Storm relative Velocity

[Kbroeren]

Hi Guys,

This is Kevin from the Netherlands. We are currently busy with a radar project based on wradlib n Python.
We have data files with 14 elevation angles, full azimuts and rage bins. This contains data of spectral, reflectivity and velocity.

Now the big question, what should be the best way to calculate the storm relative velocity, becaus we have only the base velocity available.
I have read something that you must add the shear vector to the values in the array of the data, others say to add the storm motion to the data array.

Here a sample of the output of our data files:



Thank you in advanced.
Kevin

 

[Jeff Duda]

1) Determine a storm motion vector. You can either use a column-averaged mean wind vector or have someone estimate the average motion of individual entities/cells. Or just use something arbitrary.

2) SRV = BV - SMV (where SRV - storm relative velocity, BV - base velocity, SMV - storm motion vector).

Keep in mind the above equation is a vector equation, not a scalar equation. You will have to compute components and perform the actual subtraction on the components of the vectors.

 

[Kbroeren]

Thanks for the quick answer jeff! Is it acceptable to use GFS model data for estimating the storm motion vector ? And maybe do you have an example how to compute the components and performing the actual substraction on the components of the vectors.
We works with Python, and all the data is in numpy arrays. Thank you!

 

[MClarkson]

I think if you took the average of the base velocity of all the nearby(nearby might include the entire radar site, I don't know whats best) cells, averaged over a couple of the higher levels as well like Jeff said, and used that as your storm motion, it would be more accurate than the GFS forecast data. Asking any operational forecast model to pin down the details of a mesoscale environment is pushing beyond their ability quite a bit.

 

[Kbroeren]

Thank you, so if i understand you correctly, i can get values from the data-array containing the velocity data to calculate the storm motion ? That sounds much better then using model data to me... Ok, now find out how to realize this.

 

[MClarkson]

Ya, I would start by taking the average of the all the velocity data in the lowest couple tilts to determine the mean wind, which is usually pretty close to storm motion. Keep in mind you might have to throw out the higher tilts as you get far away from the radar site and the lowest tilt right next to your site, depending on the the exact angles and range and how good your ground clutter algorithms are, for best accuracy.

Once it is working for the entire domain, you'd probably want to break it down further into smaller sectors because the mean wind can change pretty rapidly in some environments, like near a front or low pressure center.