ChristianTerry
What is the difference and what is each used for?
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Greg Blumberg
EF4
Hi Christian,
Helicity is a value that assesses how much the rotating horizontal air tubes produced by the environmental are in line with the storm relative winds. It's a way of finding out how much stream-wise vorticity a certain storm will ingest. To find out this, you HAVE know the storm motion (both speed and direction.) The value of Storm Relative Helicity is basically a value that will give you an idea of the potential for a storm's updraft to rotate. This value doesn't take into account storm mode or storm coverage, so you have to use other pieces of information to assess that, such as the Bulk Richardson Number. Often, helicity isn't considered because it's generally not consistent across the environment. Also, since you have to know the storm motion to compute it, this value sometimes isn't practical. It's difficult to predict storm motion.
As for Bulk Shear, you're looking at the difference of two winds at different levels. You have to specify which levels you are looking at. Sometimes you can be looking at the 0 - 6 km winds and other times the 0 - 1 km winds. 0 - 6 km winds greater than 40 knots are commonly seen with supercells (but greater than 30 knots are good in extreme CAPE) and the 0 - 1 km winds are often used to assess tornado potential. The values for the 0 - 1 km winds for tornadoes are usually around 15 - 20 knots.
In general, they're two values that are computed from the wind field and are used to predict supercells and tornadoes.
I used this for my post, but I encourage you to check it out as it has even more information:
http://www4.ncsu.edu/~nwsfo/storage/training/assorted/evans_fcstg supercell_tornadoes_rdu.ppt
Helicity is a value that assesses how much the rotating horizontal air tubes produced by the environmental are in line with the storm relative winds. It's a way of finding out how much stream-wise vorticity a certain storm will ingest. To find out this, you HAVE know the storm motion (both speed and direction.) The value of Storm Relative Helicity is basically a value that will give you an idea of the potential for a storm's updraft to rotate. This value doesn't take into account storm mode or storm coverage, so you have to use other pieces of information to assess that, such as the Bulk Richardson Number. Often, helicity isn't considered because it's generally not consistent across the environment. Also, since you have to know the storm motion to compute it, this value sometimes isn't practical. It's difficult to predict storm motion.
As for Bulk Shear, you're looking at the difference of two winds at different levels. You have to specify which levels you are looking at. Sometimes you can be looking at the 0 - 6 km winds and other times the 0 - 1 km winds. 0 - 6 km winds greater than 40 knots are commonly seen with supercells (but greater than 30 knots are good in extreme CAPE) and the 0 - 1 km winds are often used to assess tornado potential. The values for the 0 - 1 km winds for tornadoes are usually around 15 - 20 knots.
In general, they're two values that are computed from the wind field and are used to predict supercells and tornadoes.
I used this for my post, but I encourage you to check it out as it has even more information:
http://www4.ncsu.edu/~nwsfo/storage/training/assorted/evans_fcstg supercell_tornadoes_rdu.ppt
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- Apr 13, 2009
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That link was great, thanks! RST
ChristianTerry
Thanks for the link. Very helpful 
Tim Supinie
EF2
Strictly speaking, what Greg described is "storm-relative helicity." Helicity is simply storm-relative helicity with the assumption that the storm is stationary (storm motion is 0). This is obviously usually a bad assumption, so storm-relative helicity is considered a better quantity for determining how much stream-wise vorticity a storm is ingesting.
One other thing to mention is that storm-relative helicity is an integral quantity, meaning you get it by summing up a bunch of things: the differences between environmental wind and storm motion throughout some layer of the atmosphere. Basically, what this means is that storm-relative helicity depends on the shape of the hodograph over the layer.
However, bulk shear is a differential quantity, meaning you get it by taking the difference of two things: the winds at the top and bottom of a layer. This means the "bulk shear" over a layer doesn't depend on the shape of the hodograph in that layer, only on the winds at the top and bottom of the layer.
So I guess the point is that a hodograph could give you the same value of bulk shear over a certain depth, but different values of storm-relative helicity over the same depth.
One other thing to mention is that storm-relative helicity is an integral quantity, meaning you get it by summing up a bunch of things: the differences between environmental wind and storm motion throughout some layer of the atmosphere. Basically, what this means is that storm-relative helicity depends on the shape of the hodograph over the layer.
However, bulk shear is a differential quantity, meaning you get it by taking the difference of two things: the winds at the top and bottom of a layer. This means the "bulk shear" over a layer doesn't depend on the shape of the hodograph in that layer, only on the winds at the top and bottom of the layer.
So I guess the point is that a hodograph could give you the same value of bulk shear over a certain depth, but different values of storm-relative helicity over the same depth.
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