HAZARDOUS WEATHER 1 Dr. Julie Laity Geography 266
Violent Weather Thunderstorms Atmospheric turbulence Lightning and thunder Hail Derechos Tornadoes Tornado measurement and science Tropical Cyclones Hurricanes and typhoons
THUNDERSTORMS Globally: 40,000 per day or 14.6 million per year USA: highest frequency in the s.e. states, with Florida having the highest incidence Warm, moist air from the Gulf of Mexico and Atlantic Ocean fuels ts dev.
Ingredients for a ts Moisture Instability A lifting mechanism Moisture
Thunderstorms Figure 5.37
Lifting mechanisms 1. Convective: differential heating 2. Frontal, drylines, and outflow boundaries 3. Orographic 4. Convergence
THUNDERSTORMS Severe thunderstorms and lightning are associated with cumulonimbus clouds. Cb clouds have their base in the lower troposphere and, if humidity conditions are high and convection is vigorous, may extend to the top of the troposphere (12,000 m or more). The clouds can exist as individual towers or form a line of towers called a squall line.
Stages of an air mass ts Ist stage: towering cumulus stage: dominated by updrafts. 2nd stage: mature stage: height of 12-18 km. Strong updrafts and downdrafts coexist. Ppt, thunder, and lightning occur. At the top of the troposphere, the updrafts may build the cloud out to form an anvil shape. 3rd stage: dissipating stage. The downdraft cuts off the updraft.the storm dissipates in about an hour.
Anvil head
New Zealand
SEVERE TS Wind is in excess of 93 kph (58 mph) Hailstones larger than 1.9 cm A tornado is generated. Severe ts often appear in groups and can last from several hours to several days.
THREE TYPES OF SEVERE TS 1. Mesoscale convective complexes (MCCs) 2. Squall lines (linear belts of ts) 3. Supercells
Large clusters of self-propagating storms Long-lived: Can grow and move for periods of 12 hours or more 1. MCCs
Nocturnal in occurrence: form in early afternoon from scattered ts and persist overnight Cloud top area of 100,000 km 2 or greater Eccentricity > 0.7 (minor/ major axis) Found in USA during Spring and Summer
Consequences Heavy rainfall, flash flooding, wind, hail, lightning and possible isolated tornadoes Flooding: for example, the 1977 Johnstown, PA flood Destructive straight line winds that destroy forests (e.g., in Adirondacks)
2. Squall lines Average 500 km in length Long lines of individual storm cells 1. Along a cold front 2. Parallel to and 300 to 500 km in front of cold fronts 3. Along drylines (air masses differ in moisture content rather than temperature), in the sw USA during spring and summer.
Can produce heavy precipitation, lightning, strong straight line winds, and possible tornadoes and water spouts Bow echo
Gusty, sometimes damaging outflow winds will spread out horizontally along the ground behind the shelf cloud.
3. SUPERCELL TS A supercell is a rotating ts with a deep rotating updraft. It is the rarest of storm types, but the most dangerous. Persists for many hours. Updraft elements merge into the main rotating updraft and then explode vertically. Develop in afternoon and early evening, when effects of heating by the sun are strongest. This Maryland sc produced an F4 tornado and baseball-sized hail. The top overshot into the stratosphere.
Can occur anywhere on earth, but are most common on the Great Plains of the USA Usually found away from other ts
Char. by a rotating updraft formed by vertical wind shear, which occurs when the winds are changing direction and increasing with height.
Wind shear starts air spinning The updraft bends the spinning air upwards The updraft starts rotating with the spinning column of air
The rotation is often visible beneath the supercell.
A wall cloud may lower. This is often, but not always, a precursor to a tornado.
H: hook echo; U: main updraft
Supercell storm hazards Strong downbursts Large hail Occasional flash floods Weak to violent tornadoes Dangerous cloud-to-ground lightning Extremely dangerous to aviation High predictability of occurrence of severe events once storm is identified as a supercell.
TS HAZARDS: HAIL Damages aircraft, homes, vegetation,and cars and can be deadly to livestock and people March 28, 2000: person killed in Fort Worth, Texas when hit by grapefruit-sized hail. The sustained updrafts in supercell ts allow very large hail to develop.
Strong updrafts create a rain-free zone in the ts: they suspend rain and hail at the top.
The hailstone may take multiple journeys in the cloud, growing in size.
Updraft speeds The updraft speed of a golf ball sized hailstone is 64 mph (29 meters/sec) For a softball sized hailstone: 103 mph (46 meters/sec).
Severe property damage can result Video on www.ultimatechase.com
DOWNBURSTS: DAMAGING WINDS Wind damage is more common than tornado damage. Wind speeds reach 100 mph (161 kph) Straight-line damage is often erroneously a attributed to tornadoes.
DOWNBURSTS A strong downdraft that originates within the lower part of a cumulus cloud and descends to the ground Spreads outward, creating strong straight-line winds Winds can be >100 mph and can cause damage equivalent to F0 or F1 tornadoes
Small downbursts are termed microbursts : they are particularly hazardous to aircraft taking off or landing. Can evolve from an updraft to a downburst in a matter of minutes. Warning systems in airports save many lives.
Evolution of downbursts 1. Evaporation: rain evaporating into the unsaturated air beneath a cloud cools it. The rain cooled air is colder and denser than its environment and sinks at speeds of 40-60 mph. 2. Drag force of falling precipitation. Millions of raindrops push air ahead of them.
Curl phase
The microburst..a small downburst (<2.5 km 2 in area) Paramount, CA
Derechos Result from particularly strong downdrafts Coverage of winds: may cover up to 1,250 mi 2 (Rhode Island size), with winds covering 100s of miles before dissipating. Duration: may last over 30 minutes Speed: 50-60 mph with occasional gusts of 200 mph.