EOSC 114 Slide Notes Storms and Waves.docx

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Earth and Ocean Sciences
EOSC 114
Brett Gilley

EOSC Slide Notes Storms 17/03/2013 23:35:00 - Slide 1 – Thunderstorm Basics  Thick clouds with lightning & thunder   Cloud top near the top of troposphere (10-15 km)  Cloud base near ground (1 km)  Looks like anvil or mushroom  Strong updrafts & downdrafts (turbulent)  If very strong updrafts then dome of clouds overshoot above the anvil  Anvil can be 100s km in diameter  Main updraft (stem of mushroom) is 15 km diameter  Storm energy from temperature % humidity Thunderstorm Cells  Cumulonimbus (thunderstorms) are made of large cells that evolve during 15-30 min  Most thunderstorms contain 2 or more cells, and are called multicell thunderstorms  Sometimes a veryl arge, rotating single-cell thunderstorm forms, called supercell Tstorm. They can cause tornadoes, large hail, frequent lightning, heavy rain, strong winds  Supercells types: low precipitation, classical, high precipitation Thunderstorm Cell Life-Cycle  1) cumulus stage  2) mature stage  3) dissipating stage Storm Energy - Sun  Solar energy is absorbed at 3 different heights: o Top (thermosphere)  Absorption of non-visible light o Middle (stratopause)  Absorption of ultraviolet by ozone o Bottom (earth surface)  Light shines thru lower level, it heat the ground which then heat the air in the troposphere.  Surface Heat Budget o Some solar energy reflects back unto space  Some intercepted by clouds -> scattered back to space  Some reflected from ground o Some absorbed by the ground  warms the surface o Absorbed sunlight at ground charges into:  Sensible heat (warms air)temperature increases  Latent heat (evaporates water from lakes, vegetation etc.) humidity increases  This are the two heat sources that fuel storms o Both temperature and humidity are important  Daily cycle o Solar heating during day  input o Infrared radiation (IR) cooling at night  loss o Greatest accumulation of heat, near sunset every day o Late afternoon and early evening  most likely time of day for Tstorm formation - Slide 2 – Supercell Thunderstorm: The most violent & longest lived  Often the whole supercell rotates as a mesocyclone, and can spawn tornadoes  Rotaiton to slow to see but often you can see a barrel shaped updraft tower that looks twisted. Observing and monitoring using remote sensors  Satellite  Radar - Slide 3 – Down-bursts & Gust Fronts  Downdrafts speeds of 20 to 90 km/h  Horizontal wind speeds near ground of up to 20 km/h  Microbursts are small diameter (1 km) downbursts Downburst  Cold dense air sinking  Tstorm can create dense air where rainfalls due to precipitation drag & evaporative cooling  Risks: often invisible, but hazard to aircraft Gust front  Leading edge of straight line winds  Downburts air hits ground & spreads out  Haboob, sandstorm (if dry ground);  Arc clouds (if moist air)  Risks: can blow down large trees and destroy weak structures, hazard to aircraft during take off and landing Storm Energy – Moist Air  Storms have special organization and capability to: o Draw in humid air, o Then to cause it to condense o Release its heat into the storm o Resulting in precipitation and violent winds  Concepts: o Humidity  Air = mixture of gases  0-4% water  78% nitrogen  21 % oxygen  Humidity is the amount of water vapour in the air  Humidity variable: Mixing ratio (r)  The amount of water vapour divide by the amount of all other gases: o Saturation  An equilibrium between evaporation and condensation  Saturation value is maximum humidity that air can hold  Saturation mixing ratio increase exponentially with temperature  Warmer air can hold more water vapour o Adiabatic Cooling  When air rises it cools (10C°/km)  Cooler air can hold les water vapour  Therefore some vapour must condense into liquid droplets  But condensation releases latent heat Storm strengthen when latent heat -> sensible heat  If the saturation humidity value becomes smaller than the actual humidity, then condensation occurs  Condensation does 3 things. o Releases sensible heat into storms o Reduces the humidity down to the equilibrium value o Produces or increase liquid cloud drops, which can grow to become rain drops Rainfall (= precipitation)  Strong storms -> heavy precipitation  Strong radar reflectivity  Heavy rainfall rate (RR) o Measure by increase of depth of water in rain gauge (mm/hour)  Average warming rate o The average temperature change (T) over time interval (t) is:  A)0.33b K/mm of rain  For a Tstorm 11 km thick - Slide 3 – Recognizing Tornadoes  Tornadoe are violently-rotating columns of air, in contact with the ground  Many different types of tornadoes exist  The most violent tornadoes come from super cell thunderstorms  Most tornadoes are made visible by the funnel cloud (top of tornado, water droplets) and the bottom of the tornado the debris cloud.  Supercell storms: o strongest o most likely to have tornadoes o not all super cells spawn tornadoes  Tornado shapes o Cone o V-shaped o Wedge o Cylinder o Hour glass o Rope  Shape is independent of intensity classification (Fujita or Torro Scale) Recognizing supercell rotation  Striations visible in main updraft of supercell  Wall cloud at bottom of supercell o A wall cloud is an isolated lower of cloud base o Beneath rising cumulus towers o Tornadoes come from rotating wall clouds in supercells Speeds and Disaster Scales  Horizontal Movement (translation) of the centre of the tornado o Usually from SW toward NE in N. America o Translation speeds of centre of tornado up to 100 km/hr o Most move at speed near middle of that range o Rotational speeds around tornado are much fast than the translational speed. o These rotational winds cause damage  Classified by: o Enhanced Fujita Scale (used in N. America)  Determined by amount of damage to buildings  EF0= very weak tornado => might break a few windows  EF5= exceptionally strong tornado -> totally destroy whole buildings o Torro Scale (used in Europe)  Determined by wind speed Tornado Safety  Tonradoes are short lved  Typical damage path o Narrow o Damage paths often one to tens of kilometers long  Safest places to be: o Indoors:  Below ground, basement or storm cellar  Get out of mobile homes o Outdoors  Ditch or hole  Below line of fire, of fast moving debris o Car  Drive away from tornado  Right or left of translation direction Tornado Risk  US, Oklahoma the centre of tornado valley  Canada, greatest frequency of tornadoes in Toronto Tornado “outbreaks” = 6 tornadoes in one day and one region, or many tornadoes during about a week Tornado Evolution  Stage 1: Wall cloud and dust whirl  Stage 2: funnel cloud and dust whirl increases  Stage 3: Mature tornado  Stage 4: decaying “rope” stage  Stage 5: dissipating Tornado Forecast  Tornado Watch o 6 to 12 hors forecast o a broad region o you can continue your normal activities o you should monitor emergency announcements on news  Tornado Warning o Tornado actually detected now  Doppler radar, see vortex signature  Human spotter o Nowcast warning tells you:  Where  Predicted path  Tornado sirens o Warnings come 15 min before impact, must directly leave activities behind and find shelter - Slide 4 – Recognize: Mammatus Clouds, Flanking Line  Mammatus clouds are located on the underside of thunderstorm anvil From heat to motion  Air motions= winds o Cause damage directly o Blown in more warm, humid air (storm fuel)  Positive feedback  Longer-lasting storms  4 Factors determine air motion: o Forces  F = m * a; force= mass times acceleration  Newtons second law o Acceleration (a) = change in velocity (v) during time interval (change in t), where velocity has both speed and direction.  Acceleration is measured as velocity (m/s) change per times (s), thus giving acceleration units of (m/s^2)  A= (v new – v old) / change in t o Forecasting the Winds  Prognostic equation  V new= v old + [(F/m) * change in t ]  Applies to objects such as cannonballs, automobiles  Air parcel= hypothetical blob of air about the size of a city block  Air parcel movement = wind o Buoyancy force  Causes up and downdrafts o Pressure gradient force (PGF)  Horizontal PGF  horizontal winds Temperature alters Buoyancy to Drive Vertical Winds  Warm air rises -> updrafts  Cold air sinks -> downdrafts Buoyancy  The buoyancy of an air parcel depends on the difference between the parcel temperature and the temperature of the surrounding air  Condensation is Tstorms releases latent heat  Latent heat warms the Tstorm air, making it buoyant and causing the air to rise  It drives the violent updrafts in thunderstorms Temperature alters Pressure to Drive Horizontal Winds  Pressure (P) = force (F) per unit area (A)  P = F/A  Where we are concerned only with the component of force perpendicular to the surface area  Pressure units N/m^2  Pressure drives winds  Pressure differences o The dif
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