Transcript Corporate Profile - University of Oklahoma

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.
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.
Thermal Advection
T
T
T
 T
H
=-(u
+v
) + ( )+
t
x
y
c p p
cp
The advection term consists of a wind velocity
component and a temperature gradient component.
The spatial relationship between these two is
important.
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.
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.
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/1000500 thickness chart)
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…
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
Thermal Wind
Cold
No thermal advection:
Cold
Thermal windV2is parallel to low level wind, so
T lower and upper levels are
geostrophicV1 windVat
parallel
Warm
V1
Cold
VT
V1
V2VT
V2
Warm
Warm
Cold
V2
Cold Air Advection:
Thermal wind is to the left of the low level wind, so
geostrophic wind must back with height => CAA
VT
V1
Cold
Warm
V2
V1
VT
Cold
Warm Air Advection:
Warm
V1
Thermal wind is to the right of the low level wind, so
geostrophic wind must veer with height => WAA
VT
V2
Warm