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Thunderstorms AOSS 102: Extreme Weather 18 November 2015 Atmospheric Stability and Instability Stability of the atmosphere can be determined when comparing the environmental lapse rate with the dry adiabatic (10 K/km) and moist adiabatic lapse rates (6 K/km) 6 K / km 10 K / km Image Quiz Altitude (km) Thunderstorms develop in Absolutely Unstable or Conditionally Unstable Conditions Conditionally unstable: stable for unsaturated ascent and unstable for saturated ascent of an air parcel Fig. 5.18, p. 135 Why do Thunderclouds (Cumulonimbus clouds) form in unstable conditions? Air parcel Fig. 5.10, p. 130 Image Quiz Image Quiz Annual Average Days with Thunderstorms in the United States Annual Average Days with Thunderstorms Worldwide Thunderstorm Categories: Airmass Thunderstorm (Ordinary Cell Thunderstorm) Characteristics: ➡ can be located far from fronts ➡ forced by surface heating on sunny days, low-level warming destabilizes the environment ➡ weak vertical wind shear is present ➡ three life cycle stages ➡ relatively short-lived system ➡ intense precipitation when mature, but rarely severe ➡ lightning Airmass Thunderstorm: Mature Stage Airmass Thunderstorm: Beginning Cumulus Stage (Growth Stage) Fig. 11.2, p. 299 Airmass Thunderstorm in its Mature Stage: Distinctive Anvil at the Top Anvil: Spread-out ice cloud at the top Fig. 11.1, p. 299 Airmass Thunderstorm: Dissipating Stage • After thunderstorm enters mature stage it begins to dissipate after 15-30 minutes • Downdrafts dominate, cutting off the storm’s fuel supply (rising warm humid air) • All three stages might be over in less than 1 hour • Lower-level clouds quickly evaporate Third (Dissipating) Stage Airmass Thunderstorm in its Dissipating Stage “Weight” of a Thunderstorm Assume thunderstorm is a cube 15 km wide, deep and tall and contains about 1 gm/m3 of water. • Volume = 3.4 trillion m3. • Mass = 3.4 trillion grams or ~3.7 million tons of water. Thunderstorm Categories: Multicell Thunderstorms http://www.youtube.com/watch?v=_cl0aw87LqA https://www.youtube.com/watch?v=LYubHpEMTPM Multicell Thunderstorms Contain a number of convection cells, each in a different stage of development Moderate to strong vertical wind shear is present Tilt: updrafts rides up and over the downdrafts Overshooting top possible (intrusion into stratosphere) The longer the storm persists, the more severe Cold downdrafts under the storm reach the surface, leading to sharp temperature drops Leads to gust front and downbursts & microbursts Characteristics of a Multicell Thunderstorms Updrafts and downdrafts are tilted, supplying additional energy. Example of a Shelf Cloud Example of a Roll Cloud forming behind a gust front Exam 2: Statistics Mean: 74% Median: 75% Exam 2: Statistics Some rather big changes in the grades from exam 1 to exam 2 Exam 2: Observations Multiple Choice section: 10) Porto, Portugal averages 51 °F in December. Why is it warmer than Ann Arbor even though they are at nearly the same latitude? Image Quiz Multiple Choice Exam 2: Observations 13) What are some differences between El Niño and La Niña? A) In La Niña, the trade winds strengthen, and in El Niño they slacken or reverse B) The warm pool of ocean water is shifted to the east during La Niña C) Upwelling along the coast of Peru can be "shut down" during a La Niña La Niña El Niño D) All of the above Sea Surface Temperature Anomaly Exam 2: Observations 17.) What makes wind flow across isobars at the surface (while winds aloft generally flow parallel to them)? A. Coriolis force B. friction C. the jet stream D. gravity E. pressure gradient force Exam 2: Observations 19. The Coriolis force deflects the air motion in the Southern Hemisphere ___ near a surface high pressure system and ___ near a surface low pressure system. A. to the left, to the left B. to the left, to the right C. to the right, to the left D. to the right, to the right Always to the left of the wind in Southern Hemisphere Exam 2: Observations 28.) Direction of the PGF and Coriolis force at Mount Isa in Australia (no friction). PGF Coriolis Coriolis force always to the left (right angle) of the wind in Southern Hemisphere Exam 2: Observations 30) Direction and speed of the steering background wind northeastward 10 mph 110 mph -10 mph 110 mph 110 mph 110 mph +10 mph Exam 2: Observations 32.) Position of the warm and cold front. Warm Cold