ATOC 1050 chapter 21

ATOC 1050 chapter 21 - 2 Note stagnation core curls and runaway vortex rolls 3 4 G Microbursts 1 Downbursts ≤ 4 km in diameter(usually between 1

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Chapter 21 Rauber – Downbursts I. Downbursts A. Downburst is a downdraft that originates in the lower part of a cumulonimbus cloud and it descends to the ground B. When they hit the ground, get straight-line winds C. Winds up to 100mph D. Form by: 1. Evaporation cools the air and it sinks 2. Drag force of precipitation – precip drags air with it E. Environmental conditions conducive to downbursts: 1. Large ELR so that cool air in downburst is much cooler than the environment and it sinks faster. 1
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2. Dry air below cloud base so that evaporation of virga (what is this?) occurs which decreases temperature 3. Shallow layer of moist air near the ground. Moist air is less dense than dry air, the cooler drier air falls faster through the moist air. 4. Below freezing temps. in cloud means the ice particles in the cloud that are falling add to the coolness of the temp. F. Downburst structure 2
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1. If winds below cloud are weak, downburst more symmetrical; if not, asymmetrical
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Unformatted text preview: 2. Note stagnation core; curls, and runaway vortex rolls. 3 4 G. Microbursts: 1. Downbursts ≤ 4 km. in diameter (usually between 1 and 4km wide) 2. Usually lasts from 5 to 30 minutes 3. Two types of microbursts a). dry – no measurable precip. but can be virga. More common in western USA and Great Plains. Inverted “V” soundings indicative of dry microbursts (see sounding “A”) 5 6 b). wet – measurable rain – more common in South, Midwest, and East (see sounding “B”) 4. Microbursts are particularly dangerous to aircraft! Why? a). 1975 – JKF International; 112 dead b). 1982 – New Orleans; 152 dead c). 1985 – near Dallas; 135 dead 5. Microburst detection a). difficult – small and don’t’ last long b). low-level wind shear alert systems – but these are “nowcasts” – i.e. the wind shear is in progress c). Wind Index (WI) measures the potential for thunderstorms to produce microbursts. 7...
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This note was uploaded on 05/03/2008 for the course ATOC 1050 taught by Professor Forrest,be during the Spring '06 term at Colorado.

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ATOC 1050 chapter 21 - 2 Note stagnation core curls and runaway vortex rolls 3 4 G Microbursts 1 Downbursts ≤ 4 km in diameter(usually between 1

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