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? 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 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. Density of severe storm reports with +CG Dry line positions for June 99

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 Operations journal (with text and radar summaries) at: 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 Dopplerconfigurations

STEPS Fixed Instrumentation: Triple- Doppler Network and LMA (VHF TOA) At KGLD: -NWS -T-28 -NSSL -Electric field balloon -Mobile mesonet -MGLASS Lightning mapping/ Emission from negative leaders in positive charge regions—good for locating positive charge layers

STEPS Facilities NCAR S-pol SDSMT T-28 NWS Goodland NSSL EFM

STEPS Ops Center CSU-CHILL National Radar Facility; 10 cm polarimetric/Doppler

29 June 2000 Reflectivity Radial velocity Rotation--mesocyclone

29 June 2000 Reflectivity Radial velocity

A “static” view Reflectivity Vertical velocity Vorticity Vorticity offset from storm core on right flank Column max’s “Theta e” ridge in vicinity of storm track

Theta e ridge clearly present

LMA-derived flash rates (NMIMT algorithm); positive CG’s from NLDN

X vorticity X vorticity Broad,intenseupdraft

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 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 lightning occurs in concert with significant graupel and hail development

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 n 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. n Hypothesis is that charge separation occurred in this region via non-inductive charging. Relevant hydrometeors—hail and graupel? Graupel acquires positive charge, hail negative? n Hail fallout may provide downward bias for positive leader. Takahashi, LWC, 5 g/m 3 Afforded by broad updraft---