Severe Weather Acronyms and meanings

[JonathanBelles]

I have two acronyms that I need explinations of:
TVS: I know this is a tornado vortex signature, but what does it mean when somebody says "77kt TVS?" (CrazyC83) Does that mean the "tornado had sustained winds of 77kts?

dbz: I know that this is used on the doppler radar and the higher the worst. I also know that this is in decibels, but what does the "Z" mean? Someone explain this.

thank you in advance,
Jonathan/Fact789

[senorpepr]

Jonathan, I'm still formulating an answer to the first part of your question, involving TVS. However, I will say that "77kt TVS" DOES NOT mean it has sustained winds of 77kts. It's a shear factor. Like I said, I'll expand upon this soon.

As for dbZ: db, as you said, means decibels. Z stands for the amount of energy reflected back to the radar.

Here's a clip from a Weather Underground article about decibels in weather radar:

The scale of dBZ values is also related to the intensity of rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ, the stronger the rain rate. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rain rates which are associated to the dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Hail is a good reflector of energy and will return very high dBZ values. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.

[senorpepr]

In the example you shared, CrazyC83 mentioned a “77kt TVS”.

First, I should make it clear that the 77kt figure really doesn’t mean much to the average person and can be misleading.

That 77kt figure is the MDV or maximum delta velocity. In this case, 77kt is the greatest velocity difference of all the two-dimensional circulations.

If you use a program such as GRLevel3, you will receive this data on specific storm cells.

For instance, using GRLevel3, I analysized a cell in the Panhandle of Texas.

Type: TVS
Cell: B0
AVGDV: 52KT
LLDV: 77KT
MDV: 92KT
Hgt: 9600ft
Depth: 27300ft
Base: 3600ft
Top: 30900ft
MXSHR: 34/ks
Hgt: 9600ft

In the above data, this cell is a tornadic vortex signature (TVS). This is defined as a three-dimensional circulation with a base located on the 0.5° slice (below 600m above radar level). The depth of the circulation must be at least 1500m. Additionally, the maximum shear detected anywhere in the circulation must be at least 72kt or at least 50kt at the base of the circulation.

Cell B0 is nothing more than a way to ID the storm, much like giving a tropical storm a name.

AVGDV is the average delta velocity—the average weighted velocity difference of all the 2-D circulations.

(Delta velocity is the difference between the maximum negative velocity [inbound] and maximum positive velocity [outbound].)

LLDV is the low-level delta velocity—the greatest velocity difference of the lowest 2-D circulation.

MDV, as mentioned above, is the maximum delta velocity—the greatest velocity difference of all 2-D circulations.

Depth is the thickness of the 3-D circulation.

Base is the lowest altitude of the 3-D circulation.

Top is the highest altitude of the 3-D circulation.

MXSHR is the maximum shear—the maximum at all elevation scans of the quantity gate-togate velocity difference divided by the diameter of the TVS.

I should also point out what Greg Stumpf mentioned on another message board:

The TDA (Tornado Detection Algorithm) is a misnomer. It is not designed to "detect tornadoes". At NSSL, the algorithm was called the TVS Detection Algorithm, and bear in mind that not all TVSs are assocaited with tornadoes (only about 35-40% in the data set we had, assuming a LLDV > 25 or MXDV > 36, and depth > 1 km). Folks using inverted red traingles to indicate the locations are doing a disservice to the NSSL folks who originally developed this algorithm (yours truly was one of the scientists on the team) since it looks like a tornado when it isn't necessarily a tornado.

Hopefully this points you in the right direction.

[JonathanBelles]

your second explination is a little mind boggiling, but I'm going to do some more research, the explination on dBZ makes sense.

[senorpepr]

That 77kt figure is the MDV or maximum delta velocity. In this case, 77kt is the greatest velocity difference of all the two-dimensional circulations.

Sorry for the delay, I wanted to further explain this, but I had to take care of some other business.

Okay, you remember the velocity product on the radar? Check this link out.

Anyway, when the radar finds a storm, it will look at that velocity product. The radar is looking at how the rain droplets are moving, as compared to the radar site.

Say that within our storm we have an inbound wind of 40kt. That means within the cell, rain drops are flying toward the radar site at 40kt. Now, on the other side of the storm, we have an outbound wind of 37kt. That means that the raindrops are flying away from the radar at 37kt.

Mathematically, we look at it this way: what is the difference between -40KT (note the minus--it's an inbound wind) and +37KT? 77KT...or our MDV.

Does that help?

[JonathanBelles]

that makes much more sense.

[P.K.]

There is some stuff on estimating the preciptation from the radar returns at http://www.knmi.nl/~holleman/radarintro.html.