Event Summary
     National Weather Service, Raleigh NC

July 28, 2006 - Shear and Convective Outflow Interaction Case Study

Event Headlines

...Isolated thunderstorms with well defined outflow boundaries developed across central North Carolina...
...Convection developed along the preferred outflow flank...

Event Overview

This case study demonstrates how the interaction between the ambient low level wind shear and surface based cold pools, can dictate convective development and persistence along a preferred outflow flank. We will examine the theoretical physics of this interaction while presenting a recent example of how the atmosphere behaved as theory dictates it should.

On Friday, July 28, 2006, isolated thunderstorms developed across portions of central North Carolina in a moderately unstable airmass with minimal shear. A small cluster of thunderstorms initially developed across the central coastal plain with new convective cells developing along the eastern outflow boundary.

KRAX Radar Animation

The following is a radar animation showing multiple convective cells developing along eastward moving outflow. Note the detection of outflow, represented by lower reflectivities emanating radially approximately from Greene County (reference map). Of particular significance to this study, note that new convection is only initiating and being sustained along the eastern flank of the outflow.

KGSO RAOB 00z/July 29

The following is the observed sounding at Greensboro, NC, valid 00z/July 29. Note the magnitude (direction and speed) of the winds in the lowest 700 MB. Winds in this layer are light from the southwest at around 5 to 10 knots near the surface and gradually increase to west-southwesterly at around 20 to 25 knots at around 700 MB.

Surface Observations

Surface observations valid at 20Z/July 28 are shown below. Note the relatively uniform temperatures and dewpoints between Raleigh, Fayetteville, and Goldsboro. The rather homogenous atmosphere with temperatures in mid 90s with dewpoints in the lower 70s suggest that surface conditions would not favor the development of convection in any particular location over another.

This case is a classic example of how the ambient environmental shear profile theoretically favors new cell generation along a preferred outflow flank, which in this case is the eastern one. Why might this be the case in a relatively homogenous atmosphere?

Vorticity Fields Induced by the Environmental Shear and Thunderstorm Outflow

Image courtesy of COMET

Consider a case where winds increase with height in the lowest few kilometers, which induces a positive horizontal vorticity in the lower troposphere. Then consider both eastward/downshear and westward/upshear moving outflow, where negative vorticity is generated on the eastern flank, and positive on the western. The shear and outflow induced circulations oppose one another on the western flank, and are additive on the eastern. In other words, the induced circulations create net upward vertical motion on the eastern flank, and zero vertical motion (assuming the vorticity fields are equal in magnitude) on the western flank. Although opposite in sign vorticity fields adjacent to one another does produce upward vertical motion, their strength relative to one another dictates the depth over which the updraft remains vertically oriented. Consider a case where one vortex is much stronger than the other. The updraft would be tilted and pulled over the top of the stronger vortex, minimizing the altitude to which the updraft could extend. Thus, the ideal case for an air parcel to reach its LFC is when the vortices are EQUAL in magnitude, and OPPOSITE in sign, such is the case east of the cold pool as depicted in the above graphic..

Applying Theory to Operational Forecasting and Nowcasting

Operationally, one can determine the cold pool/low-level shear relationship by calculating the ratio of the speed of the cold pool, “C”, (which is theoretically proportional to the strength of the cold pool circulation) divided by the value of the outflow line-normal low-level vertical wind shear, “∆U”, where “low-level wind shear” should be the shear over the approximate depth of the cold pool. The average depth of cold pools has been found to be on average 1.7 km. Although shear in the cold pool layer is most important in determining the depth and strength of lift along the outflow, shear over a deeper layer is also a contributor, albeit to a lesser degree. Thus, it would be best to use the 0-3 km shear instead of 0-2 km, for example. Since the 3 km AGL is right around the 700 mb level, computing the low level shear is quick and easy - Just subtract the surface or 1000 mb wind from the 700 mb wind. The speed of the advancing outflow can easily be determined by using the distance speed tool in AWIPS.

A C/∆U ratio of 1 represents the optimal state for deep lifting by the outflow/cold pool. Values less than 1 signify that the ambient shear stronger relative to the cold pool and values greater than 1 signify that the cold pool is stronger than the ambient shear. This ratio is not only significant for anticipating the degree of lifting along outflow, but it is also important in determining the strength and longevity of linear MCS’s, since one of the primary forces driving MCS’s is their well-developed cold pools.

Case Study Team

Mike Strickler
Jonathan Blaes

For questions regarding the web site, please contact Jonathan Blaes.

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