severe convective storms -- an overview part 1 -- john monteverdi part 2 -- kathryn saussy
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Severe Convective Severe Convective Storms -- An Storms -- An
OverviewOverviewPart 1 -- John MonteverdiPart 1 -- John Monteverdi
Part 2 -- Kathryn SaussyPart 2 -- Kathryn Saussy
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Severe Convective Storms -- An Overview
Doswell, C.A. III, 2001: Severe Convective Storms -- An Overview. Severe Convective Storms, Meteor. Monogr., 28, no. 50, Meteor. Soc., 1-26
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Severe Convective Storms -- An Overview
February 1, 2006 (Monteverdi) -- Segue from previous courses (pp. 1-12)
February 8, 2006 (Saussy) -- Severe Convection (pp. 13-36)
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Severe Convection: Definitions
• Hail -- Hailstone diameter must be 3/4 inch or larger for severe convection.
• Wind -- Wind gusts must be 50 knots or greater to qualify as severe.
• Tornado -- The presence of a tornado over land qualifies as being severe convection.
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‘Events’ vs. ‘Reports’
• A severe event involves damaging winds, hailfall and tornadoes over a given area.
• Severe reports are given in terms of points.
• Is there a “mismatch” between events and reports? Do the events represent the reality of the actual events?
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Thresholds / Classification
• Do the thresholds have any physical meanings?
• There is no official threshold for precipitation.
• All tornadoes are considered severe, but there are “gray” areas of identification.
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Synoptic Observations
• Rawinsonde: Main tool for evaluating the large-scale structure of the atmosphere.
• Most of our knowledge about the atmosphere comes from radiosonde and rawinsonde soundings.
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Synoptic Observations Con’d Disadvantages with the
Rawinsonde System• Huge gaps of data in areas over water, in less-populated areas and in developing countries.
• Our view “of the synoptic-scale structures associated with severe convection tends to be dominated by continental, northern hemispheric, American and western European systems.”
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Synoptic Observations Con’d Analysis of Synoptic-scale
Events
• Most tend to be “outbreaks.”
• Outbreaks are typically uncommon throughout the year, but they contribute a large fraction of the total number of severe events.
• Intensity of outbreaks tends to be higher than non-outbreaks.
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Synoptic Observations Con’d
Outbreak-centered analysis: Composite diagram of conditions at different levels supporting favorable
conditions for severe weather.
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Synoptic Observations con’d
How do we describe a typical syonptic case for severe convection if most severe convective event days are not outbreaks?
Adopt an “ingredients-based” approach instead of generally defining synoptic-scale patterns for severe events.
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Synoptic Observations Con’d
QG-processes dominate in polar and subtropical regions
Quasi-geostrophic diagnostic omega equation
A B C • Term A: three-dimensional Laplacian of omega
• Term B: vertical variation of the geostrophic advection of the absolute geostrophic vorticity
• Term C: Laplacian of the geostrophic advection of thickness
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Synoptic Observations Con’d
Synoptic-scale Patterns within the Tropics: Geostrophic
balance plays a lesser role.
• Mesoscale systems are more important – and convection is a major player in tropical meteorology.
• Tropical regions of Africa, Latin America, Asia and Australia have monsoon-dominated convection.
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Mesoscale observations
Satellite Imagery
Satellite images display detail, but they are qualitative, rather than quantitative.
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Mesoscale observations con’d
Radar
Radar does not collect quantitative information about temperature, pressure or humidity.
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Mesoscale observations con’d
Convective Outflow
• Cooled air (from precipitation) spreads at the surface & at the storm top, as the outflow interacts with the surrounding environment.
• New convective cells can develop from an initially strong outflow.
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Mesoscale observations con’d
External Processes
• External processes include topographic origins (mountain-valley and sea-land breezes)and topographic effects such as strong downslope winds, flow changes due to surface roughness --
• Fronts, including drylines --
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Predicting Severe Convective Storms
• The accuracy of severe thunderstorm forecasts has increased since the 1950s.
• Generally, the forecasting skill has increased by 2.
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Predicting Severe Convective Storms Con’d
Disaster Mitigation
(1) The focus should be on improving the forecast, and the forecast should be verified – based on what weather events actually happened.
A severe convective storms forecast begins with a strong foundation of severe convective weather events.
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Predicting Severe Convective Storms Con’d
Disaster Mitigation
(2) Use more physical-based concepts to build forecasting methods rather than empiricism and statistical modeling.
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Predicting Severe Convective Storms Con’d
Disaster Mitigation
(3) Have a plan in place to effectively use forecasts of severe convective weather.