Conditional symmetric instability: Difference between revisions

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[[File:Northeast Snowfall Impact Scale.gif|thumb|[[Weather radar]] loop showing intense snow bands (lighter color) due to CSI ahead of a [[warm front]].]]

'''Conditional symmetric instability''', or '''CSI''', is a form of [[convective instability]] in a fluid subject to temperature differences in a uniform rotation [[frame of reference]] while it is thermally stable in the vertical and dynamically in the horizontal (inertial stability). The instability in this case develop only in an inclined plane with respect to the two axes mentioned and that is why it can give rise to a so-called "slantwise convection" if the air parcel is almost saturated and moved laterally and vertically in a CSI area. This concept is mainly used in meteorology to explain the mesoscale formation of intense [[Rainband|precipitation bands]] in an otherwise stable region, such as in front of a [[warm front]].<ref name="AMS">{{cite web | url= http://glossary.ametsoc.org/wiki/Conditional_symmetric_instability | title= Slantwise convection | publisher= [[American Meteorological Society]] | work= Meteorology Glossary | accessdate= August 23, 2019}}</ref><ref name="AMS2">{{cite web | url= http://glossary.ametsoc.org/wiki/Symmetric_instability | title= SymetricSymmetric instability | publisher= [[American Meteorological Society]] | work= Meteorology Glossary | accessdate= August 23, 2019}}</ref> The same phenomenon is also applicable to oceanography.

==Principle==

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===Slantise convection===

[[File:Instabilité symétrique conditionnelle.svg|thumb|Upward movement in an area of CSI gives clouds, downward movement clears the sky.]]

If a particle is climbing in ana CSI zone, it will cool down and the water vapor will condense upon saturation, giving cloud and precipitation by oblique convection. For example, in front of a warm front, the air mass is stable because the mild air overcomes a cold mass. The geostrophic equilibrium brings back any particle moving perpendicularly from the center of the depression towards it. However, an upwardly oblique displacement by [[synoptic scale]] upward acceleration in ana CSI layer produces parallel bands of heavy rainfall.<ref name="Schultz"/><ref>{{cite web | language = en | url= http://www.crh.noaa.gov/lmk/?n=paper-1/17/94 | title= Vertical Motion Forcing Mechanisms Responsible for the Production of a Mesoscale very heavy snow band across Northern Kentucky | publisher = [[National Weather Service]] | author= Theodore W. Funk | author2= James T. Moore}}</ref>

Conditional symmetric instability affects a layer that can be thin or very large in the vertical, similar to hydrostatic convection. The thickness of the layer determines the enhancement of convective [[precipitation]] within a region otherwise [[wikt:stratiform|stratiform]] clouds.<ref name=Schultz/> As the motion is in an area near saturation, the particle remains very close to the [[Lapse rate#Moist adiabatic lapse rate|moist adiabatic lapse rate]] which gives it a limited [[Convective available potential energy]] (CAPE). The rate of climb in a slantwise convection zone ranges from a few tens of centimeters per second to a few meters per second.<ref name=Schultz/> This is usually below the climbing speed limit in a [[cumulonimbus]], i.e. 5&nbsp;m/s, which gives [[lightning]] and limit the occurrence of it with CSI.<ref name=Schultz/> It is however possible in:<ref name=Schultz/>