CSI, IGW and Frontogenesis: Other Causes of Mesoscale Bands in Winter Storms

CSI, IGW and Frontogenesis: Other causes of mesoscale bands in winter storms

MT417 – Iowa State University – Week 3 Bill Gallus Conditional Symmetric Instability (CSI) • CSI indicates an atmospheric state where slanted convection can develop if saturated air parcels are given a bit of lift • Frontogenesis is often the process that provides the lift to create mesoscale bands of heavier precipitation in regions where CSI exists • To understand CSI, we must examine other types of instability Convective Stability/Instability (conditional gravitational….) • This is the term used often on warm summer days to indicate that thunderstorms are possible • Gravity is the restoring force • You often evaluate it by using a Skew-T chart and looking to see if the lapse rate is steeper than a moist adiabat • Can also look at cross-sections to see if Theta-E decreases with height 330

320

310 stable

300 unstable Inertial stability/instability

• Don’t hear about this much in synoptic courses – usually in dynamics courses • Coriolis force is the restoring force • To understand it, use the momentum equations, which can be written as

• DV/Dt = -f k x Va • The equation shows us that if flow is supergeostrophic, acceleration is to the right toward high pressure to slow down V

Vg Va • If flow is subgeostrophic, acc. Is to right toward low pressure to speed up

Vg Va • Since flow is usually strongly out of the west, we define momenum (or geostrophic momentum) as

Mg = ug –fy

And we can plot momentum on a cross- section 30 50

North South

If we take the blue blob with 40 units of momentum and move it to the north, it is then supergeostrophic (in an environment with only 30ish units) and would accelerate south – back to where it started. This indicates inertially stable conditions Conditional Symmetric Instability/Stability • This is a type of baroclinic instability (requires a temperature gradient) • Combines what we know about the other two types of stability/instability • Formerly was often evaluated using cross- sections of momentum and Theta-E 290

280

50 270 40

North South

Now note that if we push the parcel horizontally, it is still inertially stable and would go back to its starting point 290

280

50 270 40

North South

And if we push the parcel verticaly, it is still gravitationally stable and would go back to its starting point 290

280

50 270 40

North South But if we push the parcel along a path where it conserves its momentum, it will find itself warmer than its environment (higher theta-E value) and would continue to accelerate along this slanted path – indicating instability. We call this CSI M, Theta-E surface evaluation of CSI • If the Theta-E lines slope more than the M- lines, CSI is present • If the M-lines slope more than the Theta-E lines, we have conditional symmetric stability • Think about what strongly sloped Theta-E lines mean ….. Strong horizontal temperature gradient (front may be nearby, frontogenesis is likely strong…) MPV/EPV evaluation of CSI

• In recent years, forecasters have begun using EPV (Equivalent Potential Vorticity) more often to evaluate CSI • If EPV < 0, the atmosphere must either be Convectively unstable CSI So to evaluate CSI, we want EPV < 0 AND d(Theta-E)/dz > 0 (to rule out convective instability) What is EPV?

• Absolute vorticity dotted with Theta-E gradient • Expansion of terms leads to:

• -dVg/Dz dθe/dx + dUg/dz dθe/dy + (ζ + f) dθe/dz • Remember, we want this negative without dθe/dz < 0 • Thus, the third term will always be positive since we can’t allow Theta-E to decrease with height, and absolute vorticity is almost always positive. • In first two terms, note that vertical shear of geostrophic wind appears. Remember – this is thermal wind, and we know it is related to horizontal temperature gradients • Thus, to make environment more favorable to have CSI, we want • Strong horizontal temperature gradients • Strong horizontal moisture gradients • Straight or anticyclonic isobar curvature (to keep absolute vorticity small) • Near-neutral stability (so dθe/dz is small) Internal Gravity Waves

• Another method of getting mesoscale bands of heavy precipitation is to have high-amplitude ducted (trapped) internal gravity waves occur • These form in very unbalanced situations • Can create spectacular pressure changes of 10 mb or so in 30 minutes, with thundersnow and 4 inch per hour rates Environments supporting IGWs

• Poleward of surface front • Beneath inflection point of upstream trof/downstream ridge • Jet streak moving through trof (favorable region often in right entrance quadrant of jet streak, and left exit quadrant of geostrophic jet max) • Low-level inversion to help trap (duct) the wave More rules…

• Critical level needed in near-neutral stable layer above inversion (strong wind shear in/near top of inversion) • Low-level flow is opposite to IGW motion Forecasting?

• Can use Lagrangian Rossby Number as indicator of IGW potential • RoL = |Va| / |V| >> 0 where Va is component of ageostrophic wind normal to height contours Inflection axis IGW?

L Jet