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EESA09H3 (185)

Lecture 5

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University of Toronto Scarborough
Environmental Science
Tanzina Mohsin

EESA09H WINDLecture 5 NotesOutline of this lecturePart One Thunderstorm Primer 11 Definitions 12 Dynamics of Thunderstorms 13 Lightning 14 Hail 15 World Distribution of ThunderstormsPart Two Tornado Primer 21 Definitions and Dynamics 22 Devastation 23 Famous Canadian TornadoesPart Three Hail Research Etkin and Brun 1999 Canadas Hail Climatology Part I Thunderstorm Primer11 DefinitionsWhat is a thunderstormA thunderstorm is a convective storm This means that the storm is caused primarily by surface heating rather the upper level flow These occurred in the summer season locally This is the main feature in weather variability in Southern Ontario in the summer months It is the only type of storm that has thunder and lightning Large thunderstorms can spawn tornadoes Toronto gets between 30 and 40 thunderstorm days per year12 Dynamics of ThunderstormsA storm that produces thunder and lightning is called a thunderstorm There are four types of thunderstorms ordinary single cell thunderstorms multicell thunderstorms super cell thunderstorms and mesoscale convective complexesOrdinary thunderstorms develop in large air masses not necessarily near a frontal system with little vertical wind shear Vertical wind is the change in the speed and direction of the horizontal wind A necessary condition is that the air mass is vertical unstable Vertical instability arises when less underlies denser air Typically this occurs when warm air underlies colder air Differentially surface heating is often the trigger for ordinary thunderstorms although other triggers are possible such as topography and surface wind convergence All of the triggers forces air upward into a strong updraft If the rising air remains warmer than its immediate surroundings it continues to rise Rising air also cools due to expansion Water vapor in the air will condense once saturation is reached The condensation process releases latent heat providing an additional energy source which warms the air and adds to the instability Often thunderstorms span the entire troposphere 813 km in height and sometimes penetrate into the stable stratosphereThe condensation of the water vapor creates the cumulonimbus clouds which are a vast collection of hydrometeors ice crystals and water droplets When these hydrometeors become large enough they begin to descend This marks the beginning of the end of an ordinary thunderstorm As the droplets fall some are evaporated This uses energy and the air becomes colder and begins to sink and a downdraft starts to form Since there is little vertical wind shear the downdraft forms in the same area as the updraft thus counteracting the updraft As the downdraft gains strength the updraft dissipates With this dissipation the fuel for the storm latent heat release is also cut off The downdraft provides the surface with a blast of cool air This cool air upon encountering the surface spreads horizontally This is called the gust front Gust fronts are able to cause surrounding warm air to rise and thus are capable of inducing further thunderstorms Because of the self limiting process the downdraft dissipating the updraftordinary thunderstorms are short lived rarely lasting an hour and typically having a diameter of less than one kilometreMulticell thunderstorms are very similar to ordinary thunderstorms except there is a moderate vertical wind shear This shear causes the storm to tilt and the downdraft is formed downwind of the updraft thus the storm can last longer The gust front of the downdraft is more likely to induce another thunderstorm and a string of thunderstorms occur often at different stages of development These storms tend to occur near a frontal system of a midlatitude cycloneSupercell thunderstorms occur with strong vertical wind shear These storms form in front of a cold front of a midlatitude cyclone in the cyclones warm sector The strong wind shear results from warm tropical air from the south to southwest pushing into the region above the warm sector Above this is colder drier air moving in from the west Above this air is the jet stream often providing divergence aloft from a variety of mechanisms A small layer of stable air exists above the surface air acting like a cap or lid on the emerging updraftUnder some conditions the rising air can break through this cap Once this is done the storm grows quickly if not explosivelyThe strong wind shear insures that the downdraft that develops is downstream of the updraft thus allowing supercell thunderstorms to last longer than ordinary thunderstormsoften for more than several hours Supercells often produce large hail with diameters of as much as 10 cm The strong wind shear also enables the formation of tornadoesThunderstorms that occur ahead of a cold front often form in a line called a squall line Occasionally multiple thunderstorms organize in a circular fashion covering over 100000 square kilometers These are called mesoscale convective complexes MCCs These complexes are self sustaining and can last over 12 hours and produce heavy precipitation which has resulted in flooding Unlike supercells MCCs can form with only weak vertical wind shear
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