Severe Convective Severe Convective Storms -- An Storms -- An

OverviewOverviewPart 1 -- John MonteverdiPart 1 -- John Monteverdi

Part 2 -- Kathryn SaussyPart 2 -- Kathryn Saussy

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

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)

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.

‘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?

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.

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.

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.”

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.

Synoptic Observations Con’d

Outbreak-centered analysis: Composite diagram of conditions at different levels supporting favorable

conditions for severe weather.

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.

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

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.

Mesoscale observations

Satellite Imagery

Satellite images display detail, but they are qualitative, rather than quantitative.

Mesoscale observations con’d

Radar

Radar does not collect quantitative information about temperature, pressure or humidity.

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.

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 --

Predicting Severe Convective Storms

• The accuracy of severe thunderstorm forecasts has increased since the 1950s.

• Generally, the forecasting skill has increased by 2.

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.

Predicting Severe Convective Storms Con’d

Disaster Mitigation

(2) Use more physical-based concepts to build forecasting methods rather than empiricism and statistical modeling.

Predicting Severe Convective Storms Con’d

Disaster Mitigation

(3) Have a plan in place to effectively use forecasts of severe convective weather.