1 / 19

Symmetric instability

Symmetric instability. Sources: Texts: Martin p. 224-228 Holton p. 277-281 Meted modules: homework assignment: an operational approach to slantwise convection : highly recommended (by Kent Johnson), 28 min Try the case exercise (location: BC)

Download Presentation

Symmetric instability

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Symmetric instability Sources: Texts: Martin p. 224-228 Holton p. 277-281 Meted modules: homework assignment: an operational approach to slantwise convection : highly recommended (by Kent Johnson), 28 min Try the case exercise (location: BC) listen to in class: heavy banded snow (by J. Moore), 34 min  This is a survey of conveyor belts, trowal, and (in section 3) symmetric instability CSI pitfalls: the use and misuse of CSI : more advanced (by David Schultz), 33 min Required reading material Real-time charts (PV and SI): Canadian maps (chart description)

  2. Symmetric instability outline • examples • static and inertial instability • SI as inertial instability on isentropic surfaces • basic state energy release in an SI exchange • a computational method to determine SI (PV)

  3. Example What causes this precip?

  4. N. Dakota S. Dakota

  5. Example 2 E. Nebraska

  6. Jet M PSI?

  7. relative humidity (%)

  8. Jet relative humidity (%) 0 PV min 0 pvor (thte,wnd)

  9. Banded precipitation • Single- and multiple-banded clouds and precipitation are common, esp. in frontal systems • They are often aligned with the thickness contours (thermal wind) and occur where they are tightly packed. • possible cause: symmetric instability with moisture (PSI/MSI/CSI) • PSI and frontogenesis commonly co-exit • PSI requires EPV<0 • frontogenetic circulation requires (geostrophic) PV>0 (ellipticity condition for Sawyer-Eliassen eqn) • SI is often ‘blamed’ a posteriori, it is not prognosed well (b/o inadequate model-resolution)

  10. But: MSI only occurs if the atmosphere is potentially and inertially stable • Condition for moist or potential SI (MSI): • e lines steeper than M lines or: dqe/dz < 0 along M lines or: dM/dx <0 along qe lines • or: equivalent PV(EPV) < 0 Rimoist Emanuel (1983) PV (EPV) Example: • note: the condition for conditional instability: • dqe*/dz <0

  11. MSI: an intuitive explanation M = absolute zonal momentum 30 40 M = fy-ug dM/dy>0 60 70 see also: Jim Moore’s meted module on frontogenetic circulations & stability)

  12. Potential Potential Symmetric INstability Potential Symmetric Stability - Dash: qe Solid: Mg - - - - - - - - -

  13. Where does MSI occur? EPVg

  14. Also important for the effective release of the instability: moisture  Overlay RH And frontogenesis … why? EPVg Mapping PI, PSI, frontogenesis, and RH 900-700 mb EPVg

  15. Characteristics of bands due to the release of MSI • Two-dimensional, aligned nearly along the thermal wind. • Condition for MSI is met in the region of the bands. • This region should be close to saturation. MSI by itself is not a sufficient condition for banded precip. MSI is ubiquitous, as is upright PI. We need qe to be close to qe*, or RH close to 100%. • Bands should move at the speed of the flow at the level of MSI, in the cross-band direction. • Spacing of bands is proportional to the depth of unstable layer/slope of moist isentropes. • Ascent should be nearly along the moist adiabats.

  16. Bandedness vs MSI • Byrd 1989: 27 events in OK-KS • 80% of banded cases had EPV<0 and high RH • Xu 1992: numerical study • Initial EPV anomaly small: •  Single band develops • Initial EPV anomaly larger: •  Multiple bands develop Frontal boundary

  17. MSI predictability • Bands ~ 5-40 km wide, spacing ~twice that much • grid spacings of at most 10 km are required to capture the most unstable MSI mode (Knight and Hobbs 1988, Persson and Warner 1993) •  mesoscale models, incl the current ETA (12 km), should be able to capture most MSI-induced circulations (as well as frontogenetical flow) • Even high-resolution models tend to underpredict the rainfall variability, and also the integrated amount of rainfall

  18. Conclusion: some words of caution about CI • 1. The existence of SI alone is not sufficient to initiate convection (need moisture) • 2. SI is not a forcing mechanism for slantwise ascent over a front (frontogenesis is … SI leads to slantwise convection within the frontogenetic circulation) • 3. The terms slantwise convection and SI are not interchangeable • 4. Upright convection always prevails over slantwise convection

More Related