2009 StormsPart 2

©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”

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

MDW TDWR

20nm10nm

ORD TDWR20nm10nm

PROBLEM(s) OF THE DAY (POTD)

ORGANIZED SEVERE STORMSINGREDIENTS-BASED APPROACH

MOISTURE INSTABILITY

(sources for convective)

LIFT

SHEAR

Tornadic Supercell “Ingredients”

• Instability (varies, usually CAPE > 1000 J/Kg)

• Deep layer shear (0-6km) bulk shear > 35kt

• Low level shear (0-1km) SRH > 150 m2/s2 or pre-existing boundary)

• Relatively high RH values (in BL and aloft)• Sfc Td depressions <12°F

Some Commonly Used SPC Mesoanalysis Indices

• Effective Storm-Relative Helicity (ESRH) • Effective Bulk Shear (EBS)• Supercell Composite Parameter (SCP)• Significant Tornado Parameter (STP)

Summary of STP Assessment

• No strong signal in fixed layer STP in absolute or trending sense• Showed boldly only after event started• Underscores STP as a DIAGNOSTIC parameter

• 0-6 km shear, 0-1 km SRH (400+), and MLLCL all favorable for STP ≥1

• Primary shortcoming resulted from MLCAPE values less than 500 J/kg until 23z

• Suggests need to monitor closely for single “missing” variable when others are in place

Tornado Outbreak over N IL April 20, 2004

Friday June 19, 2009 Naper Days

Pocket of 87 mph winds

July 24, 2009

ILX

ILX

Base Velocity 1700 ft

3500 ft

LOT

Base Velocity

Aug. 9, 2009

DVN

DVN Base Velocity

DVN Storm Relative Velocity

DVN VAD Wind