Conditional Symmetric Instability---->

[RNS]

What is Conditional Symmetric Instability:

Introduction:

With winter fast approaching, and frequent discussion of potential storms increasing by the day, one may wonder what the processes responsible for the development of heavy banded snowfall are, and how they can be predicted. That said, one of the most important factors that forecasters assess on a daily basis before forming any predictions is atmospheric stability. I’m sure most of you here are aware that atmospheric stability plays a big role in forecasting during the warm season (summer), as a stable environment would not be conducive for convective initiation. The role of atmospheric stability is equally important when forecasting for a winter storm.

In winter, one main feature associated with heavy snowfall is Conditional Symmetric Instability or CSI for short. CSI and CI (convective Instability) when present in a winter storm can make the difference between precipitation type and precipitation intensity, thus correct analysis of these factors is essential in making accurate snowfall predictions, and deciding precipitation type in a given location. CSI is manifested in the development of several narrow (mesoscale (generally 30-300 KM) cloud bands (containing enhanced precipitation) in areas which are stable to normal upright convection.

CSI is a very hard factor to correctly analyze, as the mesoscale banding features particular to it are too small to be resolved by todays numerical models. For this reason, forecasters must strive to unlock the developmental process behind CSI which make the development of these mesoscale banding features possible. In other words…CSI predictors can be accurately analyzed by most models; however I would recommend the use of a mesoscale model (such as the Meso-ETA) to better analyze the location and strength of these features. This is because the grid scales of many of these models are too large to resolve the small banded features consistent with CSI.

I will try to keep this as simple and mathematically free as possible, so that one can get an idea of the basic mechanisms responsible for banded snowfall, and how to assess those factors in as basic a way as possible. Much of the information herein is taken from a case study on the effect CSI on the January 6 2002 convective snow event, which I did two years ago following that event. Other sources are sited as well.

SI (Symmetric Instability) theory:

SI is based on the concept of symmetric flow, and the assessment of the stability of that flow. “A symmetric flow is one whose basic state and perturbations are independent of a horizontal coordinate” such as an X axis - (Bluestien et al 1993). This can be visualized by assuming that an air parcel takes the form of an infinitely long tube (having no beginning and no end) extending the length of the X axis in both directions. In the parcel theory of SI, this tube is displaced to see whether or not it:

A. Is forced back to its original state before the displacement or - >
B. Becomes positively buoyant in the direction of the slantwise displacement.

The restoring forcing on the parcel is examined to determine if the parcel is unstable to slantwise displacement. If the parcel becomes positively buoyant in the direction of the displacement, then the parcel is considered unstable to slantwise displacement, indicating the presence of symmetric instability. SI relates the effects of static and inertial (gravitational and horizontal) stabilities resulted from the effects of forcing on a parcel.

Assessment of (static and inertial) stability:

Static stability describes the atmospheres ability to resist vertical displacements. The vertical buoyancy of a parcel therefore results in the vertical mixing of heat and moisture. Inertial stability on the other hand reflects the atmospheres resistance to horizontal displacement. Imbalances in the horizontal wind field can result in lateral mixing of momentum, thus to achieve SI, the environment must be both statically and intertially stable. The large-scale (synoptic) environment can largely be described in terms of inertial stability. Inertial instability exists when the absolute vorticity of the geostrophic flow is below zero, which since the earth natural vorticity is positive (as most of you know) in most cases the atmosphere will be inertially stable. However, that said, there are exceptions. It should also be noted that the conditions above can be achieved on the anti-cyclonic side of a strong speed (wind) maximum.

When the absolute vorticity of the geostrophic flow falls below zero, inertial instability automatically develops in response to the condition. This reaction allows air to horizontality mix, decreasing shear until the absolute vorticity of the flow is once again positive or inertially stable. Inertially based gravity waves at times can form in response to this as well, possibly lifting parcels to the LCL (Lifted Condensation Level) and developing precipitation. If the atmosphere is not inertially stable…then the development of precipitation bands induced by inertial gravity waves cannot be ruled out. Low positive inertial stability (Absolute vorticity of the flow is greater than zero…but only by a slight margin) would still qualify as a favorable environment for the existence of CSI given sufficient frontogenesis below the symmetrically unstable layer (see frontogenesis section)

CSI (Conditional Symmetric Instability):

Just like a car, one must have a key in order to turn it on. CSI is similar in the respect that it requires a firing mechanism, to understand what the mechanism is and how it works one seeks to understand the implications of moisture in a symmetrically unstable environment.

Symmetric instability can be present in any environment regardless of moisture, but in order for CSI to exist, one needs moisture. This would therefore imply that the Condition in CSI is the presence of moisture within the symmetrically unstable environment. Precipitation bands will develop within the symmetrically unstable environment IF there is sufficient saturation, if there is NOT sufficient saturation, banded precipitation WILL NOT develop within the symmetrically unstable environment.

CSI occurs most frequently during the winter season, in the face of strong UVM (Upward vertical motion), and sufficient saturation, in the vicinity of a warm frontal boundary. CSI is analyzed in the atmosphere by two dimensional sloped rolls aligned in the direction of the geostrophic shear or thermal wind. The rolls slope toward the cold air, and are usually 50-100 kilometers wide and 100-400 kilometers long. The rolls have spacing on the order of 50 to 100 KM or less, which are dictated by the slope of the isentropes and the depth of the symmetrically unstable layer.

Positive static stability is necessary due to the differences in convective growth rates specific to CSI and CI. Unlike normal upright convection which can develop in a matter of minutes…Slantwise Convection (the result of CSI) on the other hand requires a longer initiation period in order for mature precipitation bands to develop, occurring on the time scale of many other mesoscale features (about 3-4 hours). So in other words while it takes upright convection as little as a few minutes to develop (which most are well aware)…it takes 3-4 hours in order for precipitation banding associated with CSI to form. As a result if CSI and CI are present within the same environment, due to the developmental time scales, Upright convection will occur instead of slantwise convection even though the environment is symmetrically unstable.

A symmetrically unstable environment will persistently attempt to achieve equilibrium within a period of hours. The existence of neutral or very weak positive symmetric stability present in an environment which is well mixed, features a nearly moist adiabatic layer, would be indicative of CSI or the previous (however recent) occurrence of Slantwise convection.

CSI diagnostics:

In assessing whether or not CSI can occur in a particular environment, one must observe the presence of the factors leading to its development. These are known as the Qualitative CSI predictors. They include:

1) Strong vertical wind shear
2) A well-mixed layer, which is or close to being completely saturated.
3) The observation of multiple cloud bands aligned parallel to the direction thermal wind (1000-500 MB thickness field).
4) Observation of banded and or convective precipitation structures based on radar returns.
5) Small static and inertial Stability

The presence of predictors one through four occurring to coincide with factor number five may reveal CSI as well.

As most are aware, cloud bands and radar trends obviously cannot be observed in advance of an event, which may be characterized by CSI, however, operational forecast models (preferably those which use a mesoscale grid system), can accurately assess the predictors of CSI well in advance of the event in question.

The investigation of CSI involves the examination and analysis of vertical cross-sections displaying geostrophic angular momentum (mg), and equivalent potential temperature (theta-e). Remember, the cross-section must be normal to the middle-tropospheric temperature gradient and thermal wind profiles. For timesaving purposes, one can use the 1000-500 MB thickness field however; the cross-section must still be aligned normal to the thickness field. This is to assure that the wind shear is mostly speed shear instead of directional sheer (excerpts from Moore, in a personal dialogue). All factors must be aligned normal to the cross-section. Conditional Symmetric Instability exists in an environment where the slope of the geostrophic angular momentum surfaces (Mg) (lines of equal and consistent geostrophic angular momentum) is flatter (or less) than those of the theta-e ( e) (lines of equal and consistent equivalent potential temperature) surfaces.

In looking at the interaction between momentum surfaces, theta-e, and the resultant motions of specific air parcels undergoing, vertical, horizontal, and slantwise displacement, we then can see the relationship between how each of these features work together with one another to create CSI. The resultant forcing on air parcels undergoing vertical, horizontal, and slantwise displacements can then be narrowed down to two things, vertical (or upright) ascent and horizontal (right to left or left to right) acceleration.

As discussed in the paragraphs above, we established the premise that a symmetrically unstable environment is achieved when the theta-e surfaces possess a greater slope than those of the momentum surfaces. Horizontal accelerations are due to ether the increasing or decreasing momentum of the parcels post-displacement environment. The parcel is then forced to either speed up or slow down to match the momentum of its new environment.

Completely horizontal displacements will result in positive buoyancy; this is due to the fact that the theta-e of the parcel is greater than the theta-e of the environment, therefore the parcel is forced to decelerate in order to match the theta-e of the environment. the parcel eventually becomes colder than the surrounding environment (the theta-e of the environment is greater than the theta-e of the parcel) as it rises, the parcel will then be forced to descend due to the fact that it becomes too heavy for the environment, and as the result, the environment will no longer support it’s upward motion, so the resultant forcing on the parcel causes it to descend and warm again before returning to equilibrium, at which time the theta-e of the parcel finally becomes equal to the theta-e of the environment.

In dealing with completely upright or vertical forms of displacement, negative buoyancy becomes the resultant forcing. This is due to the fact that the theta-e of the parcel is less than the theta-e of the environment, which would indicate that the parcel would then be forced to descend, thus, the resultant action would be negative buoyancy. Combined with horizontal acceleration, and the fact that momentum increases with height, indicate that the parcel would be forced to warm and slow while descending in an attempt to match the theta-e and momentum of the environment. Once the parcel has achieved matching both the theta-e and momentum of its environment, it then is considered to have reached equilibrium.

A combination of displacements, which are characterized by both vertical and horizontal acceleration in the direction of the displacement, is likely the characteristic of a slantwise convective displacement (positive buoyancy and horizontal acceleration). In the slantwise direction of displacement, the theta-e of the parcel will always be greater than the theta-e of the environment; therefore, the parcel is forced to rise due to the fact that it is more buoyant than the environment. Also in the slantwise direction of displacement, the momentum of the parcel must be less than the momentum of the slantwise-synoptic environment; this is because the resultant action would then force the parcel to accelerate in the slantwise direction to match the momentum of the environment, these are two processes which a parcel must undergo before being able to reach equilibrium (theta-e and momentum of the parcel is equal to the theta-e and momentum of the synoptic environment).

Because slantwise convective displacements lead to the horizontal acceleration of air parcels in the direction of the displacement and positive buoyancy, continued displacement of parcels in the slantwise direction would eventually lead to the development of banded precipitation structures, which contain somewhat to very highly enhanced precipitation rates. It is the development of these features, which characterize the atmosphere’s way of restoring the parcels to equilibrium.

CSI predictors:

SCAPE (Slantwise Convective Available potential Energy):

Over the course of several years, many specific criteria for the qualitative assessment of Conditional Symmetric Instability have been formed, tested, and proven. One such method is the assessment of SCAPE or Slantwise Convective Available Potential Energy. SCAPE is very similar to the more widely known (SB/ML CAPE) or Surface Based/Mixed Layer Convective Available Potential Energy which is most commonly used in the assessing the degree of atmospheric instability when considering severe thunderstorm potential, however, the only difference is that an inertial stability analysis parameter is included in the calculation, therefore making SCAPE specific to slantwise convection. Research conducted by Emanuel (1983a) allowed for the development of a specific parcel assessment method by which the degree of SCAPE can be determined. In this method, atmospheric soundings for specific stations are created along the momentum surfaces to determine the amount of SCAPE.

As eluted to above, the computation of SCAPE is very similar to the calculation of SB/ML CAPE, except for the addition of the inertial stability assessment parameter in the SCAPE computation. Values of SCAPE measure far less than those of normal CAPE. Usually SCAPE values will range from anywhere between 50 to 300 J/kg (Joules per Kilogram). Although the values of SCAPE measure far less than those of normalized CAPE, they, nonetheless have the ability to influence a symmetrically unstable environment. Occasionally, these values of SCAPE can release any existing Conditional Gravitational Instability within any layer(s), initiating upright convection. (Jascourt et al 1997) studied the unusual configuration of very narrow, however deep layer upright convection on a day when free convection was expected. The bands of upright convection in this case were aligned parallel to the thermal wind, characteristic of CSI, however the convection was the result of upright convective displacements rather than slantwise displacements, therefore, due to the fact that the convection was indeed upright convection, although had several of the characteristics normally thought to be reserved to symmetric instability, the only explanation would be based on the fact the values of SCAPE were sufficient enough to warrant the release of gravitational instability, which then initiated the development of upright convection with characteristics of symmetric instability.

Unfortunately, the assessment of Conditional Symmetric Instability is very difficult to make based on the analysis of SCAPE due to a significant lack of temporal and spatial resolution inherent in atmospheric soundings. Therefore other methods are used in place of SCAPE to determine if CSI exists.

EPV (Equivalent potential Vorticity):

Through the use of Equivalent Potential Vorticity (or moist potential vorticity) we can make the assessment of CSI much easier than it would be when considering the complicated nature specific to using the SCAPE procedure outlined above. It is a proven fact that CSI will be the preferred form of atmospheric response to an environment, which is, characterized by negative EPV, low static stability, maximum of upward vertical motion, and sufficient saturation needed to produce clouds and precipitation.

THE REQUIREMENT THAT EPV IS NEGATIVE IS JUST AS IMPORTANT AS THE SLOPE OF THE THETA-E SURFACES BEING GREATER THAN THAT OF THE MOMENTUM SURFACES.

CSI and Frontogenesis:

Past research has proven a very intriguing relationship between frontogenesis (frontal scale forcing) and CSI. The relationship centers on a common element in frontogenesis, which is the development of an ageostrophic circulation in response to the atmosphere’s attempts to achieve equilibrium. Frontogenesis develops as a result of large-scale geostrophic confluence, which develops convergence and therefore creates a strong tightening of the horizontal thermal gradient over a particular location. The tightening of the thermal gradient causes a major disruption in the thermal wind balance resulting in the development of ageostrophic Thermally Directed Circulation (or DTC, Direct Thermal Circulation). The development of the thermally directed circulation is the result of the atmosphere’s reaction to the frontogenesis (or the disrupting factor) as it strives to re-achieve equilibrium. As the atmosphere achieves equilibrium, the frontogenesis is mixed out and the atmosphere is once again stable (returned to equilibrium).

Emanuel 1985, Sanders and Bosart 1985, and Moore / Blakely 1988, noted that when frontogenesis occurs within areas where CSI is also present, (weak positive, neutral or complete negative absolute vorticity), significant intensification of frontal-scale circulations can and usually will occur. Since the Direct Thermal Circulation associated with the frontogenesis has a trajectory up the warm side of the frontogenesis, it reduces in horizontal scale, therefore compressing the thermal gradient and causing the frontogenesis to strengthen. Even is spite of the fact that the symmetrically unstable circulation associated with CSI is the atmosphere’s way of restoring neutrality (reaching equilibrium), the existence of frontogenesis within the neutral or slightly symmetrically stable environment will re-intensify the original slantwise circulation allowing CSI to persist over a specific location as long as the frontogenesis is still present, and long after any slantwise CAPE has been used up or in the case there was little SCAPE to begin with.

In conclusion, Conditional Symmetric instability is one of the most important features specific to winter weather, and is comparable to the importance of upright convection in severe weather development during the summer.

Conditional symmetric instability and the slantwise convection associated with it are most commonly responsible for thunder and lightning observed with heavy snowfall during major winter storms. This was especially the case during the March 1993 super storm.

[ColdFront77]

RNS, thank you very very much for the detailed explanation.

[RNS]

you got it tom...

[NEwxgirl]

If i might also add, you would want the sloping frontal boundary below the symmetrically unstable layer in order for the effects of frontogenesis on the unstable layer to be maximized.

[JCT777]

Very interesting! Thanks for posting it, RNS.