CSI, IGW and Frontogenesis: Other causes of mesoscale bands in winter storms MT417 - Iowa State University - Week 3

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
    V

          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

                       40

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

    30

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

    30

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

    30

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