Thermal Wind

Overview

• The thermal wind explains the difference between geostrophic winds at different altitudes.
• It is a result of horizontal temperature gradients, such as those between the tropics and polar regions.
• Key example: Jet streams, which result from these thermal winds.

Atmospheric Science Context

• Thermal wind is a vertical wind shear hypothetical scenario.
• It embodies geostrophic balance (horizontal) and hydrostatic balance (vertical).
• Thermal wind balance is applicable to complex atmospheric flow balances like gradient wind balance.

Key Concepts

1. Geostrophic Wind

   • Flows along geopotential height contours.
   • Influenced by temperature gradients, explaining the occurrence of jet streams in the troposphere.
   • Thermally driven winds are aligned with temperature contours.

2. Wind Shear and Temperature Gradient

   • Wind shear varies with height, caused by horizontal temperature differences.
   • Baroclinic flow's variation with height is proportional to the temperature gradient.

3. Origin and Terminology

   • Term by Ernest Gold.
   • Although a misnomer, it describes wind change with height.
   • Initially used to estimate wind fields with limited data.

Physical Explanation

• Geostrophic wind results from Coriolis force and pressure-gradient force balance.
• Hydrostatic balance results from gravitational and vertical pressure-gradient forces.
• Baroclinic vs. Barotropic:
  • Barotropic atmosphere has constant geostrophic wind with height.
  • Baroclinic atmosphere has a variable geostrophic wind with height due to temperature gradients affecting isobar slopes.

Mathematical Formalism

• Geopotential thickness influenced by temperature, explained by the hypsometric equation.
• Thermal Wind Equation:
  • Derived from differentiation and integration of geostrophic wind.
  • Reveals thermal wind is perpendicular to temperature gradients, varying between hemispheres.

Applications and Effects

• Wind backing and veering indicate horizontal temperature gradients.
• Thermal wind causes atmospheric deformation and potential frontogenesis.
• Influences jet stream strength, with notable variations observed between hemispheres.
  • Strongest in regions like East Coast of North America and Eurasia due to large temperature contrasts.

Summary

• Understanding thermal wind is critical for explaining atmospheric wind patterns, particularly in the mid-latitudes.
• It provides insights into phenomena like jet streams and extratropical cyclones.