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Weather Phenomena and Rainfall Intensity: Case Studies and Analysis

Explore the application of theory to orographically forced convective rainfall with observed cases and key parameters influencing rain intensity and location. Case studies from the N. Big Thompson Flood in Colorado and Oahu Flood in Hawaii provide insights for weather analysis.

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Weather Phenomena and Rainfall Intensity: Case Studies and Analysis

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  1. Application of theory to observed cases of orographically forced convective rainfall* R. Rotunno NCAR** Boulder, CO M. M. Miglietta CNR/ISAC Italy *Miglietta & Rotunno (Mon Wea Rev, 2012) **NCAR sponsered by

  2. Dependence on external parameters of rainfall intensity/location over a simple 2D ridge* CAPE, CIN Bryan 3D Cloud Model (Bryan and Fritsch 2002 MWR) DCAPE LFC LCL *Miglietta and Rotunno (2009 JAS)

  3. Dependence on external parameters of rainfall intensity/location over a simple 2D ridge* CAPE, CIN Bryan 3D Cloud Model (Bryan and Fritsch 2002 MWR) DCAPE LFC LCL *Miglietta and Rotunno (2009 JAS)

  4. Three nondimensional parameters emerge as most important: • Triggering: 2. Orographic forcing: • Ratio of advective to convective time scale: *Miglietta and Rotunno (2009 JAS)

  5. Summary No steady orographic rain if: Ridge too short Wind weak, strong instability For steady orographic rain: / certain combinations of *Miglietta and Rotunno (2009 JAS)

  6. N Big Thompson Flood Colorado, 1976

  7. N Big Thompson Flood Colorado, 1976 low-level flow

  8. Big Thompson Flood Colorado, 1976 Caracena et al. (1979) 170mm in several hours

  9. Big Thompson Flood Simulation (2D) 10h t U 5h 0 0 x 30 km 10m/s mm/h

  10. Big Thompson Flood Simulation (2D) 10h Constant Wind t U 5h 0 0 x 30 km 10h Low-Level Wind Only t U 5h 10m/s 17m/s 0 mm/h

  11. Oahu Flood, Hawaii 1974

  12. Oahu Flood, Hawaii 1974 low-level flow

  13. Oahu Flood, Hawaii 1974 Schroeder (1977) 250mm in several hours

  14. Oahu Flood Simulations (2D) 10h t U 5h 0 0 x 30 km 10m/s mm/h

  15. Oahu Flood Simulations (2D) 10h Constant Wind t U 5h 0 0 x 30 km 10h Low-Level Wind Only t U 5h 10m/s 0 mm/h

  16. Conclusions Case-study simulations consistent / MR09 idealized analysis However…low-level wind decreasing with height doubles the rainrate and brings it closer to obs

  17. A HyMex Objective: High-Precipitation Events Ducrocq et al. (2008) Cevennes Gard Aude

  18. Summary No steady orographic rain if: Ridge too short Wind weak, strong instability For steady orographic rain: / certain combinations of (no upstream cold pool) But…low-CAPE is a special case Rainrate ~ independent of (upstream cold pool) Miglietta and Rotunno (2010)

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