Atm Ocn 100 1st Edition Exam 2 Study Guide Chapters 18 19 20 21 22 Chapters 18 19 20 21 22 And some lecture notes Thunderstorms are also known as cumulonimbus clouds that are tall vertically developing clouds that produce lightning and thunder Severe thunderstorms are ones that have the potential to threaten lives and property form wind or hail Severe thunderstorms require four elements for formation 1 source of moisture 2 conditionally unstable atmosphere 3 a mechanism to trigger the thunderstorm s updraf though lifting or heating of a surface and 4 vertical wind shear or a rapid change in wind speed and or wind direction with altitude Mesoscale convective system this refers to atmospheric processes that occur on scales ranging from the size of an individual thunderstorm to the size of the complex of clouds and precipitation composing a group of thunderstorms These are usually more common in the warm season In the cool season there are thunderstorms that can be developed along frontal boundaries these are called frontal squall lines Supercell thunderstorms if winds increase rapidly with height ahead of a strong front or a less distinct boundary thunderstorms triggered along that boundary may organize in this mode A supercell is a violent storm Supercells most often develop where strong winds are present in the upper troposphere and winds in the lowest kilometer or two of the atmosphere increase rapidly and change direction with altitude Supercells can occur in warm and cool seasons They are strong rotating cells that produce violent weather Ordinary thunderstorms can form in the absence of strong vertical wind shear because they do not produce severe conditions They tend to occur in an airmass rather than a front The typical lifetime of a single thunderstorm is an hour The winds in the thunderstorm s vicinity do not change substantially with height Ordinary thunderstorm growth First Stage Cumulus stage Cloud consists of warm buoyant rising air the updraft The updraft velocity in the cloud increases rapidly with height The clouds are composed of small liquid cloud droplets with little or no raindrops or ice crystals As the storm rises to altitudes that are colder small ice crystals begin to form These particles eventually grow large enough to precipitate and then fall through the storm Air in most thunderstorms is sufficiently buoyant so it rises to the top of the troposphere The updrafts in many storms are strong enough that the clouds will extend a short distance into the stratosphere this produces a bulge at the cloud top which is called a overshooting top As the cloud reaches the tropopause the storm enters it second stage of growth which is the mature stage In this stage air from the updraft exhausts horizontally at the tropopause which then forms a cloud feature called the anvil Mammatus clouds form at the base of thunderstorm anvils During the second stage of growth the precipitation particles fall downward through the cloud Each precipitation particle has weight and displaces air ahead of it as it falls which drags the air earthward The effect of falling precipitation is to create downdraf circulations in the middle and higher parts of the cloud Downdrafts are enhanced by evaporation as precipitation encounters dry air mixing in from the sides and the top of the cloud Downdrafs are formed by evaporative cooling and drag due to falling precipitation As evaporation occurs air is cooled and becomes denser which means it will then descend faster which enhances the downdrafts Near the surface the rain cooled air accumulates and forms a cold pool As precipitation falls downdraft circulation descends deeper into the cloud which counteracts with the updrafts bringing the warm buoyant air upward from the surface As downdrafts produced by evaporation and being dragged down they shut off the source of moisture to the storm This is when the storm enters its final stage called the dissipation stage During this stage the thunderstorm is composed of downdrafts Heavy rain and rain cooled air descend from the base of the storm as the clouds composing the storm slowly evaporate An ordinary thunderstorm will typically produce an outflow of cool air The cool air generated by evaporation of rain within downdrafts is sufficiently dense so it spreads outward which produces a gust front For mesoscale convective systems the warm season produces lifts of air along weak airmass boundaries which will trigger thunderstorms Storms that appear as a disorganized cluster at first and then organize and become more intense and progressively aligning into an arc shaped line are called squall lines A squall line is a long line of thunderstorms in which adjacent thunderstorm cells are so close together that the heavy precipitation from the cells falls in a continuous line One or more segments along the squall line may bow outward which produce a bow echo on radar Formation When the initial thunderstorms form the updrafts are relatively upright or only slightly tilted Rain evaporates as it falls into the lower atmospheres which cool the air though which it falls This is how the cold pool develops As the cold pool develops cooler air spreads outward toward the warm moist air and spreads rearward Then new updrafts preferentially form along and over the advancing cold pool air since the cool air which is denser lifts the warm air This reorganization of updrafts along the leading edge of the developing cold pool leads to the formation of the squall line At one point the cold pool becomes deep enough and cold enough that air rushes outward toward the warm air feeing into the thunderstorms The leading edge of this outrushing air is the gust front As the cold pool strengthens and the outrush of air begins air in the evaporation region at middle altitudes of the MCS flows forward toward the line of storms which creates the rear inflow jet Heavy rain in a mature MCS falls from the convective region of the storm which is located to the rear or west of the updraft region Near the end of a MCS life cycle the railing stratiform region decays which leaves a large area of clouds When this is formed there are large quantities of moisture that are condensed into rain Latent heat released during this condensation process leaves to the formation of a weak low pressure region in the middle levels within the trailing stratiform region which causes the clouds to slowly rotate The rotating clouds will remain when after an MCS