Wind Shear ?

[Jon Miller]

Of the three wind shear maps on Earl's site - Sfc-1, Sfc-3, & BL-6 - Which of the three are the most important in chase planning stratagies -Or are they of equal importance ?

http://www.wxcaster.com/conus_0012_us_models.htm

Jon Miller
KT8NDO

 

[Aaron Kennedy]

I'll give ya the short answer:

Sfc-3km and BL-6km are good for forecasting supercells (given sufficient CAPE)....

Sfc-1km is good for looking for potentially prolific tornadic supercells. I won't cancel a chase if Sfc-1 is small.... I'm just as happy with photogenic nontornadic supercells.

If I am in a hurry, I'll just glance at sfc-3km.

Aaron

 

[Michael P. Morris]

BL-6 km is important in determining the storm mode. Deep-layer shear vectors oriented perpendicular to the surface boundary are most favorable for discrete supercells.

 

[Andrea Griffa]

Originally posted by Jon Miller
Of the three wind shear maps on Earl's site - Sfc-1, Sfc-3, & BL-6 - Which of the three are the most important in chase planning stratagies -Or are they of equal importance ?

http://www.wxcaster.com/conus_0012_us_models.htm

Jon Miller
KT8NDO

If I would choose one of them and I was forced to do that, I'd probably choose BL-6km because it well expresses the supercellular potential.
However if I can, I use all those parameters

 

[Jeff Snyder]

Originally posted by Michael P. Morris
BL-6 km is important in determining the storm mode. Deep-layer shear vectors oriented perpendicular to the surface boundary are most favorable for discrete supercells.

IIRC, a normally-oriented deeplayer shear vector will favor, equally, left and right splits as they move off the convergence source. If we imagine a N-S dryline with an easterly shear vector, the right-splits will tend to move SE, while the left-splits will tend to move NE. If the dryline initiates a several supercells along its path, there may be significant storm collisions.

When there is a 45 degree difference between the convergence source (say, a dryline) and the deeplayer shear vector, right-splits tend to remain discrete longer since there will be a tendency for the right-splits to move more easterly while the left splits move more northerly or north-northeasterly. This can mean that the right-splits are able to "stay ahead of" the left-splits, thereby avoiding collisions. Regardless, the point that Michael made remains -- shear vector orientation that is more normal than parallel tends to favor discrete activity (rather than a congealing linear mess).


Slightly related to the shear-vector discussion, here's a paper highlighting the storm mode frequencies with different forcing boundaries. --> Dial, G.L., and J.P. Racy, 2004: Forecasting Short Term Convective Mode and Evolution for Severe Storms Initiated along Synoptic Boundaries. Preprints, 22nd Conf. Severe Local Storms, Hyannis MA. [104K PDF]

 

[Andrea Griffa]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

<!--QuoteBegin-Michael P. Morris

BL-6 km is important in determining the storm mode. Deep-layer shear vectors oriented perpendicular to the surface boundary are most favorable for discrete supercells.

IIRC, a normally-oriented deeplayer shear vector will favor, equally, left and right splits as they move off the convergence source. If we imagine a N-S dryline with an easterly shear vector, the right-splits will tend to move SE, while the left-splits will tend to move NE. If the dryline initiates a several supercells along its path, there may be significant storm collisions.

When there is a 45 degree difference between the convergence source (say, a dryline) and the deeplayer shear vector, right-splits tend to remain discrete longer since there will be a tendency for the right-splits to move more easterly while the left splits move more northerly or north-northeasterly. This can mean that the right-splits are able to "stay ahead of" the left-splits, thereby avoiding collisions. Regardless, the point that Michael made remains -- shear vector orientation that is more normal than parallel tends to favor discrete activity (rather than a congealing linear mess).


Slightly related to the shear-vector discussion, here's a paper highlighting the storm mode frequencies with different forcing boundaries. --> Dial, G.L., and J.P. Racy, 2004: Forecasting Short Term Convective Mode and Evolution for Severe Storms Initiated along Synoptic Boundaries. Preprints, 22nd Conf. Severe Local Storms, Hyannis MA. [104K PDF][/b]

That's so nice explanation Jeff, great! :wink:

 

[Andrea Griffa]

I report an interesting part from the paper:



"Preliminary results suggest that the orientation of the 2-6
km or 2-8 km mean wind with respect to the initiating
boundary and the component of 2-6 km or 2-8 km shear
normal to the initiating boundary are good discriminators
between those environments where storms remain
discrete within the first few hours after developing versus
those where storms evolve into lines for storms initiated
along a synoptic boundary. It would appear that when the
mean 2-6 km or 2-8 km flow and boundary are nearly
parallel, the precipitation cascades and associated
outflows of neighboring storms may merge and
consolidate more quickly. Results also suggest that when
the component of middle level shear normal to the
initiating boundary is weak, upstream development of a
cold pool may occur more readily with a faster transition
to lines than when this component of shear is strong. The
amount of low level convergence appears to discriminate,
but to a much lesser degree. Stronger low level
convergence would likely lead to the development of more
storms which would in turn increase the chances for storm
interactions. Other than geometry and convergence, the
type of initiating boundary also appears to play a role. It
appears that storms initiated along cold fronts generally
have a greater tendency to evolve into lines more quickly
than those initiated along pre-frontal troughs or drylines.
The results achieved so far remain preliminary, due
mainly to the limited data set. As more data are
accumulated within the next two years, more robust
results are anticipated. This study only included kinematic
variables as part of the mode parameter calculation.
Additional sensitivity studies will be performed using
various thermodynamic variables to assess their ability in
conjunction with kinematic variables to discriminate
between the types of mode evolution. A more in depth
examination of the physical processes governing
evolution of discrete modes into lines is also planned."




Thre's an intresting formula to calculate the mode parameter:

(A/25*S/13)
M= --------------
C/9

where A is the angle in degrees between the initiating boundary and the mean wind in the 2-6 km or 2-8 km layer (normalized with respect to 25 degrees) and S is the component of the shear in m/s within the 2-6 km or 2-8 km layer orthogonal to the initiating boundary (normalized with respect to 13ms-1). C is the convergence X 10-5s-1 in the lowest 90 mb (normalized with respect to 9 X 10-5s-1). Note that only kinematic variables are used in the current formulation of the mode parameter since these demonstrated the best discrimination skill.


To view the results there's an interesting figure in the paper(figure 6)