Tuesday, September 1, 2020

Landspouts and the meteorological ingredients that can produce them

I've had some requests to update some material about landspout tornadoes that used to be on an old web site of mine, which is the reason for this post. 

A landspout tornado is a type of non-supercell tornado that can occur with a thunderstorm that doesn't have a rotating updraft detectible on radar (a mesocyclone).  Because of this, warnings can't be issued for landspout tornadoes based on radar - they have to be reported by spotters and storm chasers, and there is no warning lead time.  Thankfully, most landspout tornadoes are relatively weak, but on occasion they can become strong in the right setting.

How do landspout tornadoes form?

First, a sharp wind shift boundary must be present.  That's what provides the vorticity (or 'spin') needed for the tornado, when the low-level wind shear that helps to generate supercell tornadoes is absent:

Second, instability (or CAPE - convective available potential energy) must be present along the boundary for a thunderstorm updraft to develop.  In particular, CAPE in the lowest 3 km above ground helps because it can facilitate low-level stretching in updrafts along the boundary due to the instability being located closer to the ground.  

Third, steep low-level lapse rates (a rapid drop off in temperature in the lowest few kilometers above ground due to strong daytime surface heating) can also help accelerate low-level air upward in updrafts, increasing low-level stretching.

Here's one example of these ingredients coming together in a setting that might generate and support landspout tornadoes, using SPC mesoanalysis page graphics:  

If a thunderstorm updraft forms on a sharp boundary in such an environment with everything coming together just right (for example, the storm updraft must develop directly over the boundary and align properly with a small 'wiggle' or pocket of 'spin' on the boundary), the thunderstorm may then generate a landspout tornado through upward stretching of that 'spin':

Here's an example of a setting for landspout tornadoes that occurred on May 25, 2018 in both southern Minnesota (MN) and eastern Nebraska (NE).  The surface map at early afternoon on 5/25/18 showed a relatively stationary wind shift boundary (thick brown dashed line) stretching from a low in southern MN into east-central NE:

SPC mesoanalysis graphics at early afternoon on 5/25/18 showed plentiful low-level instability (0-3 km CAPE, in red, 1st panel below) over southern MN near the surface low where a storm was developing on the boundary (light blue lines also show surface vorticity or 'spin' along this boundary).  The 2nd panel below showed low-level lapse rates (0-3 km) to be steep over a large area, including all along the boundary, with lapse rates > 8.0 deg C shaded in orange:

Below is the developing storm on visible satellite (yellow arrow) at about 2 pm CDT.  Less than an hour later, a landspout tornado developed on the southwest edge of this new storm (photos also below):

From the 5/25/18 surface and SPC graphics up above, notice that all of the ingredients listed earlier for supporting landspout tornadoes (boundary, instability, steep low-level lapse rates) were present in this case, increasing the possibility of a landspout tornado _if_ the storm aligned properly with the boundary and the 'spin' along it.

Later in the afternoon, other storms developed southwestward on the wind shift boundary over Iowa and Nebraska.  After 5 pm CDT, a small high-based storm on the boundary over northeast/east-central NE (yellow arrow in satellite photo below) apparently "phased' with the boundary enough to produce yet another landspout tornado (see photos below):

As can be seen from the satellite photo, there were _many_ storms that developed along the boundary by late afternoon, but only _one_ of the Nebraska storms produced a tornado.  This certainly highlights that landspout tornadoes are not really forecast-able in advance.  A meteorologist can only note that ingredients possibly supportive of landspout tornadoes (the boundary, instability, low-level lapse rates) appear to be coming together in an area, so that reports of such tornadoes are not a surprise and can be warned immediately when spotted. 

Here are a few landspout tornado cases I've posted about on my blog:

    June 21, 2020 landspout tornado in northwest Kansas

    April 17, 2019 landspout tornadoes in Oklahoma, Kansas, and Texas

    May 19, 2012 non-supercell tornado outbreak in southern Kansas

And, for more technical reference, here are some papers that discuss landspout tornadoes:

    Non-supercell tornadoes (Wakimoto and Wilson 1989)

    Tornadoes in non-mesocyclone environments with pre-existing vertical vorticity along convergence boundaries (Caruso and Davies 2005)

    Tornadoes in environments with small helicity and/or high LCL heights (Davies 2006) 

It is also important to recognize that there are some cases where both non-supercell and supercell processes appear to be at work, particularly when winds aloft are strong enough to support and organize storms into supercells while ingredients related to landspout tornadoes are also present.  This is particularly true in the High Plains of the U.S., and such "hybrid" events are difficult to categorize.  Here are a couple examples:

    June 29, 2019 long-lived tornado in southwest South Dakota

    June 6, 2018 large and long-lived tornado near Laramie, Wyoming

Hopefully, this short discussion will help those interested in understanding some of the important factors and ingredients that contribute to landspout tornadoes.

- Jon Davies 9/1/20