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Mesoscale Processes Contributing to Extreme Rainfall in a Midlatitude Warm-Season Flash Flood

This report discusses the mesoscale processes that contribute to extreme rainfall in a midlatitude warm-season flash flood event in the eastern US. It includes data analysis from radar observations, surface observations, and convection-permitting simulations.

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Mesoscale Processes Contributing to Extreme Rainfall in a Midlatitude Warm-Season Flash Flood

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  1. Mesoscale Processes Contributing to Extreme Rainfall in a Midlatitude Warm-SeasonFlash Flood Reporter: Prudence Chien Reference: Schumacher, R. S., and R. H. Johnson, 2008: Mesoscale Processes Contributing to Extreme Rainfall in a Midlatitude Warm-Season Flash Flood. Mon. Wea. Rev., 136, 3964-3986.

  2. Introduction • In eastern US, 74% warm-season extreme rainfall events were associated with MCSs. • Back-building/ quasi-stationary MCS Schumacher and Johnson (2005, 2006)

  3. Introduction • Case period: 6-7 May 2002 • Southwest of St. Louis, Missourimetropolitan area • A cluster of quasi-stationary convection produced over 300mm of rain in 9h. Rain gauge obs. 1200UTC 6 May – 1200UTC 7 May

  4. Description of the event • Data: • Rapid Update Cycle (RUC) analysesHorizontal grid space = 40km40 vertical levels • NOWrad : WSI radar mosaic product Horizontal grid space = 2kmTime interval = 15 mininutes

  5. Description of the event - MCV Shaded: η MCV MAUL PVmax MAUL: moist absolutely unstable layer

  6. Description of the event - MCV MCV MCV Shaded: CAPE (J/kg) Short barb: 850-hPa wind Solid line: isotachs (m/s)

  7. Description of the event • Radar obs. Mature stage

  8. Description of the event • Surface obs. Deep convection developed & intensified Pressure ridge Weak cold dome => Sfc cool pool was limited Solid: pressure adjusted to sea level Dashed line: sfc temperature

  9. Model configuration • WRF-ARW v2.2 • Period: 0000UTC 7 May - 0000UTC 8 May 2002 • 1KM / 3KM (NOLATENT & NOEVAP)

  10. Results • Overall structure of convection and precipitation in convection-permitting simulations • Initial of convection • Organization and maintenance of convective line • Illustration of low-level waves in idealized simulations • Development of surface low pressure and reintensification of MCV

  11. Preci.max = 309mm (obs.) Preci.max = 261mm (1KM-sim.)

  12. Results • Overall structure of convection and precipitation in convection-permitting simulations • Initial of convection • Organization and maintenance of convective line • Illustration of low-level waves in idealized simulations • Development of surface low pressure and reintensification of MCV

  13. Potential instability

  14. Results • Overall structure of convection and precipitation in convection-permitting simulations • Initial of convection • Organization and maintenance of convective line • Illustration of low-level waves in idealized simulations • Development of surface low pressure and reintensification of MCV

  15. Two mechanisms for the initiation of upstream convection: • A long-distance mechanism:Upstream parcels are lifted to their LFC by MCV-related lifting and then slowly approach the ongoing deep convection. • A “short-distance” mechanism:New cells are initiated much more quickly in close proximity to the mature convective system. Shaded: w Solid line: Div. Dashed line: Conv.

  16. Contour: 7-km vertical velocity Shaded: 0.75-km vertocal velocity

  17. Dot line: Wind speed in the direction of wave propagation TROF: surface pressure trough Shaded: Div. Surface pressure and the wind in the direction of wave propagation vary exactly in phase.

  18. Results • Overall structure of convection and precipitation in convection-permitting simulations • Initial of convection • Organization and maintenance of convective line • Illustration of low-level waves in idealized simulations • Development of surface low pressure and reintensification of MCV

  19. Results • Overall structure of convection and precipitation in convection-permitting simulations • Initial of convection • Organization and maintenance of convective line • Illustration of low-level waves in idealized simulations • Development of surface low pressure and reintensification of MCV

  20. Convergence associated with the gravity wave was most responsible for the linear organization of the convection.

  21. The diabatic heating associated with the convective system also served to reintensify the MCV.

  22. Conclusions Midlevel MCV

  23. Thanks for your listening. & Questions?

  24. Description of the event • Cutoff Low -> MCV Shaded: η

  25. Results (a.) – radar sim. Obs.

  26. Results (c.) – FIG14

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