High resolution simulations of microphysics and electrification in a hurricane-like vortex

and a TOGA COARE oceanic squall line

Alexandre Fierro

School of Meteorology, National Weather Center, University of Oklahoma

Collaborators: Leslie L. M., Mansell E., Straka J., MacGoman D., Ziegler C.

•No work done so far in terms of modeling lightning in these systems

This study will provide a first insight of electrical properties in those systems (spatial charge distribution of charge and charging rates)

Motivation continued

Katrina RitaAlso..

Deepening of the TCs is often accompanied by CG (and very likely IC) lightning bursts in eyewall (e.g., Molinari et al. 1994) Plots from Shao et al. 2006

Quick Model Description• Straka and Mansell (2005) 12 bulk hydrometeor categories (Inverse exponential distribution): cloud, rain, snow, ice (3 habits), graupel (4 densities), hail (2 sizes)

• 3-D branched lightning parameterization (Mansell et al., ,2002) with electric field initiation threshold, corona discharge and explicit treatment of small ions

• Includes 1 inductive and 5 NI charging parameterizations

Negative leader

Initial Breakdown

Positive leader-CG

2-D Lightning grid

1.Tropical oceanic squall line simulation• 9th February 1993 TOGA COARE “ship” from Petersen et al. 1999 obs.

•Δx = Δy = 600 m, Δz stretches from 100 m to 600 m above 7 km in a 132 x 210 x 22 km domain

• -7 K bubble (20 x130 x 2 km)

• CAPE/CIN = 1750 /10 J/kg

• Nearly moist adiabatic

• Westerly LLJ of 12 m/s near 800 mb

• SP98 NI charging scheme

• Moderate inductive charging strength

• Starts as a solid meridional line of convection.

• System total lightning activity of 3 –CGs and 1050 ICs: greater than obs (only 1C within range of field mill aboard the ship)

• Lifetime = 5 h, similar to obs.

-1 K

Results:

• Note: winds rotated 50° clockwise & domain rotated by 90° for display orientation of line nearly NW-SE

• 30 dBZ echo contour (with black contour) well within mixed phase layer (ML: 0-20°C). W

• Stronger updrafts in / above ML (and larger Qg values) in “mature zone” (Y=67km)

• Rapid depletion of LW ahead of line (fast coalescence)

• Stratiform region largely glaciated (snow & ice crystals)

Expect bulk of lightning activity in mature zone

Line-averaged plots (Y = 90 to 120 km)

Run

• Charge structure in mature zone resembles a normal tripole (+ - +)

• Upper positive and midlevel negative charge region: NI

• Lower positive layer: induction (largest liquid precipitation mixing ratio) & NI

• Most of the charge in stratiform region acquired Non-inductively in situ

Individual cross sections for charging

+

+ _

_+ _

• Mid-level neg. charge region attributed to (NI) charging of cloud ice and graupel

• Upper level + charge attributed to + (NI) charging of cloud ice

• Lower + charge region attributed to + NI and + inductive charging of graupel

• LW quickly depleted towards the rear of the line below 0°C limits charging magnitude in mixed phase region.

Individual cross sections for charging ctd..

2.Tropical Cyclone simulation• Composite of 00UTC 13th August 2004, at Owen Roberts airport, Grand Cayman and 00 UTC Kingston, Jamaica (above 15km). (Hurricane Charley)

•Δx = Δy = 2 km, Δz stretches from 200 m to 600 m above 7 km in a 600 x 600 x 25 km domain

• moisture/sensible heat sfc flux of Rotunno and Emanuel (1987)

• Homogeneous SST = 28°C

• Rankine type bogus vortex of max winds ~ 44 m/s

• SP98 NI charging scheme

• Moderate inductive charging

CAPE: 1473.6 J/kgCIN : 99.2 J/kg

Saharan layer

Added CIN

Reduced LL CAPE moist adiabatic

Results (at 21 h 40 min)•Well defined eye and eyewall (min psfc=952 mb)

•3 main connecting rainbands

•Asymmetric eyewall

•Bulk of total CG activity located in NW quadrant

•Exclusively –CG except 1 +CG in rainband

•IC rate ~ 300 / min

21h40 30 min CG composite

+

• Charge structure generally very complex:

• IF only considering region > 0.4 nC m-3 (i.e., involved in lightning, see later)

• Normal tripole in eyewall (+ - +)

• Strongest cells in rainband exhibit normal dipole (+ -)

--+ +

+-+ +

Charge density (nC m-3)

Positive (negative) contours are shown by the red (blue) contours by increments of 2 starting at pC m -3 s-1.

Midlevel negative charge structure: NI

charging

Upper level positive charge structure: NI

charging

Lower level positive charge structure:

Both NI and induction

Charging (pC m-3 s-1)

•Lightning initiation found at 2 levels: ~7 km and ~10 km

•The -CG flashes in eyewall propagate through the inductively and Noniductively generated lower positive charge region.

•Positive leader channels found at lower levels in rainband cell (but rarely reach ground)

+/- lightning channels

Questions ?