWF
https://gmao.gsfc.nasa.gov/forecasts/systems/fp/obs_impact/
Developing a concept of the upper-level chart
• In the real atmosphere, horizontal
temperature variability does exist.
• In the figure at right, “south” is
closer to the equator and “north” is
closer to the pole
• Because warm air occupies more space,
the 500 mb surface will be:
- higher in warm air
- lower in cold air
• The hypsometric equation can also be
used to show this relationship:
• Let the pressure at the ground be
1000 mb everywhere. Then the
distance between the ground and
the 500-mb surface depends on the
temperature of the layer
Developing a concept of the upper-level chart
• When there are horizontal variations in
temperature, the constant-pressure surface will
slope (not be flat)
• The degree of the slope depends on temperature
of the air column below it
• Resulting chart plots heights of pressure
surface
• Called a “constant pressure chart”
• The charts are more commonly referred to by the
pressure level you are showing
•
Ie, the “500-mb pressure chart” or the “500-mb chart”
• On a two-dimensional chart (like shown at
the bottom of the figure at right), the
greater the slope of the pressure surface,
the closer the lines are
• Closer lines indicate tighter height gradients
• We will see later in the course that the height
gradient is one of the main reasons why air
moves
Developing a concept of the upper-level chart
• The constant pressure surface
• Is three-dimensional
• Its shape (generally) depends on the
temperature of the air below it
• “Ridges”
• indicate regions of higher heights
• should correspond to regions of warmer air
• “Troughs”
• indicate regions of lower heights
• should correspond to regions of colder air
In the figure at right, look at how the 3-d pressure
surface (colored) shows up on a 2-d chart. Note how
the heights at 500-mb look on the chart
• The base state would have flat, east-west lines, with no
curves. That would mean isothermal conditions (constant
temperatures)
• A chart with ridges and troughs implies temperature
variations
• Ridges are where heights are higher relative to nearby values
• Troughs are where heights are lower relative to nearby heights
Let’s look at real examples of pressure surfaces
from today and try to identify troughs and ridges
http://www.pivotalweather.com/model.php?m=gfs&p=300wh
http://www.pivotalweather.com/model.php?m=gfs&p=500wh
http://www.pivotalweather.com/model.php?m=gfs&p=700wh
http://www.pivotalweather.com/model.php?m=gfs&p=850wh
Source: Univ of Arizona
Another example to connect the 3-d, wavy
upper-air surface to a 2-dimensional chart
• Figure at right shows the 850-mb
pressure surface
• Note that the ridge and trough are both
associated with temperatures
• Ridge: warmer temperatures
• Trough: colder temperatures
• Notice too, that the temperatures vary
within the ridge and trough
• Warmest temperatures are found at the
“base” (most equatorward part) of the ridge
• Coldest temperatures are found in the core
of the trough
Source: Univ of Arizona
Some general properties of troughs and
ridges
• Warmer air in ridges, colder air in troughs
• The warm and cold air masses are often deep,
occupying most of the column of air in the ridge
and trough, respectively
• At the level of the upper-air chart, temperatures in
the trough are also often colder than temperatures
in the ridge
• However, because the height of the trough and ridge
depends more on the temperature of the layer, and not
on the temperature exactly at the pressure level being
contoured, the pattern of cold and warm temps can
vary
• Winds are usually parallel to height lines
• If lines are curved, winds in gradient balance will be
parallel to the height lines
• If lines are straight (east-west or north-south),
winds in geostrophic balance will be parallel to the
height lines
Source: Univ of Arizona
Quick knowledge check
Is the pressure at Point C greater than, less than, or equal to the
pressure at Point D (you can assume that Points C and D are at
the same latitude)? How do the pressures at Points A and C
compare?
Which of the four points (A, B, C, or D) is found at the lowest
altitude above the ground, or are all four points found at the
same altitude?
The coldest air would probably be found below which of the
four points? Where would the warmest air be found?
Source: Univ of Arizona
What direction would the winds be blowing at Point C?
Quick knowledge check
Is the pressure at Point C greater than, less than, or equal to the
pressure at Point D (you can assume that Points C and D are at
the same latitude)? How do the pressures at Points A and C
compare? Pressure at all 4 points is the same. This is the 500mb chart
Which of the four points (A, B, C, or D) is found at the lowest
altitude above the ground, or are all four points found at the
same altitude? Point A is lowest (5400 m), points B and C are
same (5520 m) and point D is highest (5640 m)
The coldest air would probably be found below which of the
four points? Where would the warmest air be found? Coldest
air would most likely be found at A, and warmest air most
likely at D
Source: Univ of Arizona
What direction would the winds be blowing at Point C? Winds
at C should be from W to E (so “westerly winds”). Winds at
A would also be westerly, as at B and D.
What do troughs and ridges look like in the
Southern Hemisphere?
• Troughs are defined as lower
heights, relative to nearby
values, and ridges are defined
as higher heights, relative to
nearby values
• Generally, air is warmer closer
to the equator and cooler closer
to the poles
• Thus, when cooler air extends
from the pole toward the
equator, it typically shows up as a
trough
• Similarly, when warmer air from
the equator extends poleward, it
typically shows up as a ridge
Source: Univ of Arizona
What do troughs and ridges look like in the
Southern Hemisphere?
Source: http://wxmaps.org/fcst.php
Where are the troughs and ridges in this
500-mb chart?
Where are the troughs and ridges in this
500-mb chart?
A three-dimensional perspective
• We’ll see much more about
this later in the semester
• For now, important to
remember that upper-level
troughs and ridges are directly
related to surface features like
fronts, pressure systems and
the like
• In synoptic theory, the upperlevel waves tend to project
onto the surface, then receive
feedback from the surface.
• It’s rare to see surface features
in-absentia (eg, apart from
upper-level “support” or
“forcing”)
Source: Univ of Arizona
A three-dimensional
perspective
• Another example of the connection between
the surface (sea level pressure isobars, top
panel), the thickness (dashed 1000-500 mb
thickness lines, middle panel) and upper-air
height (solid height lines at 500 mb, bottom
panel)
• What patterns do you notice in common
between the three figures?
• What features connect across the figures? What
features do not seem to connect?
Stull 2016
A three-dimensional perspective
• Returning to the 3-cell
model: the polar jet
(boundary between
polar cell and Farrell
cell)
• The polar jet intensity
and location depends
heavily on differences
between temperature
(thickness) between
the polar and midlatitude regions
Stull 2016
A three-dimensional perspective
• What differences do
you notice between
the winter (top panel)
and summer (bottom
panel) hemispheres?
Stull 2016
Important role of troughs and
ridges: heat redistribution
• As you know from the radiation
lesson, because the Earth is a
sphere, more radiation reaches
the middle portion of the planet
(the tropics) than the poles
• Upper-air waves are the main way
the planet re-distributes that heat
• Move warm air poleward and cold
air equatorward
• Figure 11.60 (lower left) indicates
that total heat redistribution
(dark black curve) is mostly due
to atmospheric waves, then
secondarily due to ocean
circulation, and finally due to
Hadley and polar cells
Stull 2016
Important properties of upper-air charts
• Heights related to temperature of the layer (via the hypsometric equation): taller heights imply
warmer layers
• Above the surface of the Earth, winds generally blow parallel to height lines
• When winds blow parallel to the heights, and the height lines are straight (eg, no curvature), the wind is said
to be in “geostrophic balance.”
• Geostrophic balance is one of the most important balances in all of meteorology and oceanography
• You will hear about geostrophic balance many, many, many more times in your courses. In fact, it is named in the Department
Learning Objectives as one of the most important things you will learn about in the major!
• What, then, is in balance?
•
•
Pressure gradient force
Coriolis force
• When the height lines are curved, winds still typically blow parallel to the lines. This is called
“gradient balance”
• What is in balance in gradient balance? Pressure gradient force, Coriolis force, and centrifugal force
• Near the surface of the Earth, friction plays an important role and causes winds to cross (intersect
with) height lines
Utility of charts at different pressure levels
• 850mb: to identify fronts
• 700mb: intersects many clouds; moisture information is important :
intersects many clouds; moisture information is important
• 500mb: used to determine the location of short waves and long
waves associated with the ridges and troughs in the flow pattern.
waves associated with the ridges and troughs in the flow pattern.
Meteorologists examine “vorticity” (i.e. rotation of air) on this
pressure level.
• 300, 250, and 200mb: near the top of the troposphere or the lower
stratosphere; these maps are used to identify the location of
jetsreams that steer the movements of mid latitude storms
Source: Univ California Irvine
Why manual analysis?
• Tremendous amount of weather data available today when compared to 1950s
(1950s are considered the start of modern meteorology)
• Automated techniques are pretty good at plotting isolines of anything
• Temperature, pressure, dew point, precipitation, height, etc.
• But … to understand & synthesize the data in the charts requires the
meteorologist to examine the actual observations (and it requires patience)
• A manual analysis requires a meteorologist to look at every data point!
• Time consuming, yes. So is it valuable?
• Tremendous benefit in being forced to think about observational data and interpret
weather observations
Manual analysis: upper-air
• Important to also examine weather observations above the surface of
the earth
• Typically examine constant-pressure charts
• 250 mb, 500 mb, 700 mb, and 850 mb
• Height contours almost always parallel to winds (i.e., geostrophic
balance)
• Temperature gradients tend to not be as sharp above 700 mb
• Example: at the surface in winter, Florida can have temps near 30C (mid-80s
F) and New York near -10C (mid-10s F), while at 500 mb, temps near -10C
over FL may only decrease to -20C over NY.
How to conduct an upper-air analysis:
contour intervals
• 250 mb: Contour every 60 m
• Be sure to include 10800 m – so 10860, 10920, 10740, etc.
• 500 mb: Also contour every 60 m
• Be sure to include 5400 m – so 5460, 5520, 5340, etc.
• 700 mb: Contour every 30 m
• Be sure to include 3000 m – so 3030, 3060, 2970, etc.
• 850 mb: Also contour every 30 m
• Be sure to include 1500 m – so 1530, 1560, 1470, etc.
Rules of Isoplething
•
Never violate a valid data point. Only in extreme and defendable circumstances should data be omitted. Analyze for
all given data.
•
Interpolate as much as possible. Allow for extreme packing of isolines if that is defendable.
•
Smooth isolines and, whenever possible, keep pacing consistent.
•
Do not analyze for what does not exist. Do not assume data.
•
There should be no features smaller than the distance between data points.
•
Isolines cannot intersect nor can they suddenly stop. Just as data is continuous, so are isolines. The exception to this
is naturally at the end of a page.
•
Label all closed isolines with appropriate markings (i.e. "H" or "L") in bold and large letters. Label the maximum and
minimum values with a small underline.
•
Label the ends of the lines neatly and consistently. Make sure that any abbreviations are understandable. Title the
map and include time.
•
Analyze in even multiples of the interval of analysis.
•
Remember that each line must represent all areas with the specified value. On one side of the line, values will be
lower than the value on the line and on the other side, values will be higher.
•
Use a good pencil and initially sketch lines lightly. If needed, make them smooth by darkening the lines after you
know where they should be placed.
•
Have a good eraser handy.
•
Start with a line that gives you a good understanding of what is happening. This may be in the middle or near the
extremes. Use this line as a guide to draw the rest of the isolines.
•
When the lines become tricky to draw, consider all the alternatives. There may be a better way to draw the analysis.
•
Remember that the data is only a reflection of the actual atmosphere!
Adapted from College of DuPage
http://weather.cod.edu/labs/isoplething/isoplething.rules.html