Upperair circulation tied to the 3cell model We saw in Lesson 2 that differences in insolation more in the tropics less in the polar regions combined with the Earths rotation drives complex circulation patterns ID: 586449
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Slide1
SO254 – Upper-air chartsSlide2
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 patternsWhat do the upper-level wind patterns look like?
Stull 2016Slide3
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 HHHHadley/Farrell interaction: mid-latitude westerly windsSubtropical 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 wavyWhat does that waviness mean for the weather at a particular place?
How can we identify the waviness?
Stull 2016Slide4
Upper-air circulation tied to radiative imbalance
Incoming radiative flux at top of the atmosphere (
E
insol) varies by latitudeRadiative 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 ° SThis 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 2016Slide5
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 layerThe mean temperature of the layerThis relationship between thickness (Z2-Z1), mean temperature, and pressure is called the
hypsometric equationQuick 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 2016Slide6
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 levelsBut, 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?
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
0°C
0°C
0°C
0°C
0°C
1000
mb
850
mb
500
mb
700
mbSlide7
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 chartSlide8
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
Value of radiosondes to 24-h weather forecasts (and compared to other data types).
Source: NASA
https://gmao.gsfc.nasa.gov/forecasts/systems/fp/obs_impact/
Where are radiosondes typically launched? Source: ECMWF
What is in the instrument?
Source: Plymouth State
UnivSlide9
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 chartSlide10
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 chartSlide11
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 heightsSlide12
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=700wh
http://www.pivotalweather.com/model.php?m=gfs&p=500wh
http://www.pivotalweather.com/model.php?m=gfs&p=300wh
http://www.pivotalweather.com/model.php?m=gfs&p=850wh
Source:
Univ
of ArizonaSlide13
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 temperaturesTrough: colder temperaturesNotice too, that the temperatures vary within the ridge and troughWarmest 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 ArizonaSlide14
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 ridgeHowever, 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 varyWinds are usually parallel to height lines
If lines are curved, winds in gradient balance will be parallel to the height linesIf lines are straight (east-west or north-south), winds in
geostrophic balance will be parallel to the height lines
Source:
Univ
of ArizonaSlide15
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?
What direction would the winds be blowing at Point C?
Source:
Univ
of ArizonaSlide16
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 500-mb 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
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.
Source:
Univ
of ArizonaSlide17
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 troughSimilarly, when warmer air from the equator extends poleward, it typically shows up as a ridge
Source:
Univ
of ArizonaSlide18
What do troughs and ridges look like in the Southern Hemisphere?
Where are the troughs and ridges in this 500-mb chart?
Where are the troughs and ridges in this 500-mb chart?
Source:
http://wxmaps.org/fcst.php
Slide19
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 upper-level 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 ArizonaSlide20
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 2016Slide21
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 mid-latitude regions
Stull 2016Slide22
What differences do you notice between the winter (top panel) and summer (bottom panel) hemispheres?
A three-dimensional perspective
Stull 2016Slide23
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 equatorwardFigure 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 2016Slide24
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 oceanographyYou 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 forceWhen 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 linesSlide25
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 IrvineSlide26
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 anythingTemperature, 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 observationsSlide27
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 mbHeight contours almost always parallel to winds (i.e., geostrophic balance)Temperature gradients tend to not be as sharp above 700
mbExample: 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.Slide28
How to conduct an upper-air analysis: contour intervals
250
mb
: Contour every 60 mBe sure to include 10800 m – so 10860, 10920, 10740, etc.500
mb: Also contour every 60 mBe 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.Slide29
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