Thermal Wind Balance Questions and Answers

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.

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.

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.

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.

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.