0-1km EHI versus 0-3km EHI

[Jason Blum]

This is probably a dumb question, but the subject came up today so it got me curious. I looked online but couldn't find any obvious answers for what is probably obvious to anyone with more advanced meteorological knowledge than myself...

Which index is preferred for tornado forecasting? 0-1km EHI or 0-3km EHI?

Would this be dependent upon LCL's?
Or am I way, way off base?

 

[Brett Roberts]

It was shown in Rasmussen (2003) that 0-1 km SRH, and by extension 0-1 km EHI, has a stronger correlation with the tendency of a supercell to produce tornadoes than 0-3 km SRH/EHI.

As I understand it, a very rough rule of thumb is that 0-1 km SRH/EHI are most useful for forecasting tornadoes, while 0-3 km SRH/EHI are more related to the likelihood of supercells. (Of course, supercells also require sufficient deep-layer shear to separate the updraft and downdraft, so it's not a magic bullet).

 

[L.B. LaForce]

Well, these two are both composite indices, which take into account CAPE, and SRH. If I remember correctly, it is calculated by taking the product of SRH, and CAPE, and dividing that quantity by 160,000. Higher values of both correspond to higher EHI values. It is useful to look at composite indices while forecasting, but I would not rely solely on composite indices. They are meant to be used in conjunction with other parameters, and charts during a forecast process. That being said, I dont know too much about the difference between the 0-1km, and the 0-3km, except you would obviously get higher values at 0-3km. I usually look at both, but you would have to wait for a response from someone with a little more meteorological knowledge. Also in a sense, LCL's are not directly related to EHI values, as stated above, they are dependent on 2 variables.

 

[rdale]

And to add to Brett's answer - what looks good is when the 0-3 SRH is high, and the 0-1 SRH is a good percentage of it. I.E. 0-3SRH = 350, if the 0-1SRH = 200 then you're really happy since a lot of the "turning" is focused down low.

 

[Tim Supinie]

From what I've gathered, what is really important for supercells/tornadoes is the storm-relative helicity over the depth of the inflow layer to the storm. Since the depth of the inflow layer can be any value (probably somewhere between 1 and 3 km deep), 0-1 km SRH and 0-3 km SRH are used as ballpark estimates of storm-relative helicity over the depth of the inflow layer. I believe the Effective Storm-Relative Helicity product on the SPC mesoanalysis attempts to address this by actually computing the depth of the inflow layer.