Download Student Dude: But wait a minute. Way back when, you told us that for

Student Dude: But wait a minute. Way back when, you told us that
for a surface low, winds converge into the low center because
of friction. Remember that? I do.
low
center
surface winds
converging into a
low center
Student Dude: But wait a minute. Way back when, you told us that
for a surface low, winds converge into the low center because
of friction. Remember that? I do.
Wouldn't this convergence increase the mass in the column? And
wouldn't that prevent the low pressure from strengthening?
What's the deal, Professor Dude?
Student Dude: But wait a minute. Way back when, you told us that
for a surface low, winds converge into the low center because
of friction. Remember that? I do.
Wouldn't this convergence increase the mass in the column? And
wouldn't that prevent the low pressure from strengthening?
What's the deal, Professor Dude?
Professor Dude: Excellent questions, student
dude. But remember, it's really the net
convergence or divergence that matters.
Which means we also have to think about
upper levels......maybe there's divergence up
there.
●
●
Surface high and low pressure systems are associated with
ridges and troughs at upper levels
And these ridges and troughs are associated with
convergence and divergence patterns. Specifically.....
upper-level
trough
surface low
pressure system
●
●
●
To the east of an upper-level trough, we generally find
divergence
And to the west of the trough, we typically have convergence
So the question is: Where is the surface low relative to the ridge
and trough?
divergence
●
●
●
To the east of an upper-level trough, we generally find
divergence
And to the west of the trough, we typically have convergence
So the question is: Where is the surface low relative to the ridge
and trough?
convergence
●
●
●
To the east of an upper-level trough, we generally find
divergence
And to the west of the trough, we typically have convergence
So the question is: Where is the surface low relative to the ridge
and trough?
●
●
Well, as it turns out, for a developing system the upper-level
trough is typically to the west of the growing surface low
And this puts the low center directly below the region of
upper-level divergence
●
●
Well, as it turns out, for a developing system the upper-level
trough is typically to the west of the growing surface low
And this puts the low center directly below the region of
upper-level divergence
●
●
●
So.....the surface low center is more or less directly below the
region of upper-level divergence......
…..which means the divergence
at upper levels must be
countering the convergence at
the ground.....
…..and net divergence means
decreasing surface pressure
and a strengthening system!
●
●
●
…..which means the divergence
at upper levels must be
countering the convergence at
the ground.....
…..and net divergence means
decreasing surface pressure
and a strengthening system!
●
So.....the surface low center is more or less directly below the
region of upper-level divergence......
How conveeenient, you say.
●
●
●
…..which means the divergence
at upper levels must be
countering the convergence at
the ground.....
…..and net divergence means
decreasing surface pressure
and a strengthening system!
●
So.....the surface low center is more or less directly below the
region of upper-level divergence......
How conveeenient, you say. But there's a good reason why
the divergence is where it is. And it goes like this.....
●
Remember that an upper-level trough is a region of lower
heights on an upper-level pressure surface, which in turn
means that the layer of air below the surface is contracted
●
And a contracted air layer means colder temperatures
●
So where in the cyclone are the temperatures the coldest?
ridge
trough
WARM
COLD
●
Well.....the temperatures are coldest on the west (or back) side of
the cyclone, where northerly winds bring colder air southward
behind the cold front.
The coldest temperatures
are on the back side of
the low center......
cold
●
And as you might expect, the upper-level trough sits directly above
this region of colder surface air, to the west of the low center.....
....which means the
trough must be on the
back side of the low
center as well!
cold
●
…..which, in turn, puts the upper-level divergence directly above
the low pressure at the ground!
....which, in turn, means
the divergence must be
directly over the low!
cold
●
And the more the cold air plunges southward behind the front, the
deeper the upper-level trough gets. And the deeper the trough
gets, the greater the divergence above the surface low.
cold
●
And the more the cold air plunges southward behind the front, the
deeper the upper-level trough gets. And the deeper the trough
gets, the greater the divergence above the surface low.
cold
Etc, etc, etc......
cold
Summary: Development of a Mid-Latitude Cyclone
upper-level
pressure surface
(heights)
cold
surface map (pressure
& temperature)
warm
It all starts with a stationary front at the ground, separating warmer
air to the south from colder air to the north.
Summary: Development of a Mid-Latitude Cyclone
DIV
upper-level
pressure surface
(heights)
cold
surface map (pressure
& temperature)
warm
Along comes an upper-level disturbance, bringing divergence on
the front side of the trough.
Summary: Development of a Mid-Latitude Cyclone
DIV
upper-level
pressure surface
(heights)
cold
surface map (pressure
& temperature)
warm
The upper-level divergence reduces the mass in the column,
causing lower pressure at the ground.
Summary: Development of a Mid-Latitude Cyclone
DIV
upper-level
pressure surface
(heights)
cold
surface map (pressure
& temperature)
warm
The surface air begins to rotate around the new low center,
organizing into cold and warm fronts.
Summary: Development of a Mid-Latitude Cyclone
upper-level
pressure surface
(heights)
DIV
surface map (pressure
& temperature)
COLD
warm
As the cold front plunges southward, it brings even colder northern
air on the back (or west) side of the low, directly below the trough.
Summary: Development of a Mid-Latitude Cyclone
upper-level
pressure surface
(heights)
DIV
surface map (pressure
& temperature)
COLD
warm
As this cold air moves in behind the front, the whole air layer
contracts, and the upper-level trough gets deeper.
Summary: Development of a Mid-Latitude Cyclone
upper-level
pressure surface
(heights)
DIV
surface map (pressure
& temperature)
COLD
warm
And a deeper trough means stronger divergence at upper-levels.
Summary: Development of a Mid-Latitude Cyclone
upper-level
pressure surface
(heights)
DIV
surface map (pressure
& temperature)
COLD
warm
And as the upper-level divergence gets stronger, the low at the
ground must get stronger also.
Summary: Development of a Mid-Latitude Cyclone
upper-level
pressure surface
(heights)
DIV
surface map (pressure
& temperature)
COLD
warm
And on it goes, amplifying through these internal feedbacks, until
the system finally occludes.
Once the system occludes, the warm air is cut off entirely from the low
center at the ground. The trough at upper levels then moves directly
over the low.....
Once the system occludes, the warm air is cut off entirely from the low
center at the ground. The trough at upper levels then moves directly
over the low.....and the system starts to dissipate.