The first thing to understanding how Supercells can become cyclic mesocyclone producers is to understand how a Supercell works. If you look at the schematic to the right (Figure 1) you will see an analysis of the mesoscale features within a Supercell Thunderstorm. First of all, the heavy black arrow is the Storm Motion Vector, or the SM-Vector. Supercells for the most part are maintained from upper-level energy from 300mb jet streaks, mid-level inflow, and low-level heat & moisture (CAPE and Theta-E), wind shear, and streamwise vorticity (Helicity). Each and every one of these items, if found existent, can provide the ingredients for large Supercells to produce strong, long-lived Mesocyclones. If one of these key ingredients is found to be non-existent or weak, Supercells can and often still develop strong mid and low-level rotation. However, in these extreme cases, cyclic mesocyclones develop. Most often cyclic mesocyclones develop due to weak mid or low-level inflow not allowing cool, dry air from the Rear-Flank Downdraft (RFD) to wrap into the meso-low.
In Figure 1, the analyzed cold front is the leading edge of the cool, dry RFD air... If the items discussed earlier all exist, this cool, dry air is entrained into the meso-low by strong storm-relative mid and low-level inflow. The dark blue arrow shows the entrainment of this air into the meso-low. The red arrows indicate the mid and low-level inflow of warm, moist Theta-E rich air. If streamwise vorticity is positive and it's "crosswise vorticity" (Helicity) component is strong and the updraft is powerful enough, an enhanced probability for tornadoes exists. If one or more of these items are negligent or weak, then the tornado will not develop and the updraft dissipates.
However, if instability is high (high MLCAPE's) and there exists a deep layer of shear, the RFD may still be able to wrap into the meso-low. When this occurs, you may still have an enhanced tornado probablity simply due to the storm-relative environment. If a deep layer of shear exists and high MLCAPE's are producing enough bouyancy, the strength of the updraft (the meso-low) can likewise increase. Entrainment of the cool, dry RFD air occurs, wrapping into the meso-low. As long as the mid and low-level inflow maintains its strength, the RFD air continues to wrap into the meso-low, eventually wrapping itself completely around the meso-low. At this point, the low occludes. It is at this point where the mesoscale low-pressure system is at it's peak intensity; and likewise when the highest probability of tornadic potential exists. If the RFD occludes the mesocyclone, however, and mid or low-level inflow weakens the meso quickly dissipates.
If high instability still exists to the East or Southeast of the Supercell and a flanking line extends to the SW, likely bouyancy from strong surface heating and/or Theta-E rich air interacting with strong pressure falls ahead of the Supercell will aid in the development of a separate meso-low. This new meso-low will move NEward along the "cold frontal" portion of the Supercell until it encounters the area of maximum inflow (the warm frontal portion of the Supercell). This "new" meso-low is represented by the light blue "L" in Figure 2.
In Figure 3 the new low has now become dominate as the now older mesocyclone is undercut by the cooler, drier air of the RFD and accelerated West or SW-ward further developing the Supercell's flanking line. This is why in very intense and large Supercells you don't often see very long flanking lines. Cyclic mesocyclone Supercells, if compared, would probably exhibit longer flanking lines and more powerful backbuilding. As the process repeats itself, the now dominate meso-low moves somewhat northward toward a greater pressure gradient. Then, if any upper-level energy from the 300mb jet streak, mid-level inflow, or low-level heat & moisture (CAPE and Theta-E), wind shear, and streamwise vorticity (Helicity) seems to lack, the process starts over once more. Sometimes, even when these conditions exist not only will Supercells produce cyclic mesocyclones, but also cyclic tornadoes. This occurs most often when conditional instability is very high and streamwise vorticity and it's crosswise component (Helicity) are positively correlated.
