Symmetric instability
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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)

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Symmetric instability

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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)

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)


Symmetric instability

Example

What causes this precip?


Symmetric instability

N. Dakota

S. Dakota


Symmetric instability

Example 2

E. Nebraska


Symmetric instability

Jet

M

PSI?


Symmetric instability

relative humidity (%)


Symmetric instability

Jet

relative humidity (%)

0

PV min

0

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)


Symmetric instability

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


Msi an intuitive explanation

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)


Symmetric instability

Potential

Potential Symmetric INstability

Potential Symmetric Stability

-

Dash: qe

Solid: Mg

-

-

-

-

-

-

-

-

-


Symmetric instability

Where does MSI occur?

EPVg


Symmetric instability

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


Symmetric instability

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.


Bandedness vs msi

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


Msi predictability

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


Conclusion some words of caution about ci

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


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