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Impact of the Andes Cordillera on a mid-latitude cold front

Impact of the Andes Cordillera on a mid-latitude cold front. Bradford S. Barrett Department of Oceanography US Naval Academy 19 Aug 2009. What exactly is the Andean topography like?. Mean southeast Pacific winter storm track: ~90% of annual precipitation in central Chile. L.

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Impact of the Andes Cordillera on a mid-latitude cold front

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  1. Impact of the Andes Cordillera on a mid-latitude cold front Bradford S. Barrett Department of Oceanography US Naval Academy 19 Aug 2009

  2. What exactly is the Andean topography like?

  3. Mean southeast Pacific winter storm track: ~90% of annual precipitation in central Chile L

  4. Inverse relationship between gradients of terrain (brown) and annual precipitation (green)over central Chile. 5000 1000 1500 100 Terrain height (m)‏ Annual rainfall (mm)‏ 30°S 40°S

  5. Precipitation differences are noticeable!

  6. May 925 hPa winds Southeast Pacific anticyclone in May Interested in area at ~35°S nearly zonal westerly flow at mid-levels intersects continent

  7. Mendoza

  8. Simulate passage of a “typical” baroclinic zone in Central Chile • Use WRF model as a scientific tool to understand processes that occur during passage & investigate role of topography • Event summary for Santiago: • 30 hours of rainfall • 45 mm total (15% of annual total)‏ • Pronounced frontal passage • Temperature drop of 10°F in 6 hrs • Specific humidity decrease from 13 g/kg to 8 g/kg • 5mb pressure rise (949 hPa to 954 hPa)

  9. Study description: • Use WRF model as scientific tool • Mesoscale horizontal resolution • Investigate the processes that occur during passage of a typical cold front • Flow blocking • Precip timing & distribution • Examine sensitivity of model to terrain Domain 1: 10 km Δx L

  10. Three main precip regions: • Oceanic cold front (OCF) • Coastal cold front (CCF) • Orographic (OP)

  11. Result of flow blocking: • Northerly barrier jet • Extended 250 km west of the mountain crest • Centered between 875 and 950 hPa • Located above and north of surface cold front (northerly flow reaches surface) • Transported moisture from moist subtropical marine PBL

  12. Summary of control simulation • Typical baroclinic zone passage • 30 hrs steady rainfall in Santiago, followed by pronounced airmass change • Three precipitation regimes identified: • Oceanic, coastal, orographic • Forcing for precipitation strongest from convergence & orography • Flow below 700 hPa effectively blocked Now revisit the role of topography…

  13. Experiment: LOW 20% topography 10 km Δx L

  14. Compare precipitation in the full-terrain(CTL) simulation with precipitation in the 20% terrain (LOW) simulation Total 72-hr precip: LOW terrain Total 72-hr precip: CTL terrain

  15. Examine frontal timing in cross-sections of precipitation & wind: • East-west through 33°S • North-south along the coast

  16. Time-longitude (east-west) cross-sections along 33°S. LOW CTL • Long period of orographic precip over cordillera in CTL (left panel) • Front is easily identified from precip (red line) • Faster progression in LOW (right panel) Time

  17. North-South cross-sections along coastTime increases from bottom to top. LOW precip CTL precip LOW 925mb V-wind CTL 925mb V-wind

  18. High topography blocks low- and mid-tropospheric flow, resulting in northerly barrier jet • Barrier jet: • slows the progression of the cold frontal surface • increases frontal convergence over the frontal surface • Net result is heavy rainfall over coastal and valley regions of central Chile.

  19. Conclusions • Effect of high terrain: • Blocks low- and mid-level flow • Northerly barrier jet develops • Increases total amount of precipitation over Chilean coast and Cordillera • More convergence & orographic lift • Reduces northward speed of typical cold front • Longer sustained convergence • Result: • Precipitation totals over central valley of Chile 5x-15x greater than without mountains • Connection to longer-scale precip pattern: stay tuned!

  20. Thank you!Questions?

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