Question: Tornadoes and cloud bases

[J.K Oudshoorn]

Hello,

1st question: here's just a question I want to be sure about... There are much kinds of tornadoes... large cone, big wide/wedge etc... but does the cloud basis of the supercells play a roll in the kinds of tornadoes it (can) produce(s)? for example.. a very logistical thought would be that supercells with a (very) low cloud basis can easily spawn wide tornadoes, and supercells with a relatively high cloud basis spawn cone-like tornadoes?

2nd question: maybe a very stupid question, but just want to be 100% sure about this... do supercells with low cloud basis will spawn tornadoes much easier than high cloud basis?

Thanks,

Koos

 

[Alex Goldstein]

From what I understand, generally for tornadoes to occur the parent storm has to be rooted in the boundary layer, (surface based) so the lower the cloud base, the more conducive to tornadoes the storm will be.

Feel free to correct me if I am wrong.

Thanks.

 

[Bobby Prentice]

1st question: here's just a question I want to be sure about... There are much kinds of tornadoes... large cone, big wide/wedge etc... but does the cloud basis of the supercells play a roll in the kinds of tornadoes it (can) produce(s)? for example.. a very logistical thought would be that supercells with a (very) low cloud basis can easily spawn wide tornadoes, and supercells with a relatively high cloud basis spawn cone-like tornadoes?

In general, lower thunderstorm cloud bases equate to an airmass with a higher relative humidity which would make condensation within a tornado easier to achieve and equate to physically larger tornadoes. A wedge tornado is just as much a function of thunderstorm cloud base height as it is tornado width.

2nd question: maybe a very stupid question, but just want to be 100% sure about this... do supercells with low cloud basis will spawn tornadoes much easier than high cloud basis?

First, let's define how cloud bases are determined:

lifting condensation level—(Abbreviated LCL; also called isentropic condensation level.) The level at which a parcel of moist air lifted dry-adiabatically would become saturated.

On a thermodynamic diagram it is located at the point of intersection of the dry adiabat through the point representing the parcel's original pressure and temperature with the saturation mixing ratio line having the same value of the mixing ratio as the parcel. The pressure and temperature at the lifting condensation level are usually called the condensation pressure and condensation temperature, respectively, and the corresponding point on a thermodynamic diagram is called either the characteristic point, adiabatic saturation point, or adiabatic condensation point. See convective condensation level, conditional instability, saturation level.

Some atmospheric scientists believe there is a relationship between cloud base (LCL) height and significant tornadoes:

Tornado Warning Guidance: Spring 2002

II. GENERAL GUIDANCE
<snip></snip>
B. REAR FLANK DOWNDRAFT CHARACTERISTICS

The thermodynamic characteristics of the rear flank downdraft (RFD) also appear to be important to significant supercell tornadogenesis of F2 strength or greater (See Markowski, 2000). RFDs that possess a relatively low equivalent potential temperature (θe) deficit between its source region and the surface are more likely to enhance tornadogenesis. In addition, tornadogenesis is more likely as the potential buoyancy (CAPE) in the RFD increase, and as the CIN associated with RFD parcels at the surface decreases. Since direct surface measurements of RFDs are not routinely available, rough mesoscale approximations for enhanced surface humidities in the vicinity of tornadic-associated RFDs are small surface temperature/dew-point temperature spreads and low Lifting Condensation Levels (LCLs). According to Rasmussen and Blanchard (1998), the median LCL for significant tornado sounding dataset was 780 m and for nontornadic supercells, 1230 m above ground-level. LCLs above 1500 m have rarely been associated with significant tornadoes. However, there are also a significant number of nontornadic supercells with low LCLs. Additionally, as there is only a 64% correlation between LCL heights and RFD θv deficits, as reported by Markowski et al. (2002), other processes and parameters influence the eventual thermodynamic characteristics of RFDs.

To properly use LCL height as an RFD proxy, remember that low LCLs do not increase the tornado threat of a supercell by themselves. However, the presence of high (>1500m) LCLs, even with other parameters favorable for tornadoes, may be enough to dramatically lower the probabilities of a supercell to initiate significant tornadoes. Weaker tornadoes that are still significant from a tornado warning decision standpoint are not as well correlated with LCL or even RFD θe deficits. Finally, there is no established relationship between LCL heights and non-supercell tornadoes.