Download Unit 10

■
Unit 10
Vertical motions are usually much weaker
than horizontal motions
–
–
Horizontal air flow Þ 0 to 100 kt
Vertical air flow Þusually a few feet per minute
■
May exceed 5,000 ft/min in strong TS or severe clear-air
turbulence (CAT)
■ Why
Vertical motion
■ How
Vertical motion
Causes of vertical motion
See Table 5-1 p. 5-2
Rising motion Þ air expands and cools
Þ clouds & precipitation may form if the
air is sufficiently humid
■ Sinking motion Þ air is compressed
and warms Þ skies clear
■
1. Convergence & divergence
2. Topography
3. Fronts
4. Convection
5. Wave motion
6. Turbulence
■
1. Convergence and divergence
Convergence occurs when there is more
inflow to than outflow from a region
■ Divergence occurs when there is more
outflow from than inflow to a region
■ Is which of the following regions does
the wind appear to converge/diverge?
■
are vertical motions important?
do they originate?
Convergence/divergence aloft
■
■
■
Convergence causes the pressure to increase at low levels
Divergence causes the pressure to decrease at low levels
Strong divergence usually occurs just ahead of an
approaching trough, and strong convergence just behind
Conv.
tropopause
Div.
Weight of
column
increases
Weight of
column
decreases
ground
P increases
P decreases
1
Surface convergence
Example:
■
Identify the upper-level trough and ridge on the constant
pressure chart. Since troughs and ridges usually move
from west to east in mid-latitudes, where do you expect
divergence? Where might a surface low be forming?
■
■
N
10200 m
10320 m
■
■
10440 m
10560 m
250 mb
Height
Surface cyclones (low pressure
systems) tend to develop ahead
of an upper-level trough
Last time, we found that friction
causes the wind to converge
into regions of low pressure.
The converging air must go
somewhere. Since it can't go
into the ground, it rises.
Air converging over a low
pressure system will lift a layer
of air, (sometimes hundreds of
kilometers across), to higher
levels, cooling the air as it rises.
If the lifted air is moist enough,
the water vapor will condense
out to form cloud droplets.
■
■
Rising motion develops
amid divergence aloft and
surface convergence.
Sinking motion develops
amid convergence aloft
and surface divergence.
3. Fronts
Lifting
ahead of
warm &
cold
fronts
w
lo
irf
Cold
Mountain
Rising air in warm plumes (bubbles)
called thermals
■ Sinking motion between thermals
■ Occurs when the atmosphere is more is
unstable (more on this later)
■
ow
rfl
Ai
A
Cold
front
Cloud
4. Convection
rm
Wa t
fron
Co
l
fro d
nt
■
Sinking motion clear, warm and dry
surface
L
H
Rising motion Cloudy, cooler,
possible precip
250 mb
Div.
Vertical motions
associated with
convergence are typically
very small compared to
those observed inside a
TS (convection)
2. Topography
Putting everything together...
Conv.
■
Warm
Warm
Cold
Warm
front
2
Wave clouds (Altocumulus lenticularis), Greenland
5. Wave motions
■
■
Occur
downstream
from
mountains
Called
gravity
waves, or
mountain
waves
Wave
clouds
Mountain
6. Turbulence
■
■
Though turbulence can be
produced by wave motions and
mountains, it is also produced
by strong wind shear (i.e. when
the wind velocity changes with
height)
Most common near the ground
on windy days
3