Transcript AGU_2003_FINAL - Colorado State University

STEPS
Severe Thunderstorm Electrification
and Precipitation Study
May-July 2000
S. Rutledge, S. Tessendorf, K. Wiens, T. Lang, J. Miller#
Department of Atmospheric Science
Colorado State University
#National Center for Atmospheric Research
STEPS Objectives
To gain a better understanding of interactions between kinematics,
precipitation production and electrification in severe thunderstorms
over the High Plains.
What are the conditions that differentiate supercell storms into high precipitation,
moderate precipitation, and low precipitation events?
Why do some severe storms produce anonymously large amounts of positive
cloud-to-ground lightning?
Lightning Statistics from NLDN
Density of severe storm reports with +CG
Dry line positions for June 99
STEPS domain centered on area with high probability of severe weather and storms that
produce significant fractions of positive CG lightning. STEPS domain also corresponded
to climatological position of dry line. Courtesy, L. Carey, L. Barker.
POSITIVE CG MECHANISMS
SHEAR, POSITIVE CHARGE DISPLACED LATERALLY
(NORMAL POLARITY DIPOLE)
ENHANCED LOWER POSITIVE CHARGE
(INVERTED DIPOLE)
PRECIPITATION MECHANISM
(NORMAL POLARITY DIPOLE)
STEPS highlights
Observations of 8 weak events, 7 moderate storms, and 5 severe storms.
Highlights include major downburst events on 31 May and 19 June, a bow-echo
MCS on 11 June, LP supercells on 31 May and 5 July, and MP/HP supercells on
22 June, 24 June and 29 June.
STEPS information at http://www.mmm.ucar.edu/community/steps.html
Operations journal (with text and radar summaries) at:
http://www.mmm.ucar.edu/pdas/Ops-journal/ops-summary.html
Support provided by NSF and NOAA.
Key participants: NCAR, NSSL, NWS, NMIMT, SDSMT, CSU, OU, FMA
Research, Inc.
STEPS featured a unique set of observational platforms!
Radar Network
Dual-Doppler
and Triple
Doppler
configurations
STEPS Fixed Instrumentation: TripleDoppler Network and LMA (VHF TOA)
Lightning mapping/
Emission from
negative leaders
in positive charge
regions—good for
locating positive
charge layers
At KGLD:
-NWS
-T-28
-NSSL
-Electric field
balloon
-Mobile
mesonet
-MGLASS
STEPS Facilities
NCAR S-pol
NSSL EFM
NWS Goodland
SDSMT T-28
STEPS Ops Center
CSU-CHILL National Radar Facility; 10 cm polarimetric/Doppler
29 June 2000
Reflectivity
Rotation--mesocyclone
Radial velocity
29 June 2000
Reflectivity
Radial velocity
A “static” view
Reflectivity
Column max’s
Vertical velocity
Vorticity offset
from storm core
on right flank
“Theta e” ridge in vicinity of storm track
Vorticity
Theta e ridge
clearly present
LMA-derived flash rates (NMIMT algorithm); positive CG’s from NLDN
X
vorticity
Broad,
intense
updraft
Zdr column
indicative of
large drops being
lofted into main
updraft—increases
with time—
pronounced beyond
2330 UTC
Graupel and Hail Volume Time
Height History
Positive CG lightning
occurs in concert with
significant graupel
and hail development
Mean starting altitudes
of NLDN-confirmed
CG’s as revealed by
LMA
LMA reveals that
positive CG’s
are from lower levels
and co-located with a
zone of significant
hail/graupel.
“Inverted” storm.
Positive CG locations during intense phase (first peak)
Ground strike locations; starting heights of CG’s
Positive CG’s in Vertical XC
NSSL EFM Profiles
Summary
LWC, 5 g/m3
Afforded by broad
updraft---
Juxtaposition of vorticity column,
and updraft (supplying supercooled
liquid water) allowed recycling of embryos
into storm draft creating a robust mixture
of graupel and hail.

Takahashi, 1978.
Hypothesis is that charge separation occurred in this region via
non-inductive charging. Relevant hydrometeors—hail and graupel?
Graupel acquires positive charge, hail negative?


Hail fallout may provide downward bias for positive leader.