Thermal Wind Balance Questions and Answers

Q: Which measurement gives the horizontal temperature gradient without any radiosonde ascent?

Satellite sounders retrieve temperature profiles over the whole globe, including the oceans where radiosondes are scarce, and those profiles give the horizontal gradient and hence the thermal wind. This transformed analysis over the southern hemisphere from the nineteen seventies onward. Surface reports cannot be extrapolated upward, aircraft winds give the shear rather than the gradient, and rainfall is at best an indirect indicator of vertical motion.

Q: Which quantity must be known besides the shear to obtain the absolute wind?

The thermal wind gives only a difference, so an absolute wind profile requires a boundary value at some level, which in the atmosphere is usually the observed surface or upper level wind. This is the same difficulty the oceanographic dynamic method faces with its level of no motion. Surface temperature gives no wind information, and stratification and domain size do not enter the integration constant at all.

Q: If the isobars and isopycnals are parallel to each other, the flow is said to be

The flow is barotropic.In a barotropic fluid the density depends only on pressure, so surfaces of constant pressure (isobars) and constant density (isopycnals) coincide or are parallel. In a baroclinic fluid they intersect, because density also depends on temperature or salinity; this gives the solenoidal term that generates vertical shear (the thermal wind).Geostrophic describes a balance between pressure gradient and Coriolis forces, which can hold in either case, and rotational refers to vorticity, not to the relation between pressure and density surfaces.

Q: In a Barotrophic atmosphere :

In a barotropic atmosphere the geostrophic wind does not change with height.In a barotropic atmosphere density depends only on pressure, so surfaces of constant pressure and constant temperature coincide and there are no horizontal temperature gradients on pressure surfaces. By the thermal wind relation, the vertical change (shear) of the geostrophic wind is proportional to the horizontal temperature gradient, so with no gradient the geostrophic wind is the same at all heights.Geostrophic winds that increase or decrease with height require a baroclinic atmosphere. 'Pressure a function of temperature only' reverses the definition: it is density that depends on pressure alone.

101.

Which measurement gives the horizontal temperature gradient without any radiosonde ascent?

A.Satellite radiance retrievals
B.Surface station reports, from which the temperature at every level above may be extrapolated reliably
C.Aircraft wind reports
D.Rainfall observations, which indicate where the ascent associated with a thermal gradient is occurring

Answer: A

Satellite sounders retrieve temperature profiles over the whole globe, including the oceans where radiosondes are scarce, and those profiles give the horizontal gradient and hence the thermal wind. This transformed analysis over the southern hemisphere from the nineteen seventies onward. Surface reports cannot be extrapolated upward, aircraft winds give the shear rather than the gradient, and rainfall is at best an indirect indicator of vertical motion.

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102.

Which historical development most improved southern hemisphere analysis?

A.The introduction of the thermal wind relation into routine forecasting practice at operational centres
B.Satellite temperature sounding
C.The establishment of a dense network of radiosonde stations across the southern oceans after the war
D.Faster computers

Answer: B

Before satellites, the southern hemisphere had almost no upper air data over its vast oceans, and analyses were largely guesswork; satellite radiances supplied the temperature structure and with it the wind field through the thermal wind relation. No dense radiosonde network was ever established there. Faster computers helped but could not create data, and the thermal wind relation had been in use since long before.

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103.

Which quantity must be known besides the shear to obtain the absolute wind?

A.The stratification
B.The horizontal extent of the region over which the temperature gradient has been measured accurately
C.The wind at one reference level
D.The temperature at the surface, from which the wind at every level above may then be derived directly

Answer: C

The thermal wind gives only a difference, so an absolute wind profile requires a boundary value at some level, which in the atmosphere is usually the observed surface or upper level wind. This is the same difficulty the oceanographic dynamic method faces with its level of no motion. Surface temperature gives no wind information, and stratification and domain size do not enter the integration constant at all.

Discuss in Forum

104.

If the isobars and isopycnals are parallel to each other, the flow is said to be

A.baroclinic
B.barotropic
C.geostrophic
D.rotational

Exam Year(s)

GATE XE - 2024,

Answer: B

The flow is barotropic.

In a barotropic fluid the density depends only on pressure, so surfaces of constant pressure (isobars) and constant density (isopycnals) coincide or are parallel. In a baroclinic fluid they intersect, because density also depends on temperature or salinity; this gives the solenoidal term that generates vertical shear (the thermal wind).

Geostrophic describes a balance between pressure gradient and Coriolis forces, which can hold in either case, and rotational refers to vorticity, not to the relation between pressure and density surfaces.

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105.

In a Barotrophic atmosphere :

A.Geostrophic wind speed increases with height
B.Geostrophic wind speed decreases with height
C.Geostrophic wind does not change with height
D.Pressure is a function of temperature only

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

In a barotropic atmosphere the geostrophic wind does not change with height.

In a barotropic atmosphere density depends only on pressure, so surfaces of constant pressure and constant temperature coincide and there are no horizontal temperature gradients on pressure surfaces. By the thermal wind relation, the vertical change (shear) of the geostrophic wind is proportional to the horizontal temperature gradient, so with no gradient the geostrophic wind is the same at all heights.

Geostrophic winds that increase or decrease with height require a baroclinic atmosphere. 'Pressure a function of temperature only' reverses the definition: it is density that depends on pressure alone.

Discuss in Forum

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