9: Flash Floods

Question 1

What is a downburst

Answer 1

strong downdraft in a thunderstorm that strikes the surface and diverges horizontalyl as a violent wind pulse

• can be classified to macroburst, microburst and derecho

Question 2

What is a macroburst

Answer 2

downburst over a large area (>4km) typically lasting > 10 min

Question 3

What is a microburst

Answer 3

dowburst over a small area (<4km) typically lasting >10 min

Question 4

What is a derecho

Answer 4

consists of a family of straight line downbursts

Question 5

Wet vs Dry Microburst

Answer 5

• Wet microburst: large mass of cold air will splash to ground followed by a mass of precipitation
• Dry microburst: surface and bae of cloud is very large so water vapor will evaporate –> no precipitation

Question 6

What is a flash flood

Answer 6

• short term lcoalized rise in stream level due to heavy rainfall
• 6h delay between rainfall and stream flow
• persistent ascent of humid air
• stationary or slow moving long lasting thunderstorms with a warm cloud base
• can be produced by ice jams in rivers

Question 7

What is RMSE and when is it used

Answer 7

• RMSE (mm) is a measure of error that is used to calculate updraft and downdraft accurately
• lower RMSE more accurate
• if grid is more than 10km then need cumulus paramterization scheme

Question 8

Synoptic features that brings heavy preciptation in Alberta

Answer 8

• Strong cut off low over central Alberta
• Prominent ridge extending from Michigan to Yellowknife
• Rapid Cyclogenesis in lee of Alberta Rockies
• Low pressure system brings heavy precipitation
• Southernly jet transports low level moisture from the Gulf of Meico to Alberta