Conditional symmetric instability: Difference between revisions

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::* '''If <math>\theta_e</math> diminish with altitude leads to unstable airmass'''

::* '''If <math>\theta_e</math> remains the same with altitude leads to neutral airmass'''

::* '''If <math>\theta_e</math> increase with altitude leads to stable airmass.'''

===Inertial Stability===

[[File:Tourbillon total faible.png|thumb|Dark zones are regions of weak inertial stability in atmopshericatmospheric circulation.]]

In the same way, a lateral displacement of an air particle changes its absolute vorticity <math>\eta</math>. This is given by the sum of the planetary vorticity, <math>f</math>, and <math>\zeta</math>, the [[Geostrophic wind|geostrophic]] (or relative) vorticity of the parcel:<ref name="Doswell"/><ref name="MF-2">{{cite web|language=French | url= http://www.meteofrance.fr/publications/glossaire?articleId=152385 | title= Instabilité barocline | publisher= [[Météo-France]] | accessdate= August 23, 2019 | work= Glossaire météorologique }}</ref>

<{{center>|<math>\eta= \left[ \frac{\partial v}{\partial x} - \frac{\partial u}{\partial y} \right ] + f = \zeta + f \qquad \qquad </math></center>}}

Where :

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<math>\eta</math> can be positive, null or negative depending on the conditions in which the move is made. As the absolute vortex is almost always positive on the [[synoptic scale]], one can consider that the atmosphere is generally stable for lateral movement. Inertial stability is low only when <math>\eta</math> is close to zero. Since <math>f</math> is always positive, <math>\eta \le 0 </math> can be satisfied only on the anticyclonic side of a strong maximum of [[jet stream]] or in a [[Ridge (meteorology)|barometric ridge]] at altitude, where the derivative velocities in the direction of displacement in the equation give a significant negative value.<ref name="Moore" />

The variation of the [[angular momentum]] indicate the stability:<ref name="Doswell"/><ref name="Moore">{{cite web | language= en | format= ppt | url= http://www.comet.ucar.edu/class/rfc_hydromet/03-1/docs/Moore/Mesoinstab/Meso-proc.ppt | author= James T. Moore | title= Mesoscale Processes | publisher= [[University Corporation for Atmospheric Research|UCAR]] | accessdate= August 23, 2019 | date= 2001 | pages= 10–53 | archive-url= https://web.archive.org/web/20141221040317/http://www.comet.ucar.edu/class/rfc_hydromet/03-1/docs/Moore/Mesoinstab/Meso-proc.ppt | archive-date= December 21, 2014 | url-status= dead }}</ref><ref name=Schultz>{{cite journal | language = en | title= The Use and Misuse of Conditional Symmetric Instability | first1= David M. | last1 = Schultz | first2= Philip N. | last2= Schumacher | journal = [[Monthly Weather Review]] | volume = 127 | issue = 12 | pages = 2709 | date = December 1999 | doi = 10.1175/1520-0493(1999)127<2709:TUAMOC>2.0.CO;2| publisher= [[American Meteorological Society|AMS]] | s2cid= 708227 | issn = 1520-0493| doi-access = free }}</ref>

*<math>\Delta M_g = 0 </math>, the particle then remains at the new position because its momentum has not changed

*<math>\Delta M_g > 0 </math>, the particle returns to its original position because its momentum is greater than that of the environment

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;Lateral movement A

Horizontal accelerations (to the left or right of a surface <math> \scriptstyle M_g </math>) are due to an increase/decrease in the <math> \scriptstyle M_g </math> of the environment in which the particle moves. In these cases, the particle accelerates or slows down to adjust to its new environment. Particule A undergoes a horizontal acceleration that gives it positive [[buoyancy]] as it moves to colder air and decelerates as it moves to a region of smaller <math> \scriptstyle M_g </math>. The particle rises and eventually becomes colder than its new environment. At this point, sheit has negative buoyancy and begins to descend. In doing so, <math> \scriptstyle M_g </math> increases and the particle returns to its original position.<ref name = "Moore"/><ref name=Schultz/>

;Vertical displacement B

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==Potential effects==

[[File:Snowcsi.gif|thumb|Zones of CSI (solid blue) and banded snow (dash green) along dethe warm front, near the low pressure area.]]

CSI is usually embedded in large areas of vertical upward motion. The ideal situation is a geostrophic flow from the South with wind speeds that increase with height. The environment is well mixed and close to saturation. Since the flow is unidirectional, the u component of the wind can be set equal to zero, which establishes a symmetrical flow perpendicular to the temperature gradient in the air mass. This type of flow is typically found in baroclinic atmospheres with cold air to the west.<ref name=Schultz/>

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{{clr}}

===SlantiseSlantwise 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 a 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 a 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>