1、Chapter 10: Thunderstorms,Thunderstorms Tornadoes Tornadic thunderstorms,Ordinary Cell Thunderstorms,cumulus stage mature stage dissipating stage,Ordinary cell thunderstorms are sometimes called air mass thunderstorms, because they form in conditionally unstable air masses and are not necessarily as
2、sociated with fronts or severe weather.,Ordinary thunderstorms tend to form in regions of limited wind shear. Form along shallow zones where surface winds converge: topographic irregularities, sea-breeze fronts, outflow boundaries from thunderstorms producing more thunderstorms. Converging wind boun
3、daries are regions of temperature and humidity contrasts: density differences.,Fig. 10-1, p. 265,Life cycle of an ordinary thunderstorm,Cumulus stage,Mature stage,Dissipating stage,Cumulus stage: Is the growth stage, as cumulus cloud builds, transformation of water vapor to liquid and solid cloud dr
4、oplets releases latent heat that keeps the rising air inside the cloud warmer than environment.During this stage there is insufficient time for precipitation to form. Updrafts keep water droplets and ice crystals suspendedwithin the cloudThere is no lightning or thunder during this stageThe building
5、 cumulus takes on the appearance of a tower,Mature stage: As the cloud builds above the freezing level, the cloud dropletsgrow larger and heavier. Eventually the rising air can no longerkeep the particles suspended and they begin to fall. Drier air around the cloud begins to be drawn into the cloud
6、in a processes know as entrainment. The entrainment of drier air into the cloud causes some of the raindrops to evaporate, which cools the air. The air now colder begins to descend as a downdraft.The appearance of the downdraft marks the beginning of the mature stage.,Mature stage: The updraft and d
7、owndraft within the mature thunderstorm nowconstitute the cell. During the mature stage, the thunderstorm is most intense.The top of the cloud, having reached a stable region of the atmosphere, begins to take on the anvil shape as upper-levelwinds spread the clouds ice crystals horizontally. Updraft
8、s and downdrafts are at their greatest strength in the middle of the cloud, creating severe turbulence.Lightning and thunder are also present in the mature stage.,Mature stage: Heavy rain falls from the cloud, and at the surface there is a downrush of cold air with the onset of precipitation.Where t
9、he cold downdraft reaches the surface, the air spreadsout horizontally in all directions. The boundary between the coldair and the warm surface air is called a gust front.The storm begins to dissipate after 15-30 minutes after entering the mature stage.,Dissipating stage: Occurs when the updrafts we
10、aken as the gust front moves awayfrom the storm and no longer enhances the updrafts.At this stage, downdrafts dominate throughout much of the cloud.The reason the storm does not last very long is that the downdrafts inside the cloud tend to cut off the storms fuel supply by destroying the humid updr
11、afts.Not only do thunderstorms produce summer rainfall for a largeportion of the US, but they also provide momentary cooling afteran oppressively hot day.,Fig. 10-1, p. 265,Life cycle of an ordinary thunderstorm,Cumulus stage,Mature stage,Dissipating stage,Fig. 10-18, p. 277,The average number of da
12、ys each year on which thunderstorms are observed throughout the United States,Severe Thunderstorms and the Supercell,multicell storms supercell,The likelihood that a thunderstorm will become severe increaseswith the length of time the storm survives.Ordinary thunderstorms tend to from in regions of
13、low wind shear. Because of this, the storms downdraft and precipitation can fallinto the updraft which kills the storm. If there is some moderate wind shear, the precip is pushed downwind so that it does not fall into the updraft and thus the updraft is not suppressed. If the downdraft undercuts the
14、 updraft, then new cells can form,producing a long-lasting multicell storm. A storm with a clusterof cells at various stages of their life cycles.,Severe Thunderstorms,Multicell storm complex,Fig. 10-13, p. 272,Radar image of an outflow boundary,In a multicell storm, if convection is strong and updr
15、afts intense,the storm can become severe.Updrafts in severe thunderstorms can cause the cloud to reach into the stratosphere and in some cases extend up to 60,000 ft.Strong updrafts can keep hailstones suspended in the cloud long enough for them to grow to considerable size. Once they become large e
16、nough they either fall out the bottom of the cloud within a downdraft or a strong updraft may toss them out the side. Aircraft have encountered hail in clear air several kilometers awayfrom a storm.,Severe Thunderstorms,When winds become stronger aloft, the wind shear may be strong enough to create
17、horizontal spin, which when tilted into the updraft causes it to rotate.In this case, the thunderstorm may grow into a larger, long-lasting (longer than an hour), severe storm called a Supercell.The rotational aspect of supercells can lead to the formation of tornadoes.Supercell storms are typically
18、 enormous thunderstorms that consist of primarily of a single violently rotating updraft.,Supercell thunderstorm,Conditions leading to the formation of severe thunderstorms, and especially supercells,A wall cloud associated with a supercell thunderstorm spins counterclockwise,Fig. 10-4, p. 268,A sup
19、ercell thunderstorm with a tornado sweeps over Texas,A squall line forms as a line of thunderstorms, either right alonga cold front or in the warm air 100-300 km ahead of it.Pre-frontal squall-line thunderstorms represent the most severeand largest type of squall lines.There is still debate as to ex
20、actly how pre-frontal squall lines form.,Squall Line,A Doppler radar composite showing a pre-frontal squall line extending from Indiana southwestward into Arkansas,Pre-frontal squall-line thunderstorms may form ahead of an advancing cold front as the upper-air flow develops waves downwind from the c
21、old front.,Pre-frontal squall-line,Mesoscale Convective Complexes,Mesoscale Convective Complexes (MCCs) form when a number of thunderstorms grow into a single, large, circular convective weather system.They can be as large as 1000 times that of a ordinary cell thunderstorm.Can cover an entire state
22、and move slowly lasting up to 12 hoursForm during summer in regions where upper-level winds are weak.Are associated with severe weather, including hail, high winds,destructive flash floods, and tornadoes.,Mesoscale Convective Complexes,Fig. 10-10, p. 271,A simplified model describing air motions and
23、 other features associated with an intense thunderstorm that has a tilted updraft.,Fig. 10-11, p. 271,A dramatic example of a shelf cloud (or arcus cloud) associated with an intense thunderstorm,Summary of todays lecture,Life cycle of ordinary thunderstormsMulticell thunderstormsSevere thunderstormsSquall linesSupercell thunderstormsMesoscale Convective Complexes.,
