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Application of theory to observed cases of orographically forced convective rainfall*

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.

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Application of theory to observed cases of orographically forced convective rainfall*

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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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