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NATS 101 Lecture 12 Local Diurnal Winds Monsoons

NATS 101 Lecture 12 Local Diurnal Winds Monsoons. Atmospheric Scales of Motion. Ahrens, Fig 7.1. DIV. CON. Heat. Heat. Warm. Rising. Sinking. Cold. CON. DIV. Heat. Heat. Fundamental Concept Review: Thermally Direct Circulation.

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NATS 101 Lecture 12 Local Diurnal Winds Monsoons

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  1. NATS 101Lecture 12Local Diurnal WindsMonsoons

  2. Atmospheric Scales of Motion Ahrens, Fig 7.1

  3. DIV CON Heat Heat Warm Rising Sinking Cold CON DIV Heat Heat Fundamental Concept Review:Thermally Direct Circulation

  4. Sea Breeze Development(Courtesy of Mohan Ramamurthy, WW2010) 1 2 3 4

  5. DIV CON Rising Sinking CON DIV Heat Heat Sea Breeze Development(Courtesy of Mohan Ramamurthy, WW2010) 5 6 7

  6. Land-Sea Breeze • Land is colder than ocean at night • Land-sea temp contrast reverses during day • Wind forced by seasonal changes in PGFHigher SLP over land at nightOffshore flow at Surface Lower SLP over land during dayOnshore flow at Surface

  7. Land-Sea Breeze • Onshore flow leads to surface convergence Rising motion over land during summer • Offshore flow leads to surface divergence Sinking motion over land during winter • Land-Sea Breeze is Thermally Direct Circulation Warm Air Rises - Cold Air Sinks

  8. Sea Breeze versus Land Breeze (Courtesy of Mohan Ramamurthy, WW2010) PM Stronger Temperature contrast during PM than during AM Sea breezes are stronger than land breezes AM LAX Airport 4 PM upper 7 AM lower

  9. Sea Breeze • Regular feature of many coastal areas California, Florida, Gulf Coast • Occurs along large lakes-Great Lakes • Typically strongest during Spring-Summer • Can penetrate inland 50 km or more • Temperatures can drop ~10oC • Nose of cool air can trigger thunderstorms Florida Satellite Loop

  10. DIV CON Rising Sinking CON DIV Heat Heat Mountain-Valley Breeze Sun warms slopes Density decreases Air rises IR loss cools slopes Density increases Air drains constant height Ahrens, Older Ed. Mountain-Valley circulation important to Tucson Convection over Catalinas during PM summer. SE drainage flows during early AM all year.

  11. Mountain-Valley Breeze • Land is colder than free atmosphere at night • Temperature contrast reverses during day • Wind forced by seasonal changes in PGFHigher SLP over mtn slope at nightDownslope flow Lower SLP along mtn slope during dayUpslope flow

  12. Mountain-Valley Breeze • Upslope flow leads to convergence Rising on mountain crest during day • Downslope flow leads to divergence Sinking over mountain crest at night • Mountain-Valley Breeze is Thermally Direct Circulation Warm Air Rises - Cold Air Sinks

  13. PM heating AM cooling 5 AM 5 PM 5 PM 5 AM PM heating AM cooling AM TUS PM Phoenix-Tucson Diurnal Winds TUS AM PHX PHX PM

  14. Summary: Diurnal Winds • Land-Sea Breeze Due to differential heating between land-sea Gives diurnal reversal in temperature contrast Onshore winds PM - Offshore winds AM Sea Breeze PM - Land Breeze AM • Mountain-Valley Breeze Differential heating along mountain slopes Gives diurnal reversal in temperature contrast Upslope winds PM - Downslope winds AM Valley Breeze PM - Mountain Breeze AM

  15. DIV CON Rising Sinking CON DIV Heat Heat Review Land-Sea Breeze(Courtesy of Mohan Ramamurthy, WW2010) 5 6 Due to differential heating between land-sea Gives diurnal reversal in temperature contrast Onshore winds PM - Offshore winds AM Sea Breeze PM - Land Breeze AM

  16. DIV CON Rising Sinking CON DIV Heat Heat Review: Mountain-Valley Breeze Sun warms slopes Density decreases Air rises IR cools slopes Density increases Air drains Ahrens, Older Ed. Differential heating along mountain slopes Gives diurnal reversal in temperature contrast Upslope winds PM - Downslope winds AM Valley Breeze PM - Mountain Breeze AM

  17. DIV CON Heat Heat Warm Rising Sinking Cold CON DIV Heat Heat One More Time: Thermally Direct Circulation Less Dense More Dense

  18. Monsoon • Seasonal Reversal of Prevailing Wind Wind shift often accompanied by Major Change in Weather Summer Rains - Often Abrupt Onset Winter Dryness Major Monsoon occurs over Asia Weaker Monsoon occurs in North America

  19. Monsoon • Land mass is colder than ocean in winter • Land-sea temp contrast reverses in summer • Wind forced by seasonal changes in PGFHigher SLP over land in winterOffshore flow at Surface Lower SLP over land in summerOnshore flow at Surface

  20. Monsoon • Onshore flow leads to surface convergence Rising motion over land during summer • Offshore flow leads to surface divergence Sinking motion over land during winter • Monsoon is Thermally Direct Circulation Warm Air Rises - Cold Air Sinks

  21. JANUARY Ahrens Fig 7.17a

  22. JULY Ahrens Fig 7.17b

  23. Asian Winter H Cherrapunji offshore flow Aguado & Burt Fig 8-17

  24. Asian Summer L Cherrapunji onshore flow Aguado & Burt Fig 8-17

  25. Monthly Average RainCherrapunji Aguado & Burt Fig 8-18

  26. January COLD offshore

  27. July HOT onshore

  28. Geography of Region Douglas et al (1993)

  29. Terrain • Terrain (300 m) • Steep slopes of Sierra Madre Occidental • Warm Waters 20oC water High mountains Steep western slope Sea of Cortez warm Pacific cool 30oC water Mountain Barrier Stensrud et al (1995)

  30. Douglas et al (1993) Monthly Rainfall Mexican Monsoon Similar onset Similar behavior but… Much less intense Wimpy Compared to Asian Monsoon

  31. Percentage of Annual Rainfall Accounts for up to 70% total rain in monsoon core Tucson ~50% Phoenix ~40% CORE Douglas et al (1993)

  32. CCT < -38oC Frequency Proxy for Deep Cbs Centered over W. Mexico June start over Mexico AZ at northern fringes of heart of monsoon Rains reach SE Arizona by July Monsoon Evolution from Satellite June July August Douglas et al (1993)

  33. June-July 500 mb Flow Dry Dry Moist Moist Flow at 500 mb Switches from Westerly to Easterly! Douglas et al (1993)

  34. July 900 mb Flow Mountains block flow of low-level moisture from Gulf of Mexico tropical moisture Douglas et al (1993)

  35. Diurnal Winds 450 m AGL 5 PM LST 5 AM LST AZ moisture source upslope/onshore Stensrud et al (1995)

  36. Summary • Monsoons Differential Heating Between Land and Oceans Seasonal Reversal of Wind Summer Rain - Winter Aridity Thermally-Direct Circulation • Regions Major Monsoon Occurs over SE Asia Weaker Monsoon Occurs over North America Africa, Australia, South America in SH

  37. Assignment • Topic- Global Scale Winds General Circulation • Reading -Ahrens 181-189, 343-349 • Problems -7.11, 7.12, 7.13

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