This next topic is one of the most interesting aspects of meterology because of the interest generated and the fact that thunderstorms while generally beneficial, can also be destructive. Thunderstorms bring rain, often torrential, along with occasional flooding, lightning, hail and on rare instances, tornadoes.
Thunderstorms can be defined as a convective (vertical) region of intense precipitation often accompanied by lightning and thunder. Across the United States thunderstorm activity ranges from less than 20 per year along the West Coast to around 50 thunderstorms a year across much of the central and eastern United States increasing to nearly 70 thunderstorms a year near the New Mexico-Colorado border to nearly 100 a year over central Florida. The actual thunderstorm is in fact a cumulonimbus cloud which consists of updrafts (rising air) and downdrafts (descending air).
INGREDIENTS FOR THUNDERSTORM FORMATION
Thunderstorms typically develop on warm humid days within an atmosphere that is unstable (an unstable atmosphere or instability is defined as the tendency of an air mass to rise and be warmer than its surrounding environment-as long as it is warmer than its surrounding environment it will continue to rise until its temperature is the same as its surrounding envirnment-then it will begin to sink). As an air parcel rises, it cools adiabatically (unsaturated air will cool as it expands in response to the lower atmospheric pressure at increasing heights). One of the characteristics of a gas (our air parcel) is that it will cool as it expands (about 10 degrees Celsius/km-Dry Adiabatic Rate) and warm up as it descends or is compressed. As the air parcel rises, condensation occurs which is a warming process (remember how hot you feel when you perspire?) resulting in the formation of clouds. This condensation releases roughly 575 calories for every gram of liquid water that condenses. This is known as the the latent heat of condensation. The expansion cooling rate (about 7 degrees Celsius/km-Wet Adiabatic Rate) is less than the additional heat added. This allows our air parcel to continue to rise throughout the developing cumulonimbus cloud because it is warmer than the surrounding environment.
Meteorologists use these adiabatic rates as an index to measure the stability of the atmosphere and whether or not it is conducive to the formation of thunderstorms. This index is computed by using the average specific humidity (grams of water vapor per kilogram of air) from the Earth's surface to one km. Another variable used in determining the presence of instability is the maximum temperature forecast since this corresponds to the most likely time of thunderstorm activity (which is why on average most thunderstorms form during the warmest time of day between 3pm and 7pm).
The temperature of the air parcel is compared to the temperature at the 500mb level (though it varies, usually around 6 kilometers above sea level-500mb is generally the barometric pressure at this altitude). Measurements are available for dterminning the lifted index (or Convective Available Potential Energy-CAPE) by subtracting the temperature of the air parcel from the measured temperature at the 500 mb level. If the CAPE index is negative, it means the temperature of the air parcel is warmer than the surrounding environment and thus the atmosphere is unstable or has the potential to rise or lift and severe weather is possible (the more negative the index the more likely that severe thunderstorms will form). If the CAPE index is positive it means the air parcel is cooler than its surrounding environment (which will result in its sinking back to Earth) and thus severe weather is unlikely. Prior to the tornadic and severe thunderstorm outbreak of October 12-13, 2001 in the Hill Country just to the west and northwest of San Antonio the CAPE index was -2 to -4.
Moist and Dry Tounges
Another feature associated with the formation of thunderstorms is a high temperature dewpoint and associated moisture tounge interacting with dry air. The source of this moisture is the Maritime Tropical (mT) air mass from the Gulf of Mexico.
The approach of a deep midlatitude trough in the upper atmosphere and its associated cold front pulls very warm and unstable air from the Gulf and very dry Continental Tropical (cT) air mass from Mexico and the southwestern U.S. The interface of these two airmasses occurs usually near the Texas-New Mexico border. This boundary is known as the Dry Line or the Marfa Front because its mean position usually finds it near Marfa in far west Texas. While there is usuually not much temperature variation across this boundary (usually it is hotter west of the Dry Line), dewpoint temperatures are regularly in the 60's and 70's in the mT air mass while teen's and 20's dewpoints (sometimes in to the single digits or lower) are common behind the front. Because the drier, continental is denser than the tropical maritime air, it will undercut it lifting it in a similar manner of a cold front when it encounters the warm humid air mass and thus initiating thunderstorms.
Low Level Jet
Between 5000 and 7000 feet above the surface, winds have been observed blowing from south to north around 50 knots to as much as 100 knots (on the surface these winds blow from 20 to 35 knots). This is known as a low-level jet in contrast to the better known polar jet stream which blows from west to east (but can dip into the subtropics especially during winter) and is found at 40,000 feet (12km). This low-level jet is most pronounced during the late winter, spring and summer across the Southern Plains (Texas, Oklahoma, Kansas to Nebraska and Missouri). This jet functions in transporting the moisture tounge northwards in advance of the mid-latitude trough (air flow in advance of the trough is from the Gulf) and supplying wind shear necessary (more on wind shear when we discuss tornadoes) for the formation of severe thunderstorms and tornadoes.
Upper Air Divergence
As thunderstorms develop and grow, they need a continous inflow of moist air at the surface to maintain them or they will dissipate (eventually this happens anyway) by entrainment. Entrainment is the (relatively) dry air that is pulled from the surrounding environment which eventually manifests itself into downdrafts and the cool gusts you feel when the thunderstorm approaches. Entrainment can be counterbalanced by the removal of air from the top of thunderstorms. This is known as divergence which allows more moist air from the lower levels of the thunderstorm to be sucked up into giant updrafts and then diverge away from the top of the thunderstorm which allows it to maintain itself and intensify. Upper air divergence (usually accomplished with jet stream splitting and the orientation of the upper-level winds) facilitates a vacuum effect in the upper atmosphere which contributes to the formation of strong updrafts.
The jetstream is important in thunderstorm initiation because of its ability to transfer energy from a larger circulation aloft towards a smaller circulation below (this is known as the Conservation of Angular Momentum which is descibed in your text) and its influence on the parent wave cyclone (a surface low pressure which develops at the junction of cold and warm fronts) which often generates thunderstorms in the warm sector (see discussion on wave cyclones and associated weather in your text). The jet stream also supplies the developing thunderstorms energy (in the form of rotation if it is a severe thunderstorm) and direction of motion (usually to the northeast over North America).
The jet stream is not the only mechanism for generating thunderstorms it usually acts in concert with the presence of boundaries. These boundaries can be the sea breeze front, dry line, old frontal trough, the remnants of old thunderstorms and cold air outflow (outflow boundaries-acting similar to mini-cold fronts) from other thunderstorms. You may have noticed approaching thunderstorms which begin to dissipate while thunderstorms develop downwind of the dying thunderstorms. These thunderstorms become energized and probably will develop additional thunderstorms until the heat energy is exhausted. Thunderstorms also develop when outflow boundaries intersect each other. This summer I observed thunderstorms approaching from the northwest while the sea breeze front approached fron the southeast. The northwest thunderstorms dissipated with only the strong winds blowing into my neighborhood. The two converged south of downtown San Antonio and a spectacular lightning show evolved (but I didn't receive a drop of rain-typical of Texas weather and boundary layer dynamics). While boundary layers are frequently a generator of thunderstorms in conjunction with the previously discussed upper-air features, they become especially important when upper air currents are not strong.
TYPES OF THUNDERSTORMS
The above criteria describe the formation of springtime thunderstorms typical in the central part of the United States. There are other types of thunderstorms ranging from garden-variety thunderstorms to supercell thunderstorms which are capable of producing tornadoes.
1. Air Mass Thunderstorms
These types of thunderstorms generally are a product of a moist unstable atmosphere. They are usually triggered by daytime heating or via topographically induced lifting (ie; mountains) or bondary layers such as the sea breeze front. These are usually found in climates that experience hot, humid summers such as the southeastern part of the U.S. Their shelf life is usually no more than several hours and is frequently less.
2. Squall Line Thunderstorms
Convective activity which is organized into a linear feature sometimes thousands of miles long, are refered to as squall line thunderstorms. Squall lines develop in advance or along a strong cold front or the West Texas dry line as it pushes into a humid mT air mass. Typically, squall line precipiation is intense (often in excess of two inches/hour) and the most intense precipitations lasts no more than 30 minutes depending on the rate of motion which is usually in the range of 15-25mph but can range from less than 10mph to in excess of 60mph. Most squall lines are found in the midlatitudes but are also found in subtropical and tropical climates as part of tropical systems which can develop into tropical cyclones. There are certain morphological characteristics of a squall line. Often a part of the squall line will "bow" out similar to the point of a boomerang and is known by meterorologists as a bow echo. This is represents an area of very high winds and gustnadoes (short-lived tonadoes which are not as strong as those developed by supercell thunderstorms) may develop here. There are other fast-moving squall lines (movement in excess of 50mph) which are capable of producing tornadic damage and are called derechos.
3. Mesoscale Convective Complexes (MCC's)
These next type of thunderstorms act as a unit or a "commune" and are long-lasting and often cover a geographic area the size of Iowa or Nebraska. These are Mesoscale Convective Complexes (or Mesoscale Convective Systems-MCS's). MCC's can exist up to 24 hours and may move several thousand miles until they dissipate. It is not unusual for one of these complexes to develop over South Dakota and move all the way to the Atlantic or Gulf coasts. These thunderstorms appear to act as a unit and "cooperate" with each other in order to maintain themselves. As thunderstorms develop, they warm their internal environment which draws in additional moisture enabling additional warming in the storm's interior. This allows the storms to continue to expand until they may cover an entire state the size of Iowa. The heaviest precipitation occurs in the leading edge of the storms, similar to an advancing squall line. The middle and rear of the complex resembles an egg-shape and consists of moderate to heavy rain. Typically these systems drop between 2 and 4 inches of rain, but some can drop up to a foot of rain. Unlike other thunderstorms, these are primarily nighttime phenomena and are most common during the late spring and summer over the midwestern part of the U.S. The reason for their nighttime occurrence is that during the day, temperature inversions are prevelent in the lee side of the Rocky Mountains where many of these complexes develop. As night falls, the upper atmosphere cools, enhancing the temperature difference between the warming interior of the intensifying storms and the rapidly cooling the surrounding atmosphere. These storms thrive on this temperature difference and will maintain themselves unlike other thunderstorms which tend to weaken after sunset. MCC's often reach their greatest intensity after midnight and will not dissipate until late morning when the temperature differences are not as great. MCC's bring beneficial rains to the corn belt region of the U.S. and the agricultural productivity of this region would not be what it is without these types of thunderstorms. They also moderate the hot, humid climate of the region and remnants of these storms can lower consumer demand for electricity to power air conditioners (They occur in Texas but not with the frequency exhibited in the Midwest-the floods of October 1998 were a modified type of MCC). Of course as is the case with the weather you can always have too much of a good thing which is what happened during the summer of 1993 when MCC after MCC rolled across many parts of the upper and mid-Mississippi and Missouri River Valleys with some areas receiving over 30 inches of precipitation in two months resulting in devasting flooding.
When thunderstorm cell after thunderstorm cell moves over the same region particularly in a short period of time this is known as "training" because each cell resembles a train boxcar resulting in significant flooding. I say this is a modified type of MCC because while the thunderstorm complex may reach the same size as an MCC, it is usually a more linear feature (though thicker than squall line thunderstorms) with the thunderstorm cells following each other in short order (because the upper-level winds are parallel to the boundary or stationary front rather than perpendicular which allows for rapid clearing). While these types of thunderstorms do not often meet severe thunderstorm criteria, the large amount of rainfall received in a short period of time often produces widespread destruction (ie; Oct 1998 in San Antonio).
The final type of thunderstorm are supercells. We call them this because they are the tallest thunderstorms (often over 12 miles high) and they bring severe weather in the form of strong winds (over 60mph), large hail (3/4 inch in diameter or greater), flash flooding and occasionally tornadoes. Supercell thunderstorms also exhibit rotation, a signature of the potential for severe weather.
Prior to the development of severe thunderstorms, initial convective development is inhibited by the presence of a temperature inversion, a warm and dry layer of air sandwiched between unstable humid air and the colder air aloft (we usually see inversions during our hottest weather which inhibits cloud formation and traps pollutants near the ground). This dry air prevents the development of weak thunderstorms which would normally preceed the primary thunderstorm development. Meterologists often refer to this situation as an atmospheric "cap". Breaking this "cap" is critical for thunderstorm development. It also means when thunderstorms do develop, they will become severe in short order. The delayed cloud development allows the sun to warm the surface and lower atmosphere to the point that the parcel of air that penetrates the cap is so much warmer than the environment in the inversion allowing violent thunderstorms to form.
As the developing thunderstorm rapidly expands, the updrafts of the thunderstorm rise where they contact the rapidly rotating upper level winds of the jet stream. The rising clouds encounter wind shear which develops horizontal vorticies just below the axis of the jet stream. The ends of these vorticies are tilted downward in a vertical position. Eventually two rotating columns appear, a large northern one rotating anticyclonically (assuming a west-to-east movement which is typical of most thunderstorms in the midlatitudes) and a smaller more vigorous column on the south or southwest side of the storm which rotates counterclockwise. This double vortex structure allows the thunderstorm to survive the strong wind field that the storm is embedded. These strong winds will shear apart weak storms that do not have this structure. These rotating vortexes draw in additional moisture-laden air into the front part of the storm allowing it to intensify further. The larger northern anticyclonic circulation is the region where most of the precipitation falls while the southern more vigorous circulation is generally rain-free. This southern circulation is also known as the mesocyclone and is the region where a tornado is most likely to develop.
The rotation of a severe thunderstorm also effects is movement in its embedded air flow. Most severe thunderstorms will deviate about 30 degrees to the right of the direction of the upper level steering winds. These thunderstorms are known as "right movers" or "right turners" by storm chasers and are an indication that a thunderstorm is becoming severe. The reason for the turn to the right, is that the mesocyclone's rotation (our southern cyclonic circulation) becomes so violent that it forces the thunderstorm to turn cyclonically (or to the right) in the same manner that a pitcher puts spin on a baseball in order to throw a curve ball. This type of motion is known as magnus force. A northeastward moving thunderstorm would move to the east as it becomes severe. In May of 1997 over central Texas the general upper air flow was northwest, but as thunderstorms became severe, they turned to the the right or in this case to the southwest (an unusual direction for tornado producing thunderstorms) especially the supercell thunderstorm which produced a tornado that destroyed the town of Jarrel.
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