Storm Relative Velocity questions

[Dave Gallaher]

Since base velocity differentiates between motion approaching and departing the radar site, how is storm relative velocity marked? Is the azimuth the storm is following the determining line, and therefore + and - velocities are measured as perpendicular to that line? If so, which is measured as +? I know how to read the couplets, but am confused about the theory and how it differs from base velocity.

Also, I notice on GRL3 that minimum and maximum reflectivity readings will frequently be higher in the Product Details box than the colors indicate. Are they measuring different things?

Thanks from an old beginner.

 

[Jeff Snyder]

It's a lot simpler than that. The SRV simply subtracts a radar-estimate storm motion vector to the base radial velocity data. The base data show the radar-relative (i.e. ground-relative) winds towards/away from the radar. If the radar determines mean storm motion to the northeast at 30 mph, it will add subtract that wind from the radial velocity data to arrive at the "storm relative" velocity product. In such a case, it will simply add 30 mph to data that are southwest of the radar, 0 mph to data that are NW/SE of the radar (since storm motion vector has no radial component / i.e. is not toward or away from the radar / to the NW/SE of the radar), and -30 mph to data that are northeast of the radar. All areas between those 4 radials will see a subtraction of radial velocity between [-1 1]*30 (following a cosine curve). The storm motion that's being used to derive the SRV data is shown on the right side on GR3. Note that SRV data will not represent true storm-relative wind when storms are moving different directions, such as can be the case in tropical cyclones or near upper-level lows (among many other situations).

I think the min/max values are valid for all data of the PPI, which include any ground clutter that may be present. Honestly, I haven't found any use for min/max values given this. Perhaps they'd be better for higher elevation scans (i.e. 5 deg), where ground clutter shouldn't be present

 

[Dave Gallaher]

I think the min/max values are valid for all data of the PPI, which include any ground clutter that may be present. Honestly, I haven't found any use for min/max values given this. Perhaps they'd be better for higher elevation scans (i.e. 5 deg), where ground clutter shouldn't be present

Thanks, Jeff--the plus/minus now makes sense. However, in velocity modes (which is what I meant to say in the original post, rather than reflectivity--duh) the Product Details register min and max in kts rather than dBZ, so I assume they are measuring apparent velocity extremes in that particular tilt, right? The values change with different tilts. But the differential between the color table min/max and Product Details is often wide.

 

[JP Santiago]

If I might tag on a follow up question along Dave's topic- why is there a base velocity and storm relative velocity? I'm assuming that having a storm relative velocity cuts down on the radar clutter otherwise thrown up by the storm?

I was watcthing the Breckenridge tornado on the radar loops earlier this week and it seemed the rotation was visible on both storm relative velocity and base velocity.

 

[Greg Stumpf]

If I might tag on a follow up question along Dave's topic- why is there a base velocity and storm relative velocity? I'm assuming that having a storm relative velocity cuts down on the radar clutter otherwise thrown up by the storm?

By subtracting out the storm motion from the velocity field, the inbound and outbound velocities in a vortex couplet will be more balanced, making them easier to identify visually. Note that a couplet's velocity difference (delta-V) remains the same for both V and SRV.

Using SRV for convergence patterns is also good to assess the storm-relative convergence (in the mid-levels, is a precursor to downbursts). However, it should be noted that if trying to assess whether or not there are severe winds at or near the ground, you will need to go with base (or ground-relative) velocity.