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UPPER AIR DYNAMICS
MSC 243 Lecture #7, 10/15/09
High Temperature Forecasting

Three primary factors:
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ADVECTION
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ADIABATIC WARMING / COOLING
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Warm advection results in temperature rises
Cold advection results in temperature falls
Even advection above the surface can affect surface
temperatures.
(will leave until later)
DIABATIC WARMING / COOLING
Diabatic Effects

Factors affecting incoming solar radiation:

Cloud cover

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Type (thickness)
Duration
Time of Day
Ground Moisture / Vegetation

If dew points are lower than the air temperature, falling
precipitation will cool temperatures to the wet-bulb
temperature (in between temperature and dewpoint)

http://www.srh.noaa.gov/epz/wxcalc/dewpoint.shtml
Low Temperature Forecasting
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Factors that promote cool minima:
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Clear Skies
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Light Winds
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Surface decoupling
Snow Cover
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Enhanced radiational cooling
Enhanced radiational cooling / insulates surface from ground
below (traps heat below it)
Low Dew Points
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Water Vapor good absorber of IR radiation, i.e. less
radiation is absorbed in the atmosphere if dew points are low
Low Temperature Forecasting

Factors that promote warm minima:

Clouds / Fog
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Strong Winds
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Keep the boundary layer mixed
Urban Heat Island
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Absorb IR radiation emitted from ground etc.
High heat capacity of city versus country
High Dew Points

Water Vapor good absorber of infrared radiation
Upper Levels

So far, we have only looked at surface weather
features.

However, the upper levels are crucially
important for the development of weather
systems, and hence their forecasts.
Pressure Levels

Pressure is the force exerted on an object by all air
molecules that impinge on a surface area – in general,
the weight of a column of air per unit area

Pressure decreases with height.

Meteorologists concentrate on a few standard
pressure levels, plus the surface

Each of these levels are important in weather
forecasting for different reasons
Upper Level Weather Maps

Upper level weather maps are
plotted on a constant pressure
surface

Contours of equal geopotential
height are plotted (e.g. height in
meters of the 500 mb pressure
surface)

Thickness is the difference in
height between 2 pressure sfcs.
It is directly proportional to the
mean temperature of the layer
(e.g. 1000 - 500 mb). Thickness
is
useful
in
determining
precipitation type.
Ridges and Troughs Aloft

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Mountains and valleys of warm and cool air
The height of the pressure level depends on the
temperature of the column of air below it
Increasing
Height
Ridge
Ridge
500 mb
500 mb
Trough
Trough
700 mb
850 mb
Surface
Very warm
column
Cool
column
Warm
column
Very cool
column
Height of Pressure Surfaces
Pressure Surface
850 mb
700 mb
500 mb
300 mb
200 mb
Typical Height
1500 m / 5000 feet
3000 m / 10000 feet
5500 m / 18000 feet
9000 m / 30000 feet
12000 m / 39000 feet
Height on a pressure surface is analogous to
pressure on a height surface!
850 mb Chart
The 850 mb
chart is good
for estimating
surface
temperatures,
low level
moisture, and
determining
precipitation
type (rain,
snow, sleet).
850 mb Chart
The 850 mb
chart is good
for estimating
surface
temperatures,
low level
moisture, and
determining
precipitation
type (rain,
snow, sleet).
850 mb Chart
The 850 mb
chart is good
for estimating
surface
temperatures,
low level
moisture, and
determining
precipitation
type (rain,
snow, sleet).
700 mb chart
The 700 mb
chart is used to
determine
cloud cover or
rainfall, using
the relative
humidity field
and the vertical
motion field.
700 mb chart
The 700 mb
chart is used to
determine
cloud cover or
rainfall, using
the relative
humidity field
and the vertical
motion field.
700 mb chart
The 700 mb
chart is also
used to
determine
short-wave
disturbances
via the
geopotential
height field.
500 mb geopotential height
RIDGE
TROUGH
The 500 mb chart
is the forecasters’
favorite for
depicting the
motion of
weather systems.
It shows the
large-scale flow
(long waves) and
jet streams, and
also the smallscale flow (shortwaves, low level
storm systems)
250 mb Chart
The 250 mb chart
is used to locate
the jet stream.
Strong upperlevel winds help
develop surface
low pressure in
mid-latitudes.
5400m contour =
first approx for
rain/snow border
Thickness
(yellow lines):
what is it
related to?
Hydrostatic Approximation
Mass = density x Volume
Newton’s Second Law
pressure = force per unit area
Rearrange last equation to yield
hydrostatic approximation
Thickness and Temperature
Hydrostatic approximation for the atmosphere:
(p is pressure, z is height, g is gravity, and
The ideal gas law is:
(R is a constant, T is temperature)
Rearranging terms:
is density)
Thickness and Temperature
Equation from before (hydrostatic & ideal gas law):
p2
thickness
p1
Integrating through a layer with average temp Tm yields
Thus, the thickness of a layer is proportional to the
average temperature in that layer.
Convergence and Divergence
Convergence and Divergence
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(Horizontal) Convergence: more air is entering
an area than leaving it on a pressure surface
(Horizontal) Divergence: more air is leaving
an area than entering it on a pressure surface
Because mass is conserved, horizontal
divergence relates directly to vertical motion
What can we tell from a 500 mb chart?
Convergence upstream
of trough axis. Winds
coming together,
height contours
narrowing. Speed
“Upstream of
increases following
trough axis
“Downstream the flow.
of trough axis Divergence
downstream of trough
axis. Winds spreading
apart, height contours
widening. Speed
decreases following
TROUGH
the flow.

Divergence aloft is associated with rising motion and surface
low pressure
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Convergence aloft is associated with sinking motion and
surface high pressure

Surface pressure patterns are offset from troughs and ridges
aloft in developing systems
500 mb
Ridge Convergence
Divergence
Convergence
Trough
Surface
Sinking
Rising
Sinking
High
Pressure
Low
Pressure
High
Pressure
Development of Surface Low

Net convergence west (upstream) of an upper air trough and
net divergence east (downstream) of an upper air trough.

For a surface storm to intensify, the upper air trough must
be located upstream of the surface low. Divergence aloft,
convergence below = “good upper-level support”

As the upper air low moves closer to being directly over the
surface low, upper air divergence lessens and the surface
low stops deepening (intensifying).

The surface weather often improves once the 500 mb trough
axis has passed.
Conditions for surface low (L) to
develop
Vorticity
Divergence is tricky! It is difficult to accurately measure divergence,
and nearly impossible to use the observed horizontal winds to diagnose
vertical motion. Can it be related to something else – yes it can!
Vorticity is a measure of the rotation of a fluid
around a local vertical axis.
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Earth's vorticity
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The local vertical component of spin due to the rotation of the earth
Depends on latitude (greatest at poles, zero at equator)
Earth's vorticity = 2 x rate of rotation x sin(latitude)
Relative vorticity
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Vorticity generated by air motions relative to the earth
Counter-clockwise flow is positive vorticity (spin)
Clockwise flow is negative vorticity (spin)
Vorticity at 500 mb
Vorticity at 500 mb