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Thermal Wind
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Thermal wind is a vertical shear in the geostrophic wind caused by a horizontal
temperature gradient. The name is a misnomer, it is not a wind but rather a wind
shear.
Description
Physical intuition
The vertical variation of geostrophic wind in a barotropic atmosphere (a) and in
a baroclinic atmosphere (b). The blue portion of the surface denotes a cold
region while the orange portion denotes a warm region. Temperature difference
is restricted to the boundary in (a) and extends through the region in (b). The
dotted lines enclose isobaric surfaces which remain at constant slope with
increasing height in (a) and increase in slope with height in (b). This causes
thermal wind to occur only in a baroclinic atmosphere.
Geostrophic wind is proportional to the slope of geopotential on a surface of
constant pressure. In a barotropic atmosphere, one where density is a function
only of pressure, the slope of isobaric surfaces is independent of temperature, so
geostrophic wind does not increase with height.
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This does not hold true in a baroclinic atmosphere, where density is a function
of both pressure and temperature. Horizontal temperature gradients cause the
thickness of gas layers between isobaric surfaces to increase with higher
temperatures. When multiple atmospheric layers are stacked upon each other,
the slope of isobaric surfaces increases with height. This also causes the
magnitude of geostrophic wind to increase with height.
Examples
Advection turning
If a component of the geostrophic wind is parallel to the temperature gradient,
the thermal wind will cause the geostrophic wind to rotate with height. If
geostrophic wind blows from cold air to warm air (cold advection) the
geostrophic wind will turn counterclockwise with height (for the northern
hemisphere), a phenomenon known as wind backing. Otherwise, if geostrophic
wind blows from warm air to cold air (warm advection) the wind will turn
clockwise with height, also known as wind veering.
Wind backing and veering allow an estimation of the horizontal temperature
gradient with data from an atmospheric sounding.
Frontogenesis
As in the case of advection turning, when there is a cross-isothermal component
of the geostrophic wind, a sharpening of the temperature gradient results.
Thermal wind causes a deformation field and frontogenesis may occur.
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Jet stream
A horizontal temperature gradient exists while moving North-South along a
meridian because curvature of the Earth allows for more solar heating at the
equator than at the poles. This creates a westerly geostrophic wind pattern to
form in the mid-latitudes. Because thermal wind causes an increase in wind
velocity with height, the westerly pattern increases in intensity up until the
tropopause, creating a strong wind current known as the jet stream. The
Northern and Southern Hemispheres exhibit similar jet stream patterns in the
mid-latitudes.
The strongest part of jet streams should be in proximity where temperature
gradients are the largest. Due to land masses in the northern hemisphere, largest
temperature contrasts are observed on the east coast of North America
(boundary between Canadian cold air mass and the Gulf Stream/warmer
Atlantic) and Eurasia (boundary between the boreal winter monsoon/Siberian
cold air mass and the warm Pacific). Therefore, the strongest boreal winter
northern hemisphere jet streams are observed over east coast of North America
and Eurasia. Since stronger vertical shear promotes baroclinic instability, the
most rapid development of extratropical cyclones (so called bombs) is also
observed along the east coast of North America and Eurasia.
The lack of land masses in the Southern Hemisphere leads to a more constant jet
with longitude (i.e. a more zonally symmetric jet).
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