Title: Conditional Symmetric Instability CSI: Is it possible for anyone to really grasp what this is
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2Conditional Symmetric Instability -CSI
3CSI is a mechanism which creates mesoscale
banding of precipitation (not associated with
fronts)
4Courtesy of Dave Schultz (NSSL)
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6CSI works even in stabely stratified atmosphere
(dry or wet) how ??
G
G
d
T
7Recall 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
8Geostrophic balance
9The inertia circle motion
10The motion evolves into cycloids
max
11.V
V
V-Vg
Vg
12V
.V
V-Vg
Vg
13Vg
V-Vg
V
.V
14The motion evolves into cycloids
max
max
zero
15A 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
16A 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
17The geostrophic balance can be unstable if
L
meridional shear
PGF
Ug
H
18however, 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
19luckily we have vertical shear
20Recall 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
21The 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
22Symmetric 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
23Conditional 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).
24CSI - slope criterion
- (Ri/f) (f-dug/dy) Ri lt 1
- Ri stratification / shear
- satisfied in red sea trough ?
-
25Symmetric Instability
orange parcel trajectory
green acceleration (due to buoyancy)
26vertical cross-section
27Diagnosing 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)
28- 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.
29- 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 ?
30Conditional 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
31As 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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33NW
at least in the US
SE
34- 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
35Final 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
36Lets try it in our region !!!