metr 2413 20 february 2004

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METR 2413 20 February 2004 Thermal Advection 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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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?. - PowerPoint PPT Presentation

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Page 1: METR 2413 20 February 2004

METR 241320 February 2004

Thermal AdvectionThermal 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.

Page 2: METR 2413 20 February 2004

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.

Page 3: METR 2413 20 February 2004

Thermal Advection

c

H + )

p

T -

c( + )

y

Tv +

x

Tu( - =

t

T

pp

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

Page 4: METR 2413 20 February 2004

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.

Page 5: METR 2413 20 February 2004
Page 6: METR 2413 20 February 2004

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.

Page 7: METR 2413 20 February 2004

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)

Page 8: METR 2413 20 February 2004

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…

Page 9: METR 2413 20 February 2004
Page 10: METR 2413 20 February 2004

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

Page 11: METR 2413 20 February 2004

Thermal Wind

VV

V

1T

2

Warm

Cold

VV

V

1

T

2

Cold

Warm

VV

V

1T

2

Warm

Cold

V V

V

1 T

2

Cold

Warm

VV

V

1

T

2

Cold

Warm

VV

V

1T

2

Warm

Cold

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 => CAA

Warm Air Advection:

Thermal wind is to the right of the low level wind, so geostrophic wind must veer with height => WAA

Page 12: METR 2413 20 February 2004