Lecture 10: Introduction to global circulation of the atmosphere

Scientific concepts / nature phenomena presented

Global atmospheric circulation patterns (global “circulation cell” framework)

• Trade winds / easterlies

  • Persistent surface winds with an easterly component along the tropics.

• Intertropical Convergence Zone (ITCZ)

  • Converging trade winds from the Northern and Southern Hemispheres near the equator.
  • Convergence forces air to rise.

• Convective cloud formation (cumulonimbus) in the ITCZ

  • Warm, very humid air + convergence → deep convection → cumulonimbus.
  • Air rises until reaching the tropopause, then spreads and flows poleward.

• Hadley cell circulation

  • Air rises at the equator (ITCZ), then moves poleward and sinks near ~30° N and ~30° S, forming surface high pressure.
  • Subtropical highs are produced by this sinking air.

• Subtropical highs and deserts

  • Sinking air suppresses cloud formation, favoring dry climates/deserts around ~30° latitude.
  • Example: Pacific subtropical high influencing western U.S. desert regions.

• Westerlies (mid-latitude surface winds)

  • Wind direction tends to be more from the west at mid-latitudes, though the pattern is not perfectly symmetric and changes day to day.

• Mid-latitude cyclones (cyclone/comma/counterclockwise features)

  • Form along the polar front and are visible in precipitation patterns.
  • Present in both hemispheres; rotation direction differs by hemisphere (Coriolis effect).

• Jet stream

  • Strong upper-level winds (shown using ~700 mb and ~500 mb levels) embedded in the westerlies.
  • Located near the polar front, associated with strong temperature gradients and Rossby-wave-like structure.

• Polar cell / polar easterlies (described qualitatively)

  • Near the poles, sinking/descending air creates high pressure and clockwise rotation around the pole, producing polar easterlies.
  • Polar front mixing near mid-latitudes supports cyclone formation.

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Observational methods / data products used (how the phenomena are “seen”)

• Earth.nullschool / visualization of surface winds

  • Wind speed shown by color (green faster, blue slower).

• Satellite cloud depiction via infrared imagery (cloud-top temperature)

  • Infrared images measure temperature, enabling visualization of cold cloud tops from tall cumulonimbus.
  • No daylight needed; filtering can emphasize tall clouds and suppress low-level cloud effects.

• Precipitation accumulation maps (e.g., “3 hours precipitation accumulation”)

  • Used to identify the ITCZ band of heavy convective rainfall vs. mid-latitude cyclone precipitation patterns.

• Mean sea level pressure (MSLP) overlay

  • Identifies low pressure along the equatorial ITCZ and high pressure bands near subtropics.

• Water vapor / multi-frame animation

  • Water vapor and cloud features evolve over time (frames every ~3 hours), showing persistence of ITCZ convection and the reformation of subtropical dry zones.

• Google Earth cloud layer

  • Used to visually inspect where tall clouds occur (ITCZ) and where they are suppressed (subtropical high regions).

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Precipitation and climate effects driven by circulation and topography

• Rainshadow effect (topography-driven precipitation asymmetry)

  • Mountains force air to rise → cooling → more precipitation on the windward side.
  • Air descending on the leeward side warms and dries → reduced precipitation.
  • Example focus: western U.S. mountain ranges (Olympics/Cascades/Sierra Nevada/Rockies).

• Real-world precipitation contrasts

  • Wet coastal windward slopes vs. dry interior leeward zones (including Nevada-like dryness).

• Local elevation effects (New Mexico example)

  • Albuquerque / Rio Grande Valley sits in a lower elevation trough, so storms tend to weaken as they approach (unless storms arrive from special directions).
  • This reduces local snowfall/rain relative to surrounding higher terrain.

• Why mid-latitude cyclones can bring moisture inland

  • Even if storms lose moisture crossing mountains, their low-pressure circulation can access new moisture sources (e.g., Gulf of Mexico, Atlantic).
  • This enables precipitation further inland (including winter storms and summer thunderstorms).

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Methodology / logical sequence described (as presented in the lecture)

1. Identify surface wind patterns
   • Locate easterlies/trade winds near the equator.
2. Determine interaction near the equator
   • Where trade winds converge → ITCZ.
3. Infer vertical motion
   • Convergence → air forced to rise.
4. Link rise to precipitation/cloud type
   • Rising warm/humid air → cumulonimbus → heavy rainfall along ITCZ.
5. Explain pressure and large-scale sinking
   • Rising air spreads aloft, then descends near ~30° → subtropical highs.
   • Sinking air suppresses clouds → dry zones/deserts.
6. Describe mid-latitude structure
   • Westerlies dominate mid-latitudes at the surface.
   • Jet stream appears at higher altitude, adjacent to the polar front.
   • Mid-latitude cyclones form along the polar front and influence precipitation bands.
7. Add topography
   • Mountains modulate where precipitation falls via orographic lifting and the rainshadow effect.

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Researchers or sources featured

• “Mel strong” (lecture instructor; stated at the start).