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 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)

Example What causes this precip?

N. Dakota S. Dakota

Example 2 E. Nebraska

Jet M PSI?

relative humidity (%)

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

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)

Example: 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 But: MSI only occurs if the atmosphere is potentially and inertially stable Rimoist Emanuel (1983) PV (EPV) Example: note: the condition for conditional instability: dqe*/dz <0

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)

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

Where does MSI occur? EPVg

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

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 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 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 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