Case Study in
Conditional Symmetric
Instability
METR 4433: Mesoscale Meteorology
Spring 2008 Semester
Dr. Kelvin Droegemeier
School of Meteorology
University of Oklahoma
Definition and Attributes of CSI
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The combined effects of gravitational and inertial
forces leading to instability of a parcel displaced in a
slantwise direction
Conditional refers to an atmosphere near saturation
The slantwise displacement resulting from the
instability restores the atmosphere to neutrality
Banded precipitation often results from CSI owing to
the fact that the longitudinal instability occurs (i.e.,
rolls aligned with the shear or thermal wind)
– Bands typically range from 50-100 km wide and from 75-400
km long (slopes of 1:100)
– Typically last 1-3 hours and remain stationary
– Vertical velocities can be 10 m/s
Characteristics of CSI
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Occurs in regions where vertical wind profile
shows increasing speed and veering with
height (10-20 m/s over 1-2 km)
Occurs in the presence of low static stability
in the middle troposphere and a stable air
mass in the boundary-layer (north of a
surface frontal boundary)
Occurs near a warm front and ahead of an
upper-level trough
Occurs where the slope of the isentropes
(theta-e or s) are steeper (more vertical) than
the isopleths of absolute momentum (M)
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Note weak shortwave approaching area
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Shortwave reflected at sfc as inverted trough
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Atmosphere nearly saturated (good for CSI)
with strong low-level inversion
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NEXRAD VAD from Sterling, VA – note wind
veering with height and increasing in speed
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CSI often occurs in regions of frontogenesis and FG
reinforces the slantwise circulation and helps take it
to smaller scales
900-400 mb thickness
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CSI occurred between D-C, where strong stability is
present – and thus convection would not occur!