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Supercell storms

Supercell storms. Storm splitting, environmental conditions, propagation. Ordinary/multicell storm. Browning et al. (1976). Fovell and Tan (1998). Supercell (rotating) storm. Plan view at surface UD = updraft FFD = forward flank downdraft RFD = rear flank downdraft Gust front Hook echo

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Supercell storms

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  1. Supercell storms Storm splitting, environmental conditions, propagation

  2. Ordinary/multicell storm Browning et al. (1976)

  3. Fovell and Tan (1998)

  4. Supercell (rotating) storm Plan view at surface UD = updraft FFD = forward flank downdraft RFD = rear flank downdraft Gust front Hook echo Dot ~ tornado location Lemon and Doswell (1979)

  5. Beneath a rotating supercell Made using ARPS and Vis5D

  6. Animation (delcity_orig_3Dvort.MOV)

  7. Storm splitting Wilhelmson and Klemp (1981)

  8. Storm splitting (delcity_str_Vprt_QR.MOV) Two symmetric storms result from initial impulse Yields right & left movers Note rotation Domain is moving to east; storm motion subtracted Made w/ ARPS and Vis5D

  9. 24 April 2007 splitting storm(as seen from Laughlin AFB radar in SW Texas)

  10. Split L mover weaker R mover turns right R mover hook Purple = hail

  11. Tornado report Maverick, TX

  12. Dynamics of storm splitting Klemp (1987); See Houze p. 288-295

  13. Shear-induced vortex tubes…

  14. …leads to splitting and rotation Lifting of vortex tubes by updrafts create counter-rotating horizontal vortices… vertical vorticity Low pressure in the vortices establish new updrafts on original storm’s flanks The original storm decays… and voila, the storm appears to have split, yielding counter-rotating storms Note now L pressure and updraft are colocated… shear has contributed to storm stabilization

  15. Splitting – step I

  16. Splitting – step 2

  17. Splitting – step 3

  18. Splitting – step 4

  19. Precipitation is NOT involved Simulations without rain still show storm splitting

  20. Close look at two papers • Weisman and Klemp (1982) • Environmental conditions favoring ordinary/multicell storms and supercell storms • Rotunno and Klemp (1982) • Why right-movers are favored in U.S. • Why right-movers move to right of mean winds

  21. Weisman and Klemp (1982; “WK”) • “Dependence of Numerically Simulated Convective Storms on Vertical Wind Shear and Buoyancy” • 3D simulations, started with thermal • Varied CAPE by manipulating boundary layer moisture • Considered speed shear only see Houze, p. 284-288

  22. CAPE • Convective Available Potential Energy • “positive area” of a sounding • Integrated from LFC to EQL • WK and Houze (eq. 8.1, p. 283) expressions involve potential temperature • Should involve virtual potential temperature

  23. WK’s sounding CAPE varied by setting PBL moisture Standard case has 14 g/kg

  24. WK wind profiles Us = wind speed change over 10 km Note: no directional shear

  25. WK Wmax vs. time q = 14 g/kg simulations No shear case: intensifies fastest dies first Larger shear - slower development By this metric, moderate shear storm is strongest

  26. WK simulations have symmetry axis No initial flow in N-S direction; Initial bubble is radially symmetric. Therefore, storms have N-S symmetry

  27. WK results multicellular supercellular

  28. Us = 30 m/s, q = 14 g/kg Surface rainwater (g/kg) and surface positive vorticity (1/s, dashed) Low-level radar reflectivity field BWER = Bounded weak echo region

  29. WK response to initial bubble CAPE

  30. “Secondary storms” are Subsequent ordinary cell developments (i.e., multicells) “Split storms” are supercells, resulting from splitting of initial cell

  31. WK analysis “Bulk Richardson number” ∆u = avg. wind of lowest 6 km minus avg. wind of lowest 500 m “Storm strength” Wmax = largest updraft Denominator = theoretical updraft max ~ ignores pressure perturbation!

  32. WK analysis Larger BRi = initial storm stronger • more CAPE, or • less shear Largest is ~ 60% of theoretical max Multicells favored for BRi > 30 or so… Supercells • smaller BRi • relatively larger S R = BRi

  33. BRi as predictor of storm mode

  34. Concerns w/ BRi • CAPE • As formulated in WK (and Houze’s eq. 8.1), moisture in environment neglected (apart from parcel definition) • Water loading, entrainment neglected • Convective inhibition (CIN) completely ignored • Shear • Relevant depth 6 km? Always? • Directional shear completely ignored • Uniqueness of BRi • Same value for low CAPE/low shear as high CAPE/high shear. Reasonable to expect same convective mode and/or intensity? • This has been a very valuable result

  35. Rotunno and Klemp (1982; “RK”) • Most tornadoes associated with supercells • tornadoes have same sense of rotation as parent storm • Further, vast majority of tornadoes are CCW rotating • after splitting, CW left movers tend to die (and/or be non-tornadic) • So why are left movers disfavored? • it’s directional vertical shear

  36. RK hodograph

  37. Straight vs. curved hodograph Note mirror-image storms; both have hook-like echoes Note only right-mover has hook shape; left-mover weaker

  38. Straight vs. curved hodograph W at 1.5 km Without directional shear, symmetric pulse always remains symmetric With directional shear, right mover soon favored

  39. Pressure decomposition • Recall nondimensional pressure perturbation π’ subdivided into dynamic and buoyancy pressures • Dynamic pressure can be further partitioned into linear (L) and nonlinear (NL) pressures using perturbation analysis NOTE the overbars… the base state winds

  40. π’ caused by vertical shear π’ - heavy contours  - thin contours Vertical shear vector shown X-direction:

  41. Propagation to left & right On flanks, both CW & CCW cause L pressure where vort is max, min L L Net result… L pressure on flanks + L pressure ahead lead to propagation to left and right of mean winds

  42. Unidirectional shear case • Vertical accel largest on E (forward) side… storm moves E • Nonlinear p’ encourages ascent on flanks -- storm splitting • Result symmetric (neither split storm member is favored) Shear vector shown

  43. Directional shear case • RK’s analysis shows that pNL and buoy favor the left mover. • But pLfavors the right mover and dominates

  44. • Hodograph with directional shear. Thin shear vector used for lower-level shear. • Circle indicates updraft. • Linear term encourages low pressure downshear; high pressure upshear • For lower-level shear, lowered pressure to NE

  45. • Upper level shear produces pressure perturbations in different places • On south side of updraft there is lowered pressure aloft overlying raised pressure below. This flank is favored for ascent.

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