2009 StormsPart 1©2010 Fisher

Doppler Weather Radar Concepts

• Radar beam increases in altitude with distance from radar.• Radar beam increases in size with distance from radar.• Radar takes 4-5 minutes to complete full scan when in storm/precip mode.• Reliable doppler wind data limited to about 60-70 nm.• Radar velocity 100% when parallel, 0% when perpendicular to beam direction.

100% 100%

0%

0%

When the wind velocity is parallel to the radial, the full component of the wind is measured

When the radial is perpendicular to the the wind,

the radar displays zero velocity - This “zero zone” is

called the “Zero Isodop”.

What percentage of actual wind

will the radar detect?

00 = 100% - Parallel150 = 97%300 = 87%450 = 71%600 = 50%750 = 26%

900 = 0% - Perpendicular

The Zero Isodop “Problem”

Products• Base reflectivity

– how much precipitation is falling – precipitation type– assess a storm's structure and dimension

• Composite Reflectivity– Scans from all elevations, imaging precipitation intensity and storm

structure

• Base Velocity– radial wind field, speed of fronts/strong wind– range of 140 mi

• Storm relative motion– Track a circulation (show up well in doppler return) over time to

determine storm motion.– Removing the storm relative motion from base radials gives an estimate

of the flow with respect to the storm.

LOTHeight of beam above ground vs distance from radar. Green 10nmYellow 15nmRed 20nm

ORD TDWR20nm10nm

MDW TDWR

20nm10nm

What is a supercell?• Storms having deep, persistent, rotation about a vertical axis

• Although most significant tornadoes are associated with supercell thunderstorms, most supercells are not tornadic– What’s perhaps most troubling (from the perspective of

issuing warnings) is that most supercells contain low-level mesocyclones, and perhaps most supercells even have circulations that extend to the surface

nontornadic nontornadic tornadic nontornadic

What is a supercell?What are the biggest unanswered questions in the study of tornadogenesis?Why do storms with seemingly similar structure differ in their tornado production?

April 9, 2009 day 1 of 2 day storm outbreak 5PM0-6 km Shear Vector

0-3 km

0-1 km

0-1 km

KSRX Vad Winds

Storm Composite

Base Reflectivity lvl 2

Storm Relative Velocity lvl 2

Base Velocity lvl 2

April 10, 2009 2nd day of outbreak 2 PM0-6 km Shear Vector

Storm Composite

Radar HTX Base Reflectivity lvl 2 ~ 1500 ftClassic Rotating Supercell

Radar HTX Storm Relative Velocity lvl 2Strong Rotation Couplet

Radar HTX Base Reflectivity lvl 2 ~ 3200 ft

Radar HTX

HTX Base Reflectivity lvl 2 ~ 1900 ft 0.5˚

HTX Storm Relative Velocity lvl 2 ~ 1900 ft 0.5˚

Double Rotation?

HTX Base Reflectivity lvl 2 ~ 4000 ft 1.3˚

Bowing Line Tstms (L) and Classic Supercell (R)

HTX Base Velocity lvl 2 ~ 1900 ft 0.5˚

HTX Storm Relative Velocity lvl 2 ~ 4000 ft 1.3˚

Downburst (L) and Rotation Couplet (R)

Part of 5/8/09 Derecho 100+ mph S IL, 5 dead in MO/ KY

Tornadogenesis: Three ingredients

Development of a persistent, rotating updraft1

Ingestion of enhanced SRH (occasionally, large-scale SRH is sufficient) and development of strong low-level rotation

2

Development of a downdraft partially embedded in the rotation that aids in the transport of rotation to the ground, followed by focusing of that rotation through convergence if the downdraft reaches the ground with some very uncommon properties

3

Tornadogenesis in a Nutshell

The Need for Better Understanding of Tornadic Storms

• Tornado warnings– Improvements in our understanding of tornadogenesis should better

allow us to assess the likelihood of tornadoes in thunderstorms– Possible advances in our ability to forecast tornado intensity and

longevity

Tornado warning performance from 1986-2002 (adapted from Brooks 2004)

Improvements likely due to 88D network, better training, better SPC guidance, application of VORTEX1 findings?

Summary (what we know)• Supercells acquire rotation aloft by tilting horizontal vorticity (streamwise

horizontal vorticity leads to net cyclonic updraft rotation)

• Although most significant tornadoes are associated with supercell thunderstorms, most supercells are not tornadic (and the supercells with the strongest mesocyclones are not necessarily the most likely to be tornadic)

• Tornadogenesis requires a downdraft if pre-existing vertical vorticity is absent at the surface

• The temperature of the downdrafts seems to be important to tornadogenesis; downdrafts that are excessively cold apparently are unfavorable for tornadogenesis

• Environments that have large ambient low-level vertical wind shear (larger than what is found in an average supercell environment) and large ambient relative humidity favor tornadic supercells over nontornadic supercells

LSX Composite

LSX Reflectivity lvl 2

Base Velocity lvl 2

Storm Relative Velocity lvl 2

Tornado warning?

http://www.gpsvisualizer.com/geocode

http://www.capelinks.com/cape-cod/maps/geocode/