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Conditional Symmetric Instability CSI: Is it possible for anyone to really grasp what this is

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Title: Conditional Symmetric Instability CSI: Is it possible for anyone to really grasp what this is


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Conditional Symmetric Instability -CSI
3
CSI is a mechanism which creates mesoscale
banding of precipitation (not associated with
fronts)
4
Courtesy of Dave Schultz (NSSL)
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CSI works even in stabely stratified atmosphere
(dry or wet) how ??
G
G
d
T
7
Recall potential temperature surfaces
For a stable configuration of potential
temperature surfaces (potential temperature
increases with height), parcels of air tend to
return to their original potential temperature
surface if perturbed vertically.
q 310 K
q 300 K
q 305 K
S
N
8
Geostrophic balance
9
The inertia circle motion
10
The motion evolves into cycloids
max
11
.V
V
V-Vg
Vg
12
V
.V
V-Vg
Vg
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Vg
V-Vg
V
.V
14
The motion evolves into cycloids
max
max
zero
15
A closer look at angular momentum surfaces

Mg 80 m/s
Mg 70 m/s
Mg 60 m/s
N
S
  • Its actually geostrophic absolute momentum (Mg)
    thats relevant, but Mg surfaces are just angular
    momentum surfaces Mg ug - fy

16
A closer look at angular momentum surfaces
For a stable configuration of angular momentum
surfaces, parcels of air tend to return to their
original angular momentum surface if perturbed
horizontally.
Mg 80 m/s
Mg 70 m/s
Mg 60 m/s
17
The geostrophic balance can be unstable if
L
meridional shear
PGF
Ug
H
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however, this is quite unlikely (and the
relative vorticity is negative)
N
L
H
Mg 80 m/s
S
Mg 70 m/s
Mg 60 m/s
N
S
  • Its actually geostrophic absolute momentum (Mg)
    thats relevant, but Mg surfaces are just angular
    momentum surfaces Mg ug - fy

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luckily we have vertical shear
20
Recall potential temperature surfaces
For a stable configuration of potential
temperature surfaces (potential temperature
increases with height), parcels of air tend to
return to their original potential temperature
surface if perturbed vertically.
q 310 K
q 300 K
q 305 K
S
N
21
The air parcel combina - CSI
Parcels are stable with respect to both vertical
and horizontal displacements, but parcels
displaced along certain slanted paths will
accelerate away from their initial positionthis
is called symmetric instability.
q 310 K
q 305 K
Mg 80 m/s
Mg 70 m/s
q 300 K
Mg 60 m/s
S
N
22
Symmetric Instability
  • Potential temperature surfaces must be sloped
    more steeply than angular momentum surfaces, and
    the displacement must have a slope between that
    of the angular momentum and potential temperature
    surfaces

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Conditional Symmetric Instability
  • In most operational meteorology applications, we
    are concerned with the presence of this
    instability in a saturated atmospherethus, we
    replace potential temperature surfaces with
    equivalent potential temperature surfaces (qe),
    and the instability becomes known as conditional
    symmetric instability (CSI).

24
CSI - slope criterion
  • (Ri/f) (f-dug/dy) Ri lt 1
  • Ri stratification / shear
  • satisfied in red sea trough ?

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Symmetric Instability
orange parcel trajectory
  • slantwise convection

green acceleration (due to buoyancy)
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vertical cross-section
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Diagnosing Potential Symmetric Instability (PSI)
and Conditional Symmetric Instability
(CSI) Construct a cross section taken normal to
the 850-300 mb thickness isopleths with the x
axis directed towards the warm air. In the
cross-sectional plane display isentropes of ?e/
?es and isopleths of absolute angular momentum
(Mg), defined as Mg vg fx , where vg is the
geostrophic wind component normal to the cross
section, f is the Coriolis parameter, and x is
the distance along the x axis. Note that
?e/?es tends to increase both upward (in
convectively/conditionally stable air) and along
the x axis (towards the warmer air) Mg tends to
increases both upward (as the normal wind
component increases with height) and along the x
axis (as x increases)
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  • Diagnosing PSI/CSI (cont.)
  • For PSI we use ?e and Mg surfaces.
  • For CSI we use ?es and Mg surfaces.
  • You should display relative humidity on the cross
    section since CSI requires near- saturated
    conditions (i.e., RH gt 80)
  • There should also be large scale vertical motion
    in order to realize the CSI.
  • Note when the ? e surfaces fold underneath
    themselves there is convective instability.

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  • CSI is perhaps most likely to arise in the cold
    sector east of intensifying cyclones where the
    potential temperature surfaces are steeply sloped
    (perhaps implying strong isentropic ascent) and
    the midtropospheric absolute vorticity is small
    (or the relative vorticity is negative)

Relevant to Sharav low ?
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Conditional Symmetric Instability Physical
Characteristics
  • Width of the bands is approximately 50-100 km
    length of the bands is approximately 100-400 km
    time scale of the bands is approximately 3-4 h
  • Typical CSI vertical motions are on the order of
    tens of cm s-1 to a few m s-1 and thus, usually
    DO NOT produce lightning/thunder (need gt 5 m s-1
    to produce lightning)
  • However, these mesoscale bands of precipitation
    can be intense and result in significantly higher
    rain/snow fall totals than the surrounding area
  • CSI can be assessed using a parameter called the
    Equivalent Potential Vorticity (EPV) values
    less than 0.25 PVU indicate regions susceptible
    to CSI

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As gravitational or symmetric stability
decreases, the horizontal scale (width) of a
precipitation band decreases while the intensity
(reflectivity) of the band increases. Multiple
bands become established in an unstable regime.
Thus, it is very important to look for CSI and
convectively unstable areas aloft besides just
frontogenesis.
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NW
at least in the US
SE
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  • Temporal Resolution CSI occurs over time scales
    of about 3-6 hours while current upper air data
    collection is every 12 hours
  • Spatial Resolution Precipitation bands are meso-
    scale with lengths of 100-200 km and widths of
    50-100 km

35
Final comments
  • CSI is favored to occur in regions of
  • High vertical wind shear
  • Weak absolute vorticity
  • Weak convective stability
  • High mean relative humidity
  • Large scale ascent

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Lets try it in our region !!!
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