Large hail formation with updraft speed

[Marko Korosec]

Hello all,

as we had an extreme hailstones for our zones (I cannot recall stones of 6-8cm or even larger in the literature here in Slovenia in the past), I was wondering what kind of vertical speed are on going in such situations with large hail.

I know how it works for getting large hailstones, with strong vertical speeds in updrafts, entering dry intrusion for lowering the freezing level, etc... but in what kind of range for upward speeds we are talking about? There in RAOB is a MVV (WMAX) parameter, which is connected to CAPE to get a potential max vertical speed of an updraft.

Is there any other ways to estimate updraft speed? I can only imagine how insanely strong updraft speed was to maintain those large hailstones.

Also one thing might be interesting to know, convection was so explosive, that on radar scans it went from zero to 65+dBz in less than 20min. That must have been one hell of an updraft speed!

Thanks for any suggestions.

 

[Paul Knightley]

The problem with using CAPE to predict updraught speed is that it is extremely dependant on what surface parcel you are inputting, and also what the upper air is like. You can travel just small distances horizontally and find quite different surface parameters, and sometimes upper air parameters. This has an enormous bearing on what value of CAPE (and hence w_max) you calculate. Also, this parcel method does not take mixing and friction of the ascending parcel into effect; thus the actual updraught velocity will be lower. Even so, it does seem that there is a link between large CAPE values and large hailstones. Another thing that the latest issue of RAOB does is give you "hail CAPE" - that is CAPE calculated between -10 and -20C isotherms.

 

[Jeff Snyder]

In addition to what Paul wrote, it's important to remember that CAPE / thermal instability is only ONE contributor to updraft intensity. Vertical perturbation pressure gradients (a function of the vertical wind and shear profiles) can have just as great a contribution to the updraft velocity as CAPE does. Typically, while SQRT(2*CAPE) can be an estimate of velocity, it's often seen that actual updraft velocity is 1/2*(SQRT(2*CAPE)) owing to invalid assumptions used in parcel theory (i.e. caused by water loading, mixing, etc). Regardless, the role of the vertical perturbation pressure gradients helps explain why a supercell (with a meso, by definition) in an environment of 2000 j/kg CAPE (for example) can produce much larger hail than a multi-cell cluster (i.e. no meso and typically in weaker shear) with the same 2000 j/kg CAPE. Add in hail trajectory differences, and you get a complicate picture.

I know this isn't exactly what you were asking, but I figured I'd mention it anyway. I've remember hearing ~90 mph updraft to product a baseball-sized hailstone (though, again, there are many complicating factors, not the least of which is the effect of melting that occurs as a stone falls from the freezing level to the ground), but I can't give you any more specifics. I'd do a quick search of AMS publications to help you out, but I need to head out.

 

[cdcollura]

Good day all,

As with supercell updrafts, speeds of 150 to 175 MPH are not un-common.

A large hailstone (baseball sized / 2.5") has a terminal velocity of about 80-85 MPH. Grapefruit sized (4") or larger would have a velocity of about 105 MPH. Largest (recoerd breaking events, such as 5"+ in Wisconsin in June 2007 or 6"++ in Nebraska in June 2003) can approach 120 MPH (that's about the upper limit, similar to a human in free-fall (skydiving), as hail is composed of water, (or mostly water) like a person).

Note: Hail stone fall rate is not related to the size of the stone but to the weight-to-drag ratio of it. This is why a pebble falls slower than a boulder at terminal speed (in air, not vaccuum) because the pebble (although made of rock also) has a higher drag ratio due to the boulder. In other words, if a beachball sized hail stone was found, it will not fall more than about 120 MPH. Smaller stones up to grapefruit size show the biggest terminal velocity differences as size increases, but not much past that. The speed is

Below is a table for stones and their approximate terminal airspeed as MSLP...

Pea (1/4") ... 15 MPH
Quarter (1") ... 35 MPH
Golfball (1.75") ... 50 MPH
Baseball (2.5") ... 80 MPH
Grapefruit (4") ... 105 MPH

To effectively allow a large stone to grow, the hail must be bbrought above and below the freezing level and to very high altitudes (around 20,000 feet), with deviations between 10,000 and 40,000 feet ocurring very quickly and in a timely manner to allow maximum growth with little melting. This is roughly 50% to that of the stones terminal free-fall speed.

So a basball sized stone, with a terminal speed of 80 MPH, would require a minimum updraft of (80 x 1.5) about 120 MPH.