Chapter 15 Global Circulation Big Picture Idealized View

Chapter 15 Global Circulation: Big Picture: Idealized View Polar cell Ferrel cell

Fig 1. Earth’s radiation budget What do ‘surplus’ and ‘deficit’ refer to? What wavelength ranges are for shortwave and longwave? What are their sources? What is shortwave in? Why are shortwave absorbed not symmetrical with latitude? At what latitude does most heat and mass transfer occur?

Fig 2. 1735, Hadley’s circulation cell idea developed. Ideas: Latent heat release drives convection. Trade winds are Easterlies due to the rotation of the Earth, and he realized it 100 years before Coriolis wrote theory for motion in a uniformly rotating coordinate system. Main problems: No westerlies anywhere to conserve angular momentum, and the Equator to pole temperature difference is too large.

Fig. 3. Rossby theory of circulation, 1930 s clouds polar front Warm air rides up the colder air deserts doldrums clouds William Ferrel’s cell idea in 1856: Rossby clarified and explained more completely. Prevailing westerlies at mid latitudes are a wavy river of air, Rossby waves, where the Coriolis force is the restoring force, and strong horizontal gradients of pressure and temperature (baroclinic instabilities – density depends on pressure and temperature). This conceptual model captures many features of the Earth’s circulation.

Jet Streams (pink) between warm (orange) and cold (blue) regions Jet streams (shown in pink) are well-known examples of thermal wind. They arise from the horizontal temperature gradient from the warm tropics to cold polar regions. From Wikipedia

Geostrophic Wind: due to pressure gradient along x direction Thermal wind T(z) y y y Barotropic atmosphere (density depends only on pressure) xy Baroclinic atmosphere (density depends on pressure and temperature) size of temperature symbol represents value. 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. (from Wikipedia)

Fig 4 a Longitudinally averaged winds (zonal mean) for 20 years on the equinox March 21 st. Westerlies are solid lines. Note jet stream maxima at 200 h. PA.

Thermal Wind Estimation (will do derivation later)

Fig 4 b Zonal mean average circulation Note the location of the ITCZ What latitude is cloudiest? What latitude has clear conditions? Dec Jan Feb June July August

Fig 4 c Zonal mean temperature for 20 years at equinox on March 21 st. Note the strong latitudinal gradient.

Fig 7 Lorenz theory of Atmosphere Energy Cycle Energy density (units 105 Jm-2) Energy flows (Wm-2) Key points: Terminology and mechanisms at play. What are APE, KE, and eddies?

Barotropic and Baroclinic Atmosphere Barotropic Baroclinic fluid is ‘full of life as it converts thermal energy into kinetic energy’ From Marshall and Plumb