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Chapter 10 Requirements for a thunderstorm Instability 1 Adequate moisture especially near the surface 2 3 Source of vertical motion a low front flow over mountain thermal etc 4 Only required for a severe storm storms that produce at least one of the following large hail with a diameter of at least 1 inch surface wind gusts of at least 50 knots 58 mi hr or a tornado strong vertical wind shear winds change rapidly with height Florida has the most annual number of days with thunderstorms Ordinary Cell Thunderstorms Tend to form where warm humid air rises in a conditionally unstable atmosphere exists when cold dry air aloft overlies warm moist surface air and where vertical wind shear is weak Life cycle has 3 stages 1 Cumulus stage parcel of warm humid air rises cools then condenses into a single cumulus cloud or a cluster of clouds updrafts keep water droplets and ice crystals suspended hence why there is no precipitation 2 Mature stage further along in the process drier air from around the cloud is being drawn into it in a process called entrainment entrainment of drier air causes evaporation of raindrops which chills the air air is now colder and heavier than the air around it so it begins to descend as a downdraft which marks the beginning of the mature stage The downdraft and updraft within the mature thunderstorm now constitute the cell are strongest in the middle of the cloud creating severe turbulence Lightning and thunder are also present The thunderstorm is most intense at this stage The top of the cloud having reached a stable region of the atmosphere may be as high as the stratosphere takes on shape of an anvil shape as upper level winds spread the cloud s ice crystals horizontally At the surface there is often a down rush of cold air with the onset of precipitation Cold downdraft reaches the surface air spreads out horizontally in all directions The surface boundary that separates the advancing cooler air from the surrounding warmer air is called a gust front gust front forces warm humid air up into the storm enhancing the cloud s updraft 3 Dissipating stage occurs when the updrafts weaken as the gust front moves away from the storm and no longer enhances the updrafts After the storm enters the mature stage it begins to dissipate in about 15 30 minutes At this stage downdrafts dominate They tend to cut of the storm s fuel supply by destroying humid updrafts deprived of warm humid air cloud droplets no longer form lower level cloud particles evaporate rapidly sometimes leaving only the cirrus orphan anvil A single ordinary thunderstorm may go through its 3 stages in one hour or less Multicell Thunderstorms thunderstorms that contain a number of cells each in a different stage of development Tend to form in region of moderate to strong vertical wind speed shear shearing causes cell inside to tilt in a such a way that the updraft actually rides up and over the downdraft rising updraft is capable of generating new cells that go on to become mature thunderstorms Precipitation inside the storm does not fall into the updraft like with ordinary cell thunderstorms so the fuel supply is not cut off and the storm complex can survive for a long time When convection is strong and the updraft is intense the rising air may intrude into the stable stratosphere producing an overshooting top To an observer on the ground the passage of a gust front resembles that of a cold front because during its passage the temperature drops sharply and the wind becomes strong and gusty occasionally exceeding 60 mi hr these high winds behind a strong gust front are called straight line winds to distinguish them from rotating winds of a tornado They are capable of blowing trees down and overturning mobile homes As warm moist air rises along the forward edge of the gust front a shelf cloud dense arch shaped ominous looking cloud that often forms along the leading edge of a thunderstorm s gust front especially when stable air rises up and over cooler air at the surface may form Occasionally an elongated ominous looking cloud forms behind the gust front and appear to slowly spin about a horizontal axis namely a roll cloud When the atmosphere is conditionally unstable the leading edge of the gust front may force the warm moist air upward producing a complex of multicell storms each with new gust fronts these gust fronts may then urge into a huge gust front called an outflow boundary Along the outflow boundary air is forced upward generating new thunderstorms Downbursts occur beneath an intense thunderstorm The downdraft may become localized so that it hits the ground and spreads horizontally in a burst of wind like water pouring from a tap and striking the sink below A downburst with winds extending only 4km or less is called a microburst microbursts are capable of blowing down trees and inflicting damage on poorly built structures as well as sailing vessels o Dry type no rain o Wet type embedded in rain difficult to see visually detectable on Doppler radar very little warning time o Life time 10 minutes o Winds can reach 100mph Most downdrafts are cool but occasionally they can be very hot such sudden warm downbursts are called heat bursts which originate high up in the thunderstorm and warms by compressional heating as it plunges toward the surface Multi cell thunderstorm may form as a line of thunderstorms called squall lines The line of storms may form 1 Directly along a cold front and extend for hundreds of kilometers 2 Form in the warm air 100 300km out ahead of the cold front o These pre frontal squall line thunderstorms of the middle latitudes represent the largest and most severe type of squall line having huge thunderstorms causing severe weather much over its lengths They may form ahead of an advancing cold front as the upper air flow develops waves downwind from the cold front o Structure of a sever squall line 1 Strong downdrafts often form to the rear of a squall line 2 while some of the falling precipitation evaporates and chill the air If the cool air rapidly descends it may concentrate into a rather narrow band of fast flowing air called the rear flank inflow jet because it enters the storm from the west 3 Sometimes the rear inflow jet bring strong upper level winds from aloft and if these winds reach the surface they produce damaging straight line winds that may exceed 90 knots 4 As the strong winds push forward along the ground they sometimes push the squall line outward so that it


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FSU MET 1010 - Chapter 10

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