0-6 KM Shear vs. EBWD (effective Bulk wind)

[Jim wyman]

Hello All,
I am trying to determine the EBWD (effective bulk wind) from a sounding. Is it the same as the 0-6Km shear or is it not the same? IF not the same, then how is it caclulated?

Thanks,

Jim

 

[rdale]

It's close -

Effective Bulk Wind Difference = magnitude of the bulk vector shear from the "effective inflow base" upward to 50% of the equilibrium level height with the most unstable parcel in the lowest 300 mb. This parameter is similar to the 0-6 km bulk wind difference, though it accounts for storm depth (effective inflow base to mu parcel EL) and is designed to identify both surface-based and "elevated" supercell environments. Supercells become more probable as the effective bulk vector shear increases through the range of 25-40 kt and greater.

http://w1.spc.woc.noaa.gov/exper/mesoanalysis/help/begin.html

 

[Jim wyman]

Thanks. I need some math/CS wiz to show me how it is calculated from a sounding....

Jim

 

[Jim wyman]

Anyone care to take a stab at this???

 

[Chad Cowan]

Anyone care to take a stab at this???

Not entirely sure, but I think you'll need to find the most unstable parcel level on the sounding and then correlate that level to the hodograph. Lift that parcel then figure out what height is 50% of the way to the EL for that parcel. That is the other point of reference on your hodograph, and simple vector math from one point to the other will give you the value in kts.

The "effective inflow base" part throws me off a bit. I'm thinking that is roughly equivalent to the LCL.

 

[rdale]

Are you talking about doing this computatively, or just looking at it?

Inflow base is also defined (and shown in an example) on that webpage.

http://w1.spc.woc.noaa.gov/exper/mesoanalysis/help/begin.html

Effective Inflow Layer = the first layer in a sounding where all contiguous lifted parcels have CAPE >= 100 J kg^-1 and CIN >= -250 J kg^-1. The effective inflow layer is meant to represent the inflow layer for a thunderstorm, where lifted parcels have sufficient CAPE and CIN that is not excessive. The effective inflow layer technique is applicable to both surface-based and elevated thunderstorms, and this layer is used in calculations of effective storm-relative helicity. The cyan vertical bar in the sounding above (left of the temperature and dewpoint traces) marks the bottom and top of the effective inflow layer in meters above ground level.

---

So in surface based convection, you're talking about something around 0. If it's elevated, you wouldn't start at 0km but the height where your inflow is coming from.

 

[Rich Thompson]

For a typical Plains "loaded gun" sounding, the EBWD and 0-6 km bulk wind difference are almost the same. The differences become more pronounced when you have extremely tall or short storms, or the convection is rooted somewhere above the ground (so-called "elevated" storms).

The 0-6 km bulk shear (really, it's the bulk wind difference) is just the vector difference between the surface wind and the 6 km wind. A simple approximation is the difference between the surface and 500 mb winds, which works well for low-elevation sounding sites.

The EBWD is a little more complicated. You need to know two additional pieces of information: 1) the height of the equilibrium level for the most unstable parcel, and 2) the lowest level in the sounding that results in at least 100 J/kg CAPE and no more than 250 J/kg convective inhibition.

Today's 12z sounding from Slidell, LA shows how the EBWD makes a difference compared to the standard 0-6 km bulk shear:

http://www.chasetolive.com/images/ebwd_example.gif

The most unstable parcel originates at 889 mb and has an equilibrium level near 8 km, while the "effective inflow base" is very close to the most unstable parcel level (889 mb) because of the dry air below that level. If you compare the two shear values, you'll see that the 0-6 km bulk shear (62 kt) is much larger than the EBWD (39 kt).

I illustrated the 0-6 km bulk shear (cyan line) and EBWD (dark blue line) on the hodograph to show how you can "eyeball" the differences.

 

[Jim wyman]

Awesome! Thanks! Yes I was looking for a computational formula for this but I will try to disect the sounding to do this..

I have created a RUC page to take all the indicies and other sounding data to show severity of upcoming convective systems:

www.somdweather.com/ruc_run.php?rucstation=KNHK

Any comments/suggestions would be appreciated.

Thanks again for all of your help....

Jim

UPDATE: I decide to use the LFC level for the base and EL/2 as the top.... Wonder if this would suffice...

 

[Rich Thompson]

Jim,

I'd start with the pressure level for the same lifted parcel used in your other CAPE and EL calculations. The lifted parcel level will almost always be much closer to the actual "effective inflow base" compared to the LFC height, so it will give you a more realistic estimate of EBWD.

Rich T.

 

[Jim wyman]

Rich,
The CAPE and EL quantities have already been calculated in the sounding report I use..

ftp://ftp.meteo.psu.edu/pub/bufkit/RUC/latest/ruc_nhk.buf

Jim

 

[Rich Thompson]

Rich,
The CAPE and EL quantities have already been calculated in the sounding report I use..

ftp://ftp.meteo.psu.edu/pub/bufkit/RUC/latest/ruc_nhk.buf

Jim

Out of those choices, I'd use either use the LCL pressure (LCLP) or the surface. Using the surface will overestimate the shear for "elevated" storm cases, but using half of the EL height will help account for cases with very short or tall storms.

 

[Jim wyman]

Thanks Rich. Will do....