Thunderstorms Hail
In Monday's lecture, we discussed at length the mechanisms that lead to the development of lightning and thunder in cumulonimbus clouds. Our discussion was at the microphysical level. Today we will extend our dicussion of thunderstorms and talk about organized thunderstorm complexes.
Scales of thunderstorms
Thunderstorms can form on the micro, meso, and synoptic scale. Typically, thunderstorms on the micro scale are classified as air mass thunderstorms. As we will now learn, the mechanisms by which thunderstorms form can be similar yet different.
Air mass thunderstorms
The scattered, afternoon type thunderstorms that often crop up during the summertime are classified as air mass thunderstorms. Air mass thunderstorms form in regions of relative instability. During the afternoon, the strong sun heats the ground. The air above the ground is heated by both the sun and the ground. Therefore, it warms, and the lapse rate becomes unstable, which remember, is conducive to vertical motion. As the air rises, if there is moisture, it condenses into clouds. With enough moisture and enough vertical motion, cumulonimbus clouds result. If these cumulonimbus clouds grow tall enough, sufficient ice is present in the cloud for electrostatic charges to build up and a thunderstorm is born.
The vertically moving air currents that form these thunderstorms are called updrafts. In the developing stage of a cumulonimbus cloud, there exists strong updrafts.
As the storm matures, the falling rain drops and precipitation particles create downdrafts. Therefore, in a developed thunderstorm, there exist simultaneous updrafts and downdrafts. Often what will happen is once the downdraft runs into the ground, it spreads out and convergence occurs adjacent to the thunderstorm. This convergence often produces significant enough forcing for vertical motion that thunderstorms develop adjacent to the main storm.
Individual thunderstorms are called cells whereas a group of thunderstorms made up of many cells is called a complex. Thunderstorms move about in the direction of the average wind. As the amount of precipitation in the thunderstorm increases, the magnitude of the downdraft increases. Eventually, the downdraft far excedes the updraft in terms of its magnitude and the thunderstorm disintigrates. Downdrafts actually erode the base of the thunderstorm because sinking motion is accompanied by relative warming and a decrease in relative humidity, which will "dry out" the base of the thunderstorm cloud. So a downdraft will eventually weaken one cell, the outflow from the downdraft will help other cells nearby develop from convergence.
Thunderstorms often have tops on them called anvils. An anvil occurs when a cumulonimbus cloud rises up into the stratosphere, at a level where the jet stream influences can be felt. This shears the tops of the cloud and gives the thunderstorm an anvil shaped top.
Thunderstorm complexes on the mesoscale
More organized, longer-lasting thunderstorm complexes require the same initial ingredients as air mass thunderstorms: moisture, instability, and vertical motion. But another ingredient is required as well: wind shear. Vertical wind speed and direction shear helps thunderstorm complexes to develop. An example of this is during the summertime in the Plains states. At night, as the ground cools, the surface loses heat much faster than the air. As a result, the ground is cooling the air right above the surface so there is a strong vertical temperature gradient that develops. This helps to initiate a phenomena known as the low-level nocturnal jet. The jet is associated with relatively strong vertical wind speed shears which can help lift the cool and increasingly moist surface air to develop thunderstorms. Thunderstorm complexes called MCC's which stands for Mesoscale Convective Complexes often develop early in the morning as a result of the low-level nocturnal jet. They are often severe.
A hurricane can be considered a mesoscale thunderstorm complex. We will be studying hurricanes Friday and next week. They are intense areas of tropical convection.
Squall Lines
Squall lines are examples of thunderstorm complexes that predominate on the synoptic scale. They form out ahead of cold fronts (about 100-200 miles). The squall line is generated as cool, dry air moves into moist, warm air as is the typical scenario with a cold front. The cold dry air displaces and forces the warm moist air upward leading to cumulonimbus development. The individual thunderstorms are cells but the squall line itself behaves as a large, synoptic scale complex. Severe thunderstorms often develop out ahead of cold fronts in squall lines.
Severe thunderstorms
Technically, by meteorologically accepted definition, a severe thunderstorm is any storm where there are wind gusts upward of 50 miles per hour or there is 3/4" hail. Severe thunderstorms are often accompanied by very heavy and torrential rains, dangerous lightning, hail, and in some dramatic cases, tornadoes. Since severe thunderstorms can potentially kill people and often do, predicting them with accuracy is of utmost concern to meteorologists and the general public at large.
Fortunately, meteorologists are able to arm themselves with two extremely powerful tools in the effort of forecasting storms: weather radar and Skew-T diagrams. Skew-T soundings will often show characteristic signatures prior to the development of severe thunderstorms. The meteorologist can get the sense as to whether or not the proper amount of moisture is in existence at the right levels of the atmosphere and whether or not there will be enough instability for significant vertical motions to take place. Often, a meteorologist can determine by looking at a 9 AM sounding if a severe thunderstorm will develop at 6 PM.
Hail
Hail is an interesting feature of severe thunderstorms. Remember that in mature thunderstorms, there are both updrafts and downdrafts. In the updraft region of the cloud, a raindrop often gets carried up above the freezing level in the cloud and it freezes. Then it gets caught in the downdraft and migrates toward the bottom of the cloud. Then it may get carried into the updraft and brought back above the freezing level where it will be once again coated with ice. Each successive trip above the freezing level allows the hailstone to grow larger and larger until the updraft can no longer carry it upward and it falls to the ground.
Hail can be very destructive. Each year, millions of dollars of property and crop damage results from hailstorms. Hail stones can be as big as softballs and can be lethal.