Ingredients supporting non-mesocyclone tornado events         (summary by Jon Davies)
( Case studies:     4 brief case studies       Paper on non-mesocyclone tornadoes  )                Strong>      Weak>

Most non-mesocyclone tornadoes are relatively weak, but a few have been documented up to F2/F3 in intensity.  Wakimoto and Wilson (1989) and others have identified the general processes (see diagram from WW89) and ingredients that appear to be associated with non-mesocyclone tornadoes, also called "non-supercell" tornadoes or "landspouts":

• A slow-moving or stationary wind shift boundary with pockets of vertical vorticity
• Developing thunderstorm updrafts (in the convergence directly over the boundary) that can stretch the low-level vertical vorticity into tornadoes if the updrafts are properly co-located with the available vorticity.

The boundary in these cases appears to be a crucial ingredient, providing focus and convergence along with vorticity.   Such boundaries tend to be slow-moving or nearly stationary cool fronts that are weak with little temperature contrast, surface troughs, or topographically-induced convergence zones, and are often oriented NE-SW in some fashion.  The wind shift across the boundary is typically from S or SW ahead of it to W or NW behind it, generating a sharp line of vorticity.   In contrast, relevant boundaries for many supercell cases are often warm fronts or old outflow boundaries oriented W-E or NW-SE with winds shifting from S or SE to ESE or E across the boundary going northward, with storms crossing or paralleling the boundary rather than forming directly over it.  

While little has been done in operational research to clarify the thermodynamic characteristics of an environment near a boundary that would facilitate the required rapid parcel stretching in low-levels for these cases, some characteristics and general pattern features are suggested below that may help in short-term identification of settings that can support events lasting up to an hour or more involving multiple non-mesocyclone tornadoes:
                                                                                                       <(composite example by Jon Davies)
The composite above drawn from several cases shows a typical boundary, surface heat axis intersecting the boundary, and area of steep low-level lapse rates (e.g., roughly 0-2 km or 0-3 km AGL) co-located with sizable 0-3 km MLCAPE (e.g., relatively low LFC heights) and small CIN along the boundary where storms begin to develop.  The combination of steep low-level lapse rates and low-level CAPE suggests a low-level environment conducive to enhanced stretching beneath updrafts on the boundary with enough mosture depth and quality to reduce mixing and entrainment that would otherwise dilute buoyancy of lifted parcels below cloud base.  If all these ingredients (the wind shift boundary, favorable low-level thermodynamic environment, storms forming on the boundary) come together properly, non-mesocyclone tornadoes may occur, particularly where localized outflow intersects the boundary as storms evolve.  Southern-most cells seem to be favored, and tornadoes can occur rapidly and early in updraft life-cycles as stretching intensifies.  Awareness of these ingredients and where thay seem to be coming together in some cases may provide heightened short-term awareness to forecasters and warning meteorologists of an environment supportive of non-mesocyclone tornadoes, not detectable using "typical" tornado forecast parameters associated with supercells.

As further explanation of the enclosed blue area on the composite above, recent investigations of model analysis profile estimations from several non-mesocyclone tornado events suggest that the following low-level thermodynamic characteristics are important:

• Steep lapse rates that approach the dry-adiabatic lapse rate (roughly 8- 9^oC/km or more) in the lowest 2 or 3 km
• MLLCL heights that can be much higher than with supercell tornadoes (often 1500-2000m AGL)
• MLLFC heights that are relatively low (roughly below 2200-2400 m AGL) located just above the MLLCL, with little MLCIN between the two levels, resulting in sizable 0-3 km MLCAPE (roughly 30-50 J/kg or more)

Such an environment (see example skewT) would likely facilitate rapidly rising parcels of air in the boundary-layer from afternoon heating, with little CIN to retard upward parcel acceleration in low-levels, enhancing stretching beneath updrafts along the wind shift boundary.  The relatively low LFC heights resulting in notable low-level CAPE might suggest a certain depth or degree of moisture in the low-level environment to prevent near-surface parcels from "mixing out" too quickly, helping them to retain buoyancy and upward acceleration.

These case studies and papers have examples of non-mesocyclone tornado settings, both tornadic and nontornadic, that may be helpful:

                -   4 brief case studies of settings with potential for non-mesocyclone tornadoes (2003-2004,)
                -   Tornadoes in non-mesocyclone environments  (NWA online paper by Jim Caruso and Jon Davies)
                -   Tornadoes associated with small SRH or high LCL environments (informal paper by Jon Davies)

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