Symmetric instability
1 / 19

Symmetric instability - PowerPoint PPT Presentation

  • Uploaded on

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)

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about ' Symmetric instability' - avram-carrillo

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.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Symmetric instability

Symmetric instability



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)

Symmetric instability outline
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)


What causes this precip?

N. Dakota

S. Dakota

Example 2

E. Nebraska





relative humidity (%)


PV min


pvor (thte,wnd)

Banded precipitation
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)

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


Emanuel (1983)



  • note: the condition for conditional instability:

  • dqe*/dz <0

Msi an intuitive explanation
MSI: an intuitive explanation inertially

M = absolute zonal momentum



M = fy-ug




see also: Jim Moore’s meted module on frontogenetic circulations & stability)

Potential inertially

Potential Symmetric INstability

Potential Symmetric Stability


Dash: qe

Solid: Mg










Where does MSI occur? inertially


Also important for the effective release of the instability: moisture

 Overlay RH

And frontogenesis … why?


Mapping PI, PSI, frontogenesis, and RH

900-700 mb


Characteristics of bands due to the release of MSI moisture

  • 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.

Bandedness vs msi
Bandedness vs MSI moisture

  • 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

Msi predictability
MSI predictability moisture

  • 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

Conclusion some words of caution about ci
Conclusion: some words of caution about CI moisture

  • 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