Corporate Profile

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METR 2413 20 February 2004
Thermal Advection Since we do not directly
measure vertical motions, the analysis of thermal
advection on maps will provide a very useful tool
for determining the vertical motions currently
occurring in the atmosphere.
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Why use Temp. Advection?

• The temperature at a location may change in two
  ways
• The air parcel which is being sampled might
  change its thermodynamic state. For example,
  sunlight might increase its internal energy, and
  hence its temperature will rise.
• The air parcel might be replaced by a different
  parcel with a different thermodynamic state as
  the wind blows past the station. This process is
  called advection.
• In practise, both processes will operate.
  However, on the synoptic scale, temperature
  changes on timescales less than a few days are
  dominated by advection effects.

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Thermal Advection
The advection term consists of a wind velocity
component and a temperature gradient component.
The spatial relationship between these two is
important.
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Thermal Advection

• Spatial relation between wind and temperature
  gradients
• (Geostrophic) wind is parallel to isobars.
• Temperature gradients are represented by
  isotherms.
• The magnitude of the pressure gradient and
  temperature gradient and angle between the two,
  isobars (wind) and isotherms, determines the
  strength of advection.

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Solenoids

• When the wind crosses the temperature gradient at
  nearly a 90 degree angle the boxes formed on
  the weather map are called Solenoids. Solenoids
  analyzed on a weather map indicate the presence
  of strong advection and vertical motions.

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Solenoids

• Thermal advection Solenoids can be identified on
  850 mb charts by comparing isotherms and
  isohypses.
• Also by comparing 1000-500 mb thickness and
  surface pressure isobars, which have historically
  been plotted together on weather charts
  (MSLP/1000-500 thickness chart)

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500-1000mb Thickness

• In addition to isotherms on a constant pressure
  surface, we can look at thickness compared to
  surface pressure
• Remember the hypsometric eqn?
• Thickness between 2 pressure surfaces is directly
  related to mean layer temp!
• Increase mean temp, increase thickness
• Decrease mean temp, decrease thickness
• This can be used as an additional tool when
  analyzing thermal advection

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Why use thickness?

• Heard of the Thermal Wind?
• Not really a wind at all, but a vector difference
  between the geostrophic wind at different heights
• The Thermal Wind is always parallel to contours
  of thickness, with cold air to the left and warm
  to the right
• If we plot thickness along with surface
  pressures, and assume that surface winds are
  somewhat parallel to surface isobars, then we
  have 2 pieces of information
• 1) Surface wind vector
• 2) Thermal Wind vector
• The difference between the two is the geostrophic
  wind above the surface, so now we know how the
  geostrophic wind changes with height

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Thermal Wind
No thermal advection Thermal wind is parallel to
low level wind, so geostrophic wind at lower and
upper levels are parallel Cold Air
Advection Thermal wind is to the left of the low
level wind, so geostrophic wind must back with
height gt CAA Warm Air Advection Thermal wind
is to the right of the low level wind, so
geostrophic wind must veer with height gt WAA
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