BRN Shear

[Metsfan]

The other day I was looking at SPC OKX 12z sounding, and wanted more information on BRN Shear. I tried to come up with the exact value that it had on the sounding, but was not getting it.

 

This is what I did.

 

0-6km shear: (45-0)=45

0-.5km shear: (10-0)=10

 

magnitude: sqrt(45^2+10^2)=46

 

.5*46=23 m^2/s^2

 

The sounding had 18 m^2/s^2

 

Am I missing something or doing something wrong?

 

[Chinook]

First of all, I have always found this to be confusing, and I tend to look at the 0-6km shear because I understand it.

 

Somehow density (and/or pressure) give different weights to the 0-500m wind and 6km wind. My best guess is, mathematically, 1/2*[ (5.5-6km wind)^2*(weighting factor) + (0-500m wind)^2*(weighting factor) ]

 

Here is my take on it. There are all sorts of convective situations that are possible. They may be categorized like this:

 

1. there is too much CAPE for shear (ordinary cell, multicell)

2. the CAPE and shear balance is about right for a supercell

3. there is too much shear for the CAPE, storms tend to be ripped apart by shear

 

The BRN should be able to classify these situations, but it won't be perfect.

 

wikipedia article for the regular (not bulk) Richardson Number:

http://en.wikipedia.org/wiki/Richardson_number

 

 

http://www.spc.noaa.gov/exper/soundings/help/shear.html

 

The denominator of the bulk Richardson number, known as the BRN shear term. The BRN shear term is the square of the bulk vector difference between the 0-500 m AGL mean wind and the 5500-6000 m AGL mean wind (both pressure weighted), multiplied by one half.

 

http://www.theweatherprediction.com/habyhints/315/

2. How is BRN determined?

The basic formula is CAPE / SHEAR. CAPE is the amount of pure instability present relative to a rising surface parcel. SHEAR is a mean wind vector between the surface to 500 m above surface average wind speed and the surface to 6 kilometer average wind speed.

BRN= CAPE / (0.5*(shear differential)^2)