All images and text © copyright Gene
unless otherwise indicated.
It all begins with cumulus clouds, but we generally know what those are so allow me to start with the ones that build into towering cumulus and then into storms. Severe and tornadic storms require increasing wind shear with height in the atmosphere. Both directional and speed shear are beneficial. The introduction of shear not only aids in the development and intensity of long lived severe storms, but it contributes to the picturesque beauty. Tropical storm near the equator "go up and fall down"; the supercell storm stays strong for hours as the dry shearing wind shapes and configures the cloud to amaze all that watch.
These are two examples of this type of shear. On the first image note
the low level wind in coming into the cloud from the right (south) and the
upper level winds are westerly. This is directional shear with height. That is,
the wind changes from south to west or about 90 degrees of turning with height.
This turning of the wind may result in the rotation of the storm. Increasing
speed shear or higher winds further up in the atmosphere also aid the
development of severe storms. This all assumes the other requirements are
present for storms like deep moisture and instability, or warm temperatures at
the surface and cold dry air above the moist layer. In these images deep
moisture is assumed to be in place or these type of towering cumulus would not
occur. Incidently, the buildup in the first image went on to produce a damaging
tornado in northeast Nebraska after dark. The second buildup was part of a line
of storms that caused severe weather and a tornado near Wichita, Kansas. A
meager beginning for such powerful storms.
The first image is a flanking line of new cumulus towers feeding into the back of a severe storm. The top of the image is the anvil canopy. The bottom of the storm is liquid water from cumulus clouds and the top is partly ice as the anvil of the storm glaciates and moves east. The strongest winds are at the anvil level of the developing storm. The second image is an good example of how the dryline looks under exceptional shearing conditions aloft. The shot was taken looking north and the storm is shearing (east) in the westerlies. Note the dramatic chimney effect as the thermals break through the afternoon temperature inversion. On both days tornadoes were produced.
The left image is a severe thunderstorm near Abilene, Texas.
Note the cumulus towers feeding into the back of the storm as the anvil spreads
out. Strong storms usually have a very solid anvil appearance, but this is not
always the case. In this cell the updraft is so strong that some of the cumulus
in the anvil pushing down as the updraft "folds over". The second image is
looking north and the jet stream westerlies are coming from left to right. The
storm is just beginning to back shear into the prevailing westerlies.
Indicating a strong updraft in the storm.
This image depicts a penetrating, or overshooting storm top above the anvil. The anvil is the equilibrium level for the updraft and the area where the updraft gets sheared off. An overshooting top seen during the spring indicates a strong updraft able to punch through the westerly jet stream winds and past the equilibrium level of the storm. A series of tops are shown in this image downstream from the current penetrating top. They tend to blow over and downstream in the jet stream winds. As the updrafts blow downstream part of the precipitation stays in the anvil and some of it falls back to earth as rain and hail.
If you can see a continuing series of penetrating tops rise and fall
downstream the storm may be pulsing. A strong supercell thunderstorm will
generally hold one overshooting top in position for a long time, as new towers
replace the falling part of the top. In extreme tornadic situations you may
witness an overshooting top "roll over" or fold over, as new activity climbs up
the back while the front of the overshoot collapses into the storm. This may
occur while a violent tornado is on the ground. Last, a slow falling
penetrating top does not mean the storm is dying. During many documented long
track tornadoes the top will "net collapse" during the tornadoes'' time on the
ground. In one situation the storm top dropped from 50,000 feet to 38,000 feet
by the time the tornado ended.
This view, courtesy of NOAA, shows a classic
A horizontal view of the thunderstorm shows the common features in a tornadic storm as it would be seen looking west, provided the visibility were excellent.
These two images illustrate like, but visually different processes that both contribute to the persistence of the storm. In the first image the rain falling out of the center of the storm is commonly called the core, or the main precipitation area. It contains rain and sometimes hail. Much of the wind a storm produces is a result of the cool precipitation falling and spreading out. This storm has a shelf cloud that moves into the wind and lifts the air condensing it to form a menacing low cloud base. Along this cloud base, often called a gust front, the wind will shift into the direction of the approaching storm and usually cool dramatically. These gust fronts lift warm moist air forming new convective clouds on the front of the storm increasing its intensity.
The second image has a slightly different process going on. There is an outflow from the cool rain area, but it's not surging out from the storm.. It remains closer to the storm as a boundary of cool moist air near the rain or down draft area. Converging winds are lifted, cooled and taken into the storm at one main area of the updraft. The cooler moist air from the outflow combines with the warm air entering the updraft. Since it's cooler it condenses earlier making a new lower base on the storm. This (updraft) structure may begin to rotate and is called a wall cloud. Wall clouds have both up motion and rotational motion. Spotters need to confirm that the lowered base is rotating before using the term wall cloud.
Many times rotating wall clouds are called mesocyclones, actually they are not. A wall cloud is only a part or subset of the mesocyclone which is much larger, usually miles across or storm scale. Mesocyclone is a "radar definition" and therefor cannot be visually seen in the field. It is the cyclonic (low pressure) circulation we see on radar that forms in supercell thunderstorms. Occasionally I use the term mesocyclone when referring to a wall cloud. Actually this is incorrect as the wall cloud is only part of this circulation; although, on occasion very large wall clouds may account for much of the mesocyclone circulation. Some mesocyclones never produce tornadoes but they are capable of causing high wind. Additionally, chasers are careful not to get directly down wind of a strong wall cloud or known mesocyclone because that area of the storm is most prone to large hail.
CONTINUE TO STORM IDENTITY - PART 2