Major gap in inbound vs. outbond couplet

[Jason Harris]

Major gap in inbound vs. outbound couplet

Watching the readings on GR3 for the supercell N of Winterset, IA. Just saw the couplet go from a balance of 52.5 kts for inbound/outbound to now an imbalance of 67.5 outbound/15 inbound. Should one assume the chance of the tornado dissipating goes up with such a reading and can the much stronger outbound winds be tied to RFD occlusion? I assume I'm making at least a couple of mistakes here, so please clear me up on things. Currently the TVS is sitting right on the couplet.

* And now the inbound is gone altogether. No more couplet. But TVS remains. (everything on .5 SRV1) And now TVS gone too.

 

[rdale]

Watch the speed you are using for cell motion, that may have a lot to do with it. Adjust that for the scan in question to make sure that's on track. Otherwise your "balance" will be out of whack. What might be easier to worry over is the delta-V, the total change (i.e. outbound + inbound, divided by 2.)

 

[Jason Harris]

Also ahaberlie just made the point on the NOW thread "looks like the outflow of the adjacent storm to the south cut off the inflow." What's the "speed" setting you're referring to? Whole storm is getting close to the radar too, so I assume that's why we lost some of the hail markers, unless there was a major cloudburst (omaha, NE still has it, so seems to just be the approach to KDMX is too close). Scan is pretty low, under 500 feet for much of storm. I see that 651 CDt still have report of tornado. 7 miles southwest of desmoines moving east at 50 mph. This is despite losing the couplet reading.

 

[rdale]

You need to set the storm's motion in order to determine the SRV. If you aren't using Level II data, you are simply using an average motion for all storms and that can change quite quickly and not be accurate for the storm you are watching.

 

[Jason Harris]

So GR3 just doesn't have the precision to accurately know whether SRV for a couplet actually indicates a clear inbound/outbound distinction? The pixel scan with the mouse reading isn't reliable?

 

[rdale]

Well, I'm not sure what you're asking Quick answer -- it depends. If the storm motion being used by the 88D is the same as the actual storm motion of the cell you're watching, it'll be pretty accurate. If it's not, the change in velocity will still be correct but the in vs out will be offset.

 

[Jeff Snyder]

So GR3 just doesn't have the precision to accurately know whether SRV for a couplet actually indicates a clear inbound/outbound distinction? The pixel scan with the mouse reading isn't reliable?

On the right side of the window, when you have an SRV product open, you'll see the storm motion characteristics using to calculate storm-relative velocity from the (measured) ground-relative velocities. It's not so much a GR3 thing as much as a Level 3 radar thing (which applies to all Level 3 SRV data). In addition, you sometimes need to be careful when using the SRV products because it is calculated by subtracting a single storm motion (speed + direction) from the base velocity data. However, not all storms within the radar's view may be moving the exact same direction and speed. For example, considering a hurricane coming on shore -- the radar has to "assume" one single storm motion, even though the landfalling cyclone likely has storm "cells" that are moving in all directions (typically cyclonically about the eye or center of the hurricane). You're also likely to see inaccuracies in the derived SRV data when you have supercells that have varying yet deviating motions (considering a supercell splitting -- the left split will typically move left of and more quickly than previous, while the right split will typically move right of and more slowly than previously).

 

[Jason Harris]

So, my main question is, assuming that the storm relative velocity is relatively close to the storm speed (for instance, right now cell W8 is moving at 36 kts. and the estimated speed for SRV on GR3 right now is 37. 6 kts.) when I saw the big discrepancy between the outbound vs. inbound wind right before the couplet vanished on radar on that cell (the Winterset cell earlier), can this actually be an indication of RFD occlusion or is that not a relevant speculation? Assuming the reading is accurate, what does such an imbalance suggest?

 

[Jeff Snyder]

Jason -- I haven't looked at the particular data you mentioned. In general, for a reasonably axisymmetric vortex, there should be a nearly "symmetric" inbound-outbound couplet (with maximum in magnitude gate-to-gate) in a vortex-relative sense. I say vortex-relative since even if the "storm motion" estimate is perfect, the actual vortex may have a different motion than the main cell echo. At any rate, you will often see general horizontal shear (characterized by a change in radial velocity between adjacent radials in most cases) associated with a storm's primary inflow or outflow. As such, you may be able to see the rear-flank downdraft surge and wrap around the existing mesocyclone, which often results in a very asymmetric radial velocity appearance.

 

[Jeff Duda]

It sounds to me like looking at storm relative velocity would help you understand this a lot better than using base velocity. GR3 does have storm relative velocity products on the same tilts as it has base reflectivity and base velocity. Just go to the products menu in the menu bar and click on the storm-relative velocity products to view them. In level 3 data, you don't have the option of setting the storm motion vector. It is done by someone else (either an average of all active cells pegged by SCIT on that scan or by the radar operator at the WFO operating the radar you're viewing). In GR2 and GR2AE you can manually set the storm motion vector. I'd be willing to bet the reason you saw the imbalance between inbounds and outbounds was because you were looking at base velocity instead of storm-relative velocity. Like Jeff said a few comments back, if the storm motion vector set in the program is not the same as the true motion of the storm you're viewing, you will still get imbalances. That's a result of not having a proper storm motion vector set.

 

[Jason Harris]

Thanks for the responses Rob, Jeff Snyder, and Jeff Duda.

So, with the asymmetry in the couplet that might indicate the RFD wrapping around the meso, would that be manifest more in the outbound wind value of the couplet assuming the storm is to the south of the radar as was the case earlier today?

you may be able to see the rear-flank downdraft surge and wrap around the existing mesocyclone, which often results in a very asymmetric radial velocity appearance.

Also, regarding this, it was definitely not BV I was looking at: it was SRV1 on GR3.

I'd be willing to bet the reason you saw the imbalance between inbounds and outbounds was because you were looking at base velocity instead of storm-relative velocity

I was toggling up and down from SRV4 down to SRV1, not looking at BV1. Like I said for a while during the tornado warning the SRV1 inbound/outbound values of the couplet were identical: 52.5 kts. Then just a little before ahaberlie made the comment on the "NOW" thread about the couplet disappearing, I noticed the inbound color had switched from the stronger blue to green and the value was down to 15kts while the outbound was a white color (using the spectrum I have preselected for more contrast than just brightness of green & white) according to the pixel scan with the mouse the outbound was 67.5 kts. After the couplet had disappeared then saw ahaberlie's post that ""looks like the outflow of the adjacent storm to the south cut off the inflow."

There was still an area on SRV of 67.5kts. for outbound winds even in the absence of the couplet afterwards.

Also, another thing I was wondering about was the report from NWS that emergency workers continued to report a tornado although the couplet was no longer showing on radar. Was the message text a leftover artifact of the earlier report or was there still a tornado visible despite radar indicating no couplet?

AT 651 PM CDT…EMERGENCY MANAGEMENT REPORTED A TORNADO. THISTORNADO WAS LOCATED NEAR NORWALK…OR 7 MILES SOUTHWEST OF DESMOINES…MOVING EAST AT 50 MPH.
LOCATIONS IMPACTED INCLUDE… DES MOINES…WINDSOR HEIGHTS…CARLISLE…PLEASANT HILL… ALTOONA…BONDURANT…DES MOINES INT AIRPORT…HARTFORD…RUNNELLS AND MITCHELLVILLE.

The news story also claims tornado was still visible, which would have been after couplet dissipated on radar, particularly with the time gap in data:

Emergency management officials reported a tornado on the ground north of Norwalk at about 6:50 p.m.

http://blogs.desmoinesregister.com/dmr/index.php/2011/03/22/tornadoes-batter-winterset-creston-areas-d-m-metro-endures-hail/

Anyway, I'd think someone else could just go over radar scans from about 6:40-6:55 CDT and check out to see how asymetry develops near the end of that couplet. Is it even really at all unusual? Wouldn't one expect some asymetry as circulation gets disrupted with most mesos lifespans?

(In case there's any confusion, this was the cell near Winterset, not the Creston cell that there are reports of already on this site. Haven't seen any of the Winterset pics yet, but looks like all the Madison reports are of that on SPC.

 

[Jeff Duda]

Alright, Jason, I had a look at the scans you mentioned. I apologize for misunderstanding the products you were viewing. Regarding the explanation for the appearance of the velocity field, I am not the expert on this forum (Jeff Snyder will probably have a better explanation for this), but I can list some factors that will affect how you see it:

1) There was about 4 minutes between scans during this period of time (2335 - 0000 UTC). Especially during the cooler months of the year, a lot can happen in a 4-minute block of time and it is possible that a tornado occurred very briefly during the 4 minutes between scans and simply was missed by the radar.

2) Although the storm was quite close to the radar (the 0.5 degree tilt scan was between about 700 and 1100 ft ARL during this time period), know that this storm was crossing (or as I like to call it, "tripping") over a fairly narrow warm frontal boundary. The mesoscale circulation associated with the front could've been doing some strange things during the short time period (probably no more than 30 minuets) that the storm (not necessarily the mesocyclone or tornadic portion of it) was crossing the boundary. Also, given the surface conditions (T/Td of and ~61/54 and 62/54 at the nearest observation site, KDSM - the Des Moines Intl. Airport - at 2344 and 2354 UTC), there was a fairly low T/Td spread indicating low cloud bases, so it is possible that the tornado actually occurred below the beam on the 0.5 deg. tilt (although I think this is unlikely).

3) Tornadoes are not very large and are rarely explicitly resolved on WSR-88Ds. Doing some rudimentary calculations, the area of a bin at about the distance from the radar as the feature in question is about 55,000 m^2. That may seem large, but a tornado with a diameter of 100 m is only about 62,000 m^2. I haven't seen any pictures of the tornado(es) from this cell, but if it/they was/were anything like the Creston tornado (I did see pictures of that in the REPORTS thread) then this tornado was likely small in diameter. Thus it is possible that the tornado either fit entirely within one bin of radar data or was larger than one bin but only by a little. Thus due to the averaging done when velocity is shown, no extreme velocity values appeared despite the presence of a tornado. FYI, when you see velocity plotted on radar it is the AVERAGE velocity of all the objects in the pulse volume (i.e., in that bin); the spectrum width is the standard deviation of the set of individual velocity values measured within that pulse volume.

4) While also unlikely, I suppose it is also possible that there were some small errors in the transmitting, receiving, and/or processing of the radar signal during that pulse and at that range and azimuth. The radar site itself was in the FFD of the supercell during most of this period, so attenuation or other meteorological factors could also cause some small errors.

The above regard possible scientific reasons for the discrepancy in your understanding. Now I'll discuss some non-scientific reasons to explain this all.

1) The NWS isn't perfect. There could be many small errors that add up to give somewhat of a not-totally-accurate report. Perhaps the report came into the DMX WFO at 6:50 when the emergency management officials saw the tornado a few minutes earlier. Perhaps the officials themselves were north of Norwalk when they made the report and the tornado itself was not at their location, but at some other location (I believe this does sometimes happen when reporting LSRs).

2) Looking at the 2351 UTC scan from DMX, it doesn't look likely that there would be a tornado on the ground anyway because for one, the center of the old couplet looks to still be about 1 mile west of I-35 at the same latitude as Cumming, a good 5 miles west or so of "north of Norwalk". Two, I see no hint of rotation or even any "dirty" velocity signatures (i.e., I see only minimal variation of velocity with respect to changes in azimuth or range instead of lots of variation over small spatial distances like you see farther west in the region of the former couplet). With this scale of distance and time, enough small errors in the reporting or the data can add up to something seemingly significant in our minds when we interpret this image.

Like I said, I'm probably not the most knowledgeful about this on this forum, but I think these are all possible explanations for why you saw what you saw on the radar and what might've happened.