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Storm Relative Helicity
- Thread starter MikeD
- Start date
Mike Johnston
EF5
Helicity is the product of low level shearing (known as streamwise vorticity) and storm inflow directly into the streamwise vorticity. The Helicity is storm relative which means the Helicity is calculated from the storm's frame of reference
Marshall Stoner
EF1
This is a good definition, but it's difficult to understand without a picture or some kind of 3D animation. I also think the OP might be more interested in what kind of numbers to look for on a forecast chart in terms of supercell/tornado potential. I'm not particularly experienced in forecasting supercells/tornadoes though so I don't really know what the rule of thumb is.
rdale
EF5
Winner of the most helpful response of the year.
The only problem is that it doesn’t help explain (in English, not professional meteorologist language) what SRH is.
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Quincy Vagell
EF4
If you're familiar much with vertical wind profiles or at least how wind can change speed and direction with height, that helps understand some of what I'll post below. Some of this is testing my understanding a bit too, so if anything is not quite accurate, feel free to chime in. I'm not a teacher, so I apologize in advance if this may be a little difficult to follow.
SPC defines SRH (storm relative helicity) as the "measure of the potential for cyclonic updraft rotation in right-moving supercells..." Simply stated, the more SRH there is, the more likely it is that an updraft will be able to rotate. If winds veer and strengthen with height, meaning that winds turn from, for example, a southerly direction near the surface to westerly in the upper levels of the atmosphere and winds get stronger with height, then SRH will be higher/larger. This wind flow, as ingested by developing convection, could support a supercell, especially if there is adequate SRH in the 0-3km layer. For tornado formation (tornadogenesis), the 0-1km SRH layer is arguably most helpful, as it focuses on rotation in the lowest 1km above ground level (AGL).
This might be a bit abstract depending on your knowledge level, but looking at a hodograph helps visualize SRH.
Tonight's 00z sounding from Topeka (TOP) shows a relatively large amount of SRH, but for the dismay of chasers, the boundary layer is far too chilly to be worrying about any thunderstorm development, let alone supercells.
Winds shift from out of the southeast at 7 knots near the surface, to southwest around 30 knots at 1km AGL to about 30 knots out of the northwest at 2km AGL. When using a hodograph, to determine the line that is traced (large, looping traces are most indicative of environments favorable for supercell tornadoes), start at the center of the graph and trace in the direction that the wind is flowing toward, so a southeast wind will need a trace from the middle, toward the top-left. Where does the trace stop? Since the wind is only 7 knots near the surface, plot a starting point 7 knots "away" from the center. Repeat the steps for winds at different levels and at the very end, trace the dots to create the hodograph loop. (it will probably not be a clean loop most of the time)
Once the hodograph is finished, use the storm motion (speed and direction) to be able to figure out SRH. Calculating storm motion is a bit more involved, but forecast (and observed) soundings will generally give you this information. Since we're assuming right-moving supercells given a clockwise looping hodograph, in the case of TOP, the storm motion is 33 knots out of the northwest. Start at the center and there is a small hatched circle plotted where the storm motion vector points toward.
Now, connect two points to evaluate 0-1km SRH:
1. Starting point to the storm motion plot.
2. Storm motion plot to the 1km hodograph plot.
The area under the curve in the 0-1km layer, shaded in pink, is your 0-1km SRH. In this example, the hodograph gets messy beyond 2km, so the curve does not really enlarge much at all beyond that. As a result, the area under the curve in the 0-3km layer is similar to 0-1km and the resultant 0-3km SRH is only slightly larger than 0-1km SRH. Note that the grey arrows (wind vectors) are how the hodograph was drawn in the first place.
Values via sounding:
0-1km SRH: 434 m2/s2 (relatively large)
0-3km SRH: 494 m2/s2 (only slightly larger than 0-1km)
I don't know how helpful this is at all, so please refer to a pair of links below for a more professional explanation:
http://twister.ou.edu/MM2005/Supercell_3.ppt
https://training.weather.gov/wdtd/courses/rac/severe/hodograph/presentation_html5.html
SPC defines SRH (storm relative helicity) as the "measure of the potential for cyclonic updraft rotation in right-moving supercells..." Simply stated, the more SRH there is, the more likely it is that an updraft will be able to rotate. If winds veer and strengthen with height, meaning that winds turn from, for example, a southerly direction near the surface to westerly in the upper levels of the atmosphere and winds get stronger with height, then SRH will be higher/larger. This wind flow, as ingested by developing convection, could support a supercell, especially if there is adequate SRH in the 0-3km layer. For tornado formation (tornadogenesis), the 0-1km SRH layer is arguably most helpful, as it focuses on rotation in the lowest 1km above ground level (AGL).
This might be a bit abstract depending on your knowledge level, but looking at a hodograph helps visualize SRH.
Tonight's 00z sounding from Topeka (TOP) shows a relatively large amount of SRH, but for the dismay of chasers, the boundary layer is far too chilly to be worrying about any thunderstorm development, let alone supercells.
Winds shift from out of the southeast at 7 knots near the surface, to southwest around 30 knots at 1km AGL to about 30 knots out of the northwest at 2km AGL. When using a hodograph, to determine the line that is traced (large, looping traces are most indicative of environments favorable for supercell tornadoes), start at the center of the graph and trace in the direction that the wind is flowing toward, so a southeast wind will need a trace from the middle, toward the top-left. Where does the trace stop? Since the wind is only 7 knots near the surface, plot a starting point 7 knots "away" from the center. Repeat the steps for winds at different levels and at the very end, trace the dots to create the hodograph loop. (it will probably not be a clean loop most of the time)
Once the hodograph is finished, use the storm motion (speed and direction) to be able to figure out SRH. Calculating storm motion is a bit more involved, but forecast (and observed) soundings will generally give you this information. Since we're assuming right-moving supercells given a clockwise looping hodograph, in the case of TOP, the storm motion is 33 knots out of the northwest. Start at the center and there is a small hatched circle plotted where the storm motion vector points toward.
Now, connect two points to evaluate 0-1km SRH:
1. Starting point to the storm motion plot.
2. Storm motion plot to the 1km hodograph plot.
The area under the curve in the 0-1km layer, shaded in pink, is your 0-1km SRH. In this example, the hodograph gets messy beyond 2km, so the curve does not really enlarge much at all beyond that. As a result, the area under the curve in the 0-3km layer is similar to 0-1km and the resultant 0-3km SRH is only slightly larger than 0-1km SRH. Note that the grey arrows (wind vectors) are how the hodograph was drawn in the first place.
Values via sounding:
0-1km SRH: 434 m2/s2 (relatively large)
0-3km SRH: 494 m2/s2 (only slightly larger than 0-1km)
I don't know how helpful this is at all, so please refer to a pair of links below for a more professional explanation:
http://twister.ou.edu/MM2005/Supercell_3.ppt
https://training.weather.gov/wdtd/courses/rac/severe/hodograph/presentation_html5.html
Paul Knightley
EF5
If you want to simply 'picture' it in your head, think of a phone cord which is coiled. SRH is a measure of the tendency of the air to trace a spiral path in a horizontal direction. Updraughts can tilt and stretch this into the vertical, which can go on to help low-level mesocyclones form.
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Perhaps thinking of a toilet flushing helps. The water that comes out of a toilet comes out at an angle. If the water was aiming straight down it wouldn’t help the water get rotating much at all. Now, the more of an angle and the faster the water comes out, the quicker and more intensely your toilet water will spin. That water coming around the sides of your toilet would be your SRH.
Miniature mesocyclones with your phone cord. 
I like it...
Thanks, but I’m pretty sure you can come up with something less gross.
I like it...Thanks, but I’m pretty sure you can come up with something less gross.
Marshall Stoner
EF1
The phone cord is a good visual aid. Just pretend in the picture of the phone cord you are looking due north. Imagine the phone cord coil is exactly 1 km thick. The surface winds are out of the southeast, and the winds 1 km above the surface are out of the southwest. You can imagine if an air parcel was forced to bob up and down it would move towards the NE while at the top of the layer, then as it descended it would move towards the NW, then towards the NE again as it moved up again. This forms a corkscrew motion. The mean motion of the 0-1 km layer is to the north, so the air parcel is moving north over time.
Now imagine the flow around 6 km is W or WNW, so that the storm motion is due east. Since the storm motion is from west to east, any parcel in the southerly 0-1 km layer will be scooped up into the updraft. If the horizontally bobbing corkscrew motion of each inflow parcel is turned into the vertical, it now moves in a cyclonic spiral as it rises. At this point, looking at all the parcels together, it will look like the phone cord is turned into a vertical direction and you now have a cyclonic spiral (i.e. mesocyclone).
Note that it's important that the storm motion is at least partially against the mean 0-1 km flow. If the storm was moving in the same speed and direction as the 0-1 km flow, you won't have any helicity because the storm-relative inflow is no longer from the south. The corkscrew-like motion must be incorporated into the updraft as inflow. The greater the storm-relative inflow in the direction of the corkscrew, the greater the helicity. If the 6 km flow is more southwesterly and the storm motion is therefore more NE, then the inflow component from the south will be less, therefore there is less helicity.
Helicity really takes into account three things. 1.) The magnitude of the low level shear. 2.) The degree to which the vector normal to the low level shear (i.e. the vector pointing along the phone cord corkscrew) is aligned with the storm's inflow. 3.) The strength of that storm relative inflow.
One additional caveat... the horizontal corkscrew often doesn't literally exist. A literal corkscrew motion may occur in cumulus cloud streets that develop before storm initiation, but it doesn't have to occur for a supercell to develop. Even if you just imagine two layers of air sliding past each other in the 0-1 km layer, you can kind of imagine that if an updraft suddenly scoops up a small section of that sheared layer, it will also rotate. It's just not quite as easy to visualize as the literal horizontal corkscrew though. It's actually difficult to see in your mind how it occurs without looking at a 3D computer simulation. That's why it's best to imagine a literal horizontal corkscrew first.
Hope this helps.
Now imagine the flow around 6 km is W or WNW, so that the storm motion is due east. Since the storm motion is from west to east, any parcel in the southerly 0-1 km layer will be scooped up into the updraft. If the horizontally bobbing corkscrew motion of each inflow parcel is turned into the vertical, it now moves in a cyclonic spiral as it rises. At this point, looking at all the parcels together, it will look like the phone cord is turned into a vertical direction and you now have a cyclonic spiral (i.e. mesocyclone).
Note that it's important that the storm motion is at least partially against the mean 0-1 km flow. If the storm was moving in the same speed and direction as the 0-1 km flow, you won't have any helicity because the storm-relative inflow is no longer from the south. The corkscrew-like motion must be incorporated into the updraft as inflow. The greater the storm-relative inflow in the direction of the corkscrew, the greater the helicity. If the 6 km flow is more southwesterly and the storm motion is therefore more NE, then the inflow component from the south will be less, therefore there is less helicity.
Helicity really takes into account three things. 1.) The magnitude of the low level shear. 2.) The degree to which the vector normal to the low level shear (i.e. the vector pointing along the phone cord corkscrew) is aligned with the storm's inflow. 3.) The strength of that storm relative inflow.
One additional caveat... the horizontal corkscrew often doesn't literally exist. A literal corkscrew motion may occur in cumulus cloud streets that develop before storm initiation, but it doesn't have to occur for a supercell to develop. Even if you just imagine two layers of air sliding past each other in the 0-1 km layer, you can kind of imagine that if an updraft suddenly scoops up a small section of that sheared layer, it will also rotate. It's just not quite as easy to visualize as the literal horizontal corkscrew though. It's actually difficult to see in your mind how it occurs without looking at a 3D computer simulation. That's why it's best to imagine a literal horizontal corkscrew first.
Hope this helps.
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Also, other question, what is the difference between Bulk Shear 1/6 km and Bulk Shear effective? Which is more helpful and what should I look at to get the general idea of how good supercells that day will be? Which do you use to determine a good chase day?
Completely unrelated question: how do you tell if the day is leaning towards LP, CLASSIC, or HP? Whenever I chase, there’s usually someone else doing it.
April 13 might look like a good chase day. Upper trough, Southwest jet, a low, and a cold front
Completely unrelated question: how do you tell if the day is leaning towards LP, CLASSIC, or HP? Whenever I chase, there’s usually someone else doing it.
April 13 might look like a good chase day. Upper trough, Southwest jet, a low, and a cold front
Marshall Stoner
EF1
I'm not really an experienced severe weather forecaster, but I personally don't look much at effective bulk shear. The difference is how the lower and upper levels are selected. Bulk shear always uses the wind closest to the ground as the lowest layer. This can be problematic in situations where the instability is elevated. To remedy this, effective bulk shear uses the lowest unstable layer as the bottom layer, rather than the layer closest to the ground (which might actually be stable). It might also be useful to use effective bulk shear to forecast severe weather when the boundary layer is deep and the cloud bases are high.
Since tornadoes usually form only when there's surface instability around and cloud bases are relatively low, effective bulk shear is irrelevant most of the time. In most plains outbreaks the values will be pretty similar anyways. The exception might be low-topped supercells. If the tropopause is unusually low, the equilibrium level will be somewhat lower and effective bulk-shear will take this into account better.
It might not be completely useless for storm chasing, but I personally don't pay much attention to effective bulk shear. Usually I just look at the normal bulk shear and assume anything over 40 kts is sufficient for supercells, though you can sometimes get away with even less shear when there's a dryline or outflow boudary around. I've noticed that bulk shear over 60 kts is sometimes superfluous. Extreme shear seems to increase the likelihood of long track F4 tornadoes, but it can also lead to a bust if the cap is too strong or there isn't a good enough synoptic trigger. 50-60 kts does the job very well in most cases. 70 or 80 kts is extreme, but it may or may not increase chances of tornadoes. The most destructive tornadoes do often occur in conjunction with extreme shear though.
Since tornadoes usually form only when there's surface instability around and cloud bases are relatively low, effective bulk shear is irrelevant most of the time. In most plains outbreaks the values will be pretty similar anyways. The exception might be low-topped supercells. If the tropopause is unusually low, the equilibrium level will be somewhat lower and effective bulk-shear will take this into account better.
It might not be completely useless for storm chasing, but I personally don't pay much attention to effective bulk shear. Usually I just look at the normal bulk shear and assume anything over 40 kts is sufficient for supercells, though you can sometimes get away with even less shear when there's a dryline or outflow boudary around. I've noticed that bulk shear over 60 kts is sometimes superfluous. Extreme shear seems to increase the likelihood of long track F4 tornadoes, but it can also lead to a bust if the cap is too strong or there isn't a good enough synoptic trigger. 50-60 kts does the job very well in most cases. 70 or 80 kts is extreme, but it may or may not increase chances of tornadoes. The most destructive tornadoes do often occur in conjunction with extreme shear though.
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Ethan Lang
EF2
The University of Wisconsin at Madison has a youtube video of the supercomputer simulation of a radar and it shows streamwise vorticity and relative storm inflow. That video will definitely help
Another thing... I’m kinda confused about the wall/tail/beavers tail/vault concept. Correct me if I’m wrong.
From what I understand, the vault is the section between the mesocyclone and the FFD, and the inflow tail, the tail, and beavers tail are the same thing.
I’m also confused wether the tail is connected to the mesocyclone or the wall cloud. Does there have to be a wall cloud for a tail cloud to form? After doing some research, I conclude that the mesocyclone is connected with the wall cloud, which is then connected to the tail cloud, so the tail cloud is not connected with the mesocyclone?
From what I understand, the vault is the section between the mesocyclone and the FFD, and the inflow tail, the tail, and beavers tail are the same thing.
I’m also confused wether the tail is connected to the mesocyclone or the wall cloud. Does there have to be a wall cloud for a tail cloud to form? After doing some research, I conclude that the mesocyclone is connected with the wall cloud, which is then connected to the tail cloud, so the tail cloud is not connected with the mesocyclone?
Darren Lo
EF0
- Joined
- Feb 25, 2012
- Messages
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The beaver's tail is an inflow band at the level of the updraft base that forms along the "pseudo warm front" between the inflow sector and the FFD. It can be very long and usually shows some degree of cyclonic curvature. Here's a horrible picture but it shows you the idea -- the updraft base is to the left behind the power lines, and the beaver's tail is curving overhead on the right side (it actually went back way farther than this picture shows). The photo is taken from a position in the inflow sector, and the FFD is all behind the beaver's tail.
A tail cloud is an inflow band that attaches to the wall cloud, thus it is vertically at a lower level than the beaver's tail and does not attach directly to the updraft base.
A tail cloud is an inflow band that attaches to the wall cloud, thus it is vertically at a lower level than the beaver's tail and does not attach directly to the updraft base.
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