UPPER AIR DYNAMICS

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UPPER AIR DYNAMICS

• MSC 243 Lecture 7, 10/15/09

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High Temperature Forecasting

• Three primary factors
• ADVECTION
• Warm advection results in temperature rises
• Cold advection results in temperature falls
• Even advection above the surface can affect
  surface temperatures.
• ADIABATIC WARMING / COOLING
• (will leave until later)
• DIABATIC WARMING / COOLING

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Diabatic Effects

• Factors affecting incoming solar radiation
• Cloud cover
• 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

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Low Temperature Forecasting

• Factors that promote cool minima
• Clear Skies
• Enhanced radiational cooling
• Light Winds
• Surface decoupling
• Snow Cover
• Enhanced radiational cooling / insulates surface
  from ground below (traps heat below it)
• Low Dew Points
• Water Vapor good absorber of IR radiation, i.e.
  less radiation is absorbed in the atmosphere if
  dew points are low

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Low Temperature Forecasting

• Factors that promote warm minima
• Clouds / Fog
• Absorb IR radiation emitted from ground etc.
• Strong Winds
• Keep the boundary layer mixed
• Urban Heat Island
• High heat capacity of city versus country
• High Dew Points
• Water Vapor good absorber of infrared radiation

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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.

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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

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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.

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Ridges and Troughs Aloft

• 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

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Height of Pressure Surfaces

• Pressure Surface Typical Height
• 850 mb 1500 m / 5000 feet
• 700 mb 3000 m / 10000 feet
• 500 mb 5500 m / 18000 feet
• 300 mb 9000 m / 30000 feet
• 200 mb 12000 m / 39000 feet

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Height on a pressure surface is analogous to
pressure on a height surface!
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850 mb Chart
The 850 mb chart is good for estimating surface
temperatures, low level moisture, and determining
precipitation type (rain, snow, sleet).
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850 mb Chart
The 850 mb chart is good for estimating surface
temperatures, low level moisture, and determining
precipitation type (rain, snow, sleet).
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850 mb Chart
The 850 mb chart is good for estimating surface
temperatures, low level moisture, and determining
precipitation type (rain, snow, sleet).
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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.
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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.
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700 mb chart
The 700 mb chart is also used to determine
short-wave disturbances via the geopotential
height field.
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500 mb geopotential height
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 small-scale flow
(short-waves, low level storm systems)
RIDGE
TROUGH
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250 mb Chart
The 250 mb chart is used to locate the jet
stream. Strong upper- level winds help develop
surface low pressure in mid-latitudes.
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Thickness (yellow lines) what is it related to?
5400m contour first approx for rain/snow border
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Hydrostatic Approximation
Mass density x Volume
Newtons Second Law
pressure force per unit area
Rearrange last equation to yield hydrostatic
approximation
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Thickness and Temperature

• Hydrostatic approximation for the atmosphere
• (p is pressure, z is height, g is gravity, and
  is density)
• The ideal gas law is
• (R is a constant, T is temperature)
• Rearranging terms

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Thickness and Temperature

• Equation from before (hydrostatic ideal gas
  law)
• Integrating through a layer with average temp Tm
  yields
• Thus, the thickness of a layer is proportional to
  the average temperature in that layer.

p2
thickness
p1
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Convergence and Divergence
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Convergence and Divergence

• (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

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What can we tell from a 500 mb chart?
Convergence upstream of trough axis. Winds
coming together, height contours narrowing.
Speed increases following the flow. Divergence
downstream of trough axis. Winds spreading
apart, height contours widening. Speed decreases
following the flow.
Upstream of trough axis
Downstream of trough axis
TROUGH
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• Divergence aloft is associated with rising motion
  and surface low pressure
• 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
Convergence
Divergence
Trough
Rising
Sinking
Sinking
Surface
High Pressure
Low Pressure
High Pressure
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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.

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Conditions for surface low (L) to develop
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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.
• Earth's vorticity
• 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
• Vorticity generated by air motions relative to
  the earth
• Counter-clockwise flow is positive vorticity
  (spin)
• Clockwise flow is negative vorticity (spin)

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Vorticity at 500 mb
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Vorticity at 500 mb