ENVIRONMENTAL FLUID MECHANICS Troposphere & Weathercushman/courses/engs151/EFM-weather.pdfinto a...
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1 ENVIRONMENTAL FLUID MECHANICS Troposphere & Weather Benoit Cushman-Roisin Thayer School of Engineering Dartmouth College www.fas.org/irp/imint/docs The story begins with the fact that the earth’s surface is curved and therefore different places see the sun differently. As a rule, the tropics get the most heat while the mid- latitudes get a little less and the polar regions get the least.
Transcript of ENVIRONMENTAL FLUID MECHANICS Troposphere & Weathercushman/courses/engs151/EFM-weather.pdfinto a...
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ENVIRONMENTAL FLUID MECHANICS
Troposphere & Weather
Benoit Cushman-RoisinThayer School of Engineering
Dartmouth College
ww
w.fas.org/irp
/imint/docs
The story begins with the fact that the earth’s surface is curved and therefore different places see the sun differently.
As a rule, the tropics get the most heat while the mid-latitudes get a little less and the polar regions get the least.
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On the other hand, the escape of heat to space is nearly uniform with latitude, making the tropics receive more than they lose and the poles lose more heat than they receive. Heat balance on earth demands that there be a net flow of heat from low to high latitudes to bring the tropical excess to the poles.
This hemispheric flow of heat is effected by convective cells, not one spanning the latitudinal range but three of them:
- The first cell is vertical, extending from Equator to about 30oN or S (Hadley cell)
- The second “cell” in mid-latitudes is really a band with active weather patterns, from 30o to 60o
- The third cell is vertical, extending from 60o to the pole.
To these north-south cells need to be added the zonal circulation:
Because of the rotation of the earth, winds have a tendency to veer,to the right in the Northern hemisphereto the left in the Southern Hemisphere.
At ground level, this creates the Trade Winds in the tropical regions, the Westerlies at mid-latitudes, and the Polar Easterlies(also polar anticyclone) near the poles.
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LOWPRESSURE
HIGHPRESSURE
cyclone anticyclone
(Arrows shown for Northern Hemisphere – Reversed arrows for Southern Hemisphere)
The upper-level of the mid-latitude cell is occupied by a very strong westerly (= eastward!) jet called the Polar Jet and most familiarly known as the Jet Stream.
(http://ww
w.srh.noaa.gov/jetstream
/global/jet.htm)
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nasa.gov
Meandering of the Jet Stream
“trough”
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To understand this major instability of the Jet Stream system(wiggling and vortex shedding),We must consider conservation of potential vorticity.
3) Conservation of circulation
in a horizontal loop
4) Conservation of volume
4) Combination leads to conservation of potential vorticity
f dsh ds
f ds f
h ds h
1) With L>>H, U>>W, vorticity is essentially vertical.
2) Vorticity includes f = 2 sin(latitude) of ambient rotation.
Meanders of the Jet Stream are unstable and grow.
The name of the process isBaroclinic Instability.
Each unstable wave develops into a familiar mid-latitude weather pattern.
_________________
Baroclinic instability is a complicated process that involves stratification, a vertically sheared jet, and the constraint of potential-vorticity conservation.
It is a central subject in Geophysical Fluid Dynamics.
B. C
ushman-R
oisin & J.-M
. Beckers, Introduction to G
eophysical Fluid D
ynamics, A
cademic P
ress, 2011.
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http
s://ww
w.fa
s.org/irp
/imin
t/docs/rst/S
ect14/S
ect14_1d.h
tml
3D diagram of a developing weather system
Instability occurs when the Low at the surface lies to the East of the Low in the upper troposphere because then the lower and upper waves create mutual amplification.
Handy rule to remember!
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Nimbostratus clouds Cirrus clouds
Warm Front
Cumulonimbus clouds
Altocumulus cloud
Cold Front
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How it looks halfway up in the atmosphere (550 mb level)
Chart of Tuesday 12 February 2019
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Upper level500-mb charts
A textbook-like example – Stages of growth
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Now, the same again but with upper and lower charts side by side.
Stage of maturation
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The end of the episode
Let’s have a look at the last few days: Start with Wednesday 6 February 2019
Upper level (550 mb)
Surface chart
Smooth over eastern US; big trough over western US,with Low P situated over Idaho.
Surface Low P over Idaho, coinciding with Low P at 500 mb, but there is another Low P further east. This one is “ahead” of the 500-mb Low P.
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Thursday 7 February 2019
Upper level (550 mb)
Surface chart
The trough (Low P) formerly over Idaho has moved eastward over the Dakotas.
The ground level Low P has not moved significantly, now putting itself in interaction with the 500-mb Low P above.
Friday 8 February 2019
Upper level (550 mb)
Surface chart
The Low P above has strengthened, too and has remained “behind” the Low P at ground level.
The low pressure at ground level has greatly intensified! This was because of the interaction with the 500-mb Low P above.
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Saturday 9 February 2019
Upper level (550 mb)
Surface chart
The low pressure at ground level has moved out of the map.
The Low P above is moving out, too.
Sunday 10 February 2019
Upper level (550 mb)
Surface chart
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Monday 11 February 2019
Upper level (550 mb)
Surface chart
The low pressure has disappeared at 500 mb.
But now watch this other trough further west, over Wyoming.
At the surface, the Low P seems to coincide with the position of trough above.
Tuesday 12 February 2019
Upper level (550 mb)
Surface chart
At 500 mb, the trough (Low P) is just west of the Great Lakes.
At the surface, the Low P is further east, south of the Great Lakes.
This other Low P further east is bringing humid air from over the ocean.Hence, snow!
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Precipitation
Tuesday 12 February 2019
Surface pressure and fronts
Wednesday 13 February 2019
Upper level (550 mb)
Surface chart
At 500 mb, the Low P has closed into a defined cyclone. This is a sign of intensification.
At the surface, the location of the Low P cyclone coincides with that above. This is a sign that there will not be a further intesification.
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Analysis for today, 14 February 2019
Forecast for 15-16 February 2019
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500mb Forecast17-21 February 2019
