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ATM 612 Part 1: General Theme:

ATM 612 Part 1: General Theme:. ….Consider the evolution of convection in the absence of significant larger-scale forcing influences…or even boundary layer features…. The spectrum of convective storms and convective systems can largely be explained based on just two environmental parameters:

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ATM 612 Part 1: General Theme:

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  1. ATM 612 Part 1: General Theme: ….Consider the evolution of convection in the absence of significant larger-scale forcing influences…or even boundary layer features…. The spectrum of convective storms and convective systems can largely be explained based on just two environmental parameters: …..Buoyancy ….Vertical Wind Shear

  2. Archetypes: Building blocks of the observed spectrum Ordinary Cells:short lived (30-60 min), propagate with the mean wind Multicells:long-lived group of ordinary cells Supercells:quasi-steady, rotating, propagate right or left of the vertical wind shear vector

  3. Ordinary Cell:

  4. Multicell:

  5. Supercell:

  6. Physical processes controlling cell types: • Buoyancy processes: basic updraft/downdraft, (ordinary cells) • Gust front processes: triggering of new cells, upscale growth, (multicells) • Dynamic processes: rotating updraft, dynamic vertical pressure gradient forcing, (supercells)

  7. http://www.meted.ucar.edu/convectn/csmatrix/

  8. Buoyancy: What Goes Up……

  9. Must Come Down

  10. Ordinary Cell Evolution: …in the absence of vertical wind shear, controlled by buoyant processes

  11. What does it really look like?

  12. Kingsmill and Wakimoto 1991

  13. Buoyancy Force: Archimedes Principal: Buoyancy is simply the difference between the weight of a body and the fluid it displaces.

  14. Basic Equations: -- (Buoyancy) + ice….

  15. Basic Equations: (ignoring Coriolis, mixing) (Buoyancy)

  16. So, where does the buoyancy term come from? …linearize about a hydrostatic base state: …final form -precip loading …or -precip loading

  17. CAPE: Parcel Theory: Undilute parcel ascent curve …following a parcel, ignoring p’ CAPE = .5 Wmax = (2CAPE)

  18. So, what about pressure effects? **Impact of buoyancy is scale dependent! e.g., in the hydrostatic limit (large scales), the buoyancy force is balanced by the vertical perturbation pressure gradient, eliminating vertical accelerations…

  19. …real warm bubble in 3D simulation Small bubble produces stronger vertical motions than large bubble Buoyancy is Scale-Dependent!!!

  20. Diagnostic Pressure: Dynamic Pressure: Buoyancy Pressure: **For wavelike disturbances:

  21. The basic response of the pressure field to a warm bubble is to generate high pressure above the bubble and low pressure beneath Buoyancy is Scale-Dependent!!!

  22. Vertical Momentum Eq. (rewritten) (dynamic) + (buoyancy) …accounting for scale effects

  23. Basic 2D Equations: ⁄ -- Or, more simply, consider the 2D horizontal vorticity equation: where

  24. Buoyant Processes: Buoyancy is Scale-Dependent!!!

  25. DCAPE DCAPE = …following a parcel downward Wmin = (2DCAPE)

  26. Cold Pools: Density Currents

  27. Droegemeier and Wilhelmson, JAS, 1987 …2D …30 – 40 km …100 – 200 m You’ve all heard of “Kelvin” Helmholtz instability…????

  28. Shallow (Trapped) Wave-Like Disturbances Internal Bore of Wavelength Density Current • Gravity-wave related phenomena can be excited by antecedent convection • Statically stable nocturnal PBL provides an environment where such • disturbances can maintain coherence From Simpson (1997), An Introduction to Atmospheric Density Currents

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