The three ingredients listed above are necessary for the development of thunderstorms. Recent research has found that if the environment (wind, moisture, or instability) of a storm is changed, then the type of storm (multicell, supercell, etc.) which is favored to exist may change as well. The amount of vertical wind shear in the storm's environment is critical in determining what type of storm will form. Vertical wind shear is defined as a change in wind direction or speed with height. If the amount of vertical wind shear is low (little change in wind speed or direction), then multicellular storms with short-lived updrafts will be favored. Low values of vertical wind shear result in weak inflow to a storm. Because the inflow is weak, the outflow from the rainy downdraft area will push the gust front out away from the storm. This, in turn, will cut off the storm's source of warm, moist air, resulting in a storm with short-lived updrafts. Precipitation which is produced will fall through the storm's updraft and contribute to the updraft being short-lived. Figure 4 depicts a storm which developed in a low-shear environment.
As the vertical wind shear increases, storms with longer lived updrafts will be favored. Stronger vertical wind shear results in stronger inflow to the storm. The gust front will be "held" close to the storm, and the storm will have access to the source of warm, moist air for a much longer time. As a result, the storm's updraft will tend to last longer when the environment has strong vertical wind shear. Precipitation will tend to fall downwind from the updraft rather than through the updraft. This enables the updraft to continue for relatively long periods of time. Figure 5 shows a storm which developed in a high-shear environment.
Closely related to the concept of vertical wind shear is the veering of the wind with height in the lowest mile or so of the atmosphere. Veering is defined as a clockwise turning of the wind direction as we move up through the atmosphere. It is possible to make a rough check of veering winds while spotting. If there are two layers of clouds in the lower levels of the atmosphere, look closely at the directions in which the cloud layers are moving. If the direction turns clockwise between the lower and upper layers, then veering is present. Computer simulations and observational studies have suggested that veering of the low-level wind is instrumental in the production of storm rotation. If the wind speed is sufficiently strong (usually 30 miles an hour or greater) and veering of the wind with height is present, then horizontally-oriented "rolls" may develop in the lower levels of the atmosphere. These horizontal "rolls" may then be tilted into a vertically-oriented rotation by a storm's updraft. The updraft can also "stretch" the vertical rotation and increase the rate of rotation. Once this vertical rotation has been established, a mesocyclone (see chapter V) can develop which may produce a tornado or significant severe weather. Variations in moisture or instability can also have an effect on thunder- storms. If the amount of moisture in the atmosphere is low (as might be found on the High Plains), the storms will tend to have high cloud bases. Small amounts of precipitation will fall from the storms, but they will typically have strong downdrafts. If moisture levels in the atmos- phere are high (as might be found in the Southeast), then storms will have low cloud bases. Copious amounts of precipitation will reach the ground usually accompanied by weak downdrafts. A rule of thumb to keep in mind is: the higher the cloud base, the better the chance for dry microbursts. The lower the cloud base, the better the chance for flash flood-producing rainfall. The amount of instability which is present plays an important role in the strength of a thunderstorm's updraft and downdraft. If the instability is low, then a storm's drafts will probably not be strong enough to produce severe weather. If the storm's environment has high instability, then the storm's drafts will be stronger, and the storm will have a better chance of producing severe weather. Another important factor in the storm's environment, although not as critical as the above-mentioned factors, is the presence of a mid-level capping inversion. The mid-level capping inversion is a thin layer of warm air between the low-level moist air and the upper-level cold (usually dry) air. If the mid-level cap is weak or is not present, then storms will usually form early in the day before the sun's strong heating can produce high amounts of instability. A number of storms may form, but the storms will generally be weak and poorly organized. If the mid-level cap is strong, then storms may not form at all. The very warm mid-level temperatures will literally act as a lid, preventing updrafts from growing above the cap. A mid-level cap of moderate strength is preferred for the
development of severe thunderstorms. A moderate cap will prevent weak storms from forming,
thus "saving up" the atmosphere's instability. When storms do form, usually in
the mid to late afternoon, only the strongest few updrafts will be able to break through
the cap and continue to develop. These few storms can take advantage of the high
instability which is present, with little competition from nearby storms, and possibly
develop into severe thunderstorms. |
