RADAR STUDIES OF RADAR STUDIES OF LIGHTNING PRODUCING LIGHTNING PRODUCING

CLOUDSCLOUDS

Prof. Steven A. RutledgeDepartment of Atmospheric Science

Colorado State University

ILMC 2014Tucson, AZ

I want to thank….

• CSU students Brett Basarab, Nick Beavis and Brody Fuchs

• Timothy Lang (NASA/MSFC) and Walt Lyons (FMA Research)

• Steve Cummer and colleagues at Duke• V. N. Bringi and Pat Kennedy at CSU• Eric Bruning (Texas Tech), Paul Krehbiel, Bill Rison and

Ron Thomas (New Mexico Tech) and Matt Kumjian (Penn State)

• National Science Foundation for financial support

Outline• Regional and seasonal characteristics of large

impulse charge moment change events, and in relation to Mesoscale Convective Systems

• Distilling some properties of storms with inverted (anomalous) charge structures; regional lightning studies afforded by multiple LMA networks, looking at how environmental parameters relate to storm electrical characteristics

• Results from DC3: flash rates and NOx production and a quick look at electrified pyrocumulus

Transient Luminous Events

TLE’s clearly linked to large impulse charge moment change events. Alsoobvious is a link between large positive iCMC’s and Mesoscale Convective Systems Courtesy W. Lyons

Detects ELF radiation from vertical channel segments of lightning.Examine 5 year climatology of iCMC observations from 2007-2012. Regional and seasonal evaluations.

Cummer et al. (2013)

Using the National Charge Moment Change Network

iCMC (annual) Density; > 100 C km “Large” > 300 C km “Sprite class”

+

+

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_ X 0.1 for > 300 C km10-2 km-2yr-1

Large +iCMC density, Seasonal, > 100 C km

10-2 km-2yr-1

Sprite-Class +iCMCs, Seasonal, > 300 C km

Zajac and Rutledge, 2001

Sprite-class iCMC’s maximized in March-August time period and occur in “MCS alley”

Ashley et al. 2003 Mon. Wea. Rev.

10-2 km-2yr-1

Sprite-Class -iCMCs, Seasonal

Sprite class negative iCMC’s do not follow the MCS climatology.Peak density about factor of 10 less than peak density forpositive iCMC’s. Negative iCMC’s are generated by non-MCSprecipitation, especially in the SE U.S.

10-2 km-2yr-1

Mesoscale Convective Systems`

Charge advected into stratiform region plus generated locally

June 16 2011

Largest iCMC rates occur during growth phase of stratiform region

April 30 2012

Again see iCMC ramping up as as stratiform area blossoms

April 30 2012

Intense convection necessary

Builds stratiform region and contributescharge via charge advection

Strong convection leads to mesoscale ascent instratiform region which also contributes to charge via local non-inductive charging

MCS stratiform region can easily provide requisite charge volumes with modest charge densities

0.1 C/km3 x 1 km depth x 25 km x 25 km x 5 km = 300 C km Charge Moment Change

iCMC’s concurrent with active convectiveprecipitation and building stratiform region

Convective regionbehavior

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Regional Environmental/Lightning

StudiesTo examine relationship between environmental parameters (CAPE, warm clouddepth, LCL, etc) and charge structure / lightning characteristics

Normal charge structureAnomalous charge

structure

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LMA mode-40 °C -20 °C

Use LMA source density profile used to infer charge structure, how do storms develop mid-level or low level

dominant positive charge; why are these storms confined to specific geographical locations?

Temperature (°C)

0

-10

-20

-30

-40

18Williams et al. (2005)

Williams et al. demonstrated Flash Rate linked to cloud base height for tropicallocations. They also suggested that optimal intersection of sufficientlylarge CAPE and significantly elevated cloud base heights may lead to superlativeelectrification and storms producing dominant positive CG lightning. These storms have inverted or anomalous charge structures (Wiens et al. 2005).

• Colorado region; highest flash rates

• DC region lowest

19Flash rates via clustering algorithm developed by E. Bruning and others…

Now examine environmental variables in these regions

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NCAPE: CAPE divided by theheight difference between theLFC and Equilibrium Level. J/kg/m. NCAPE is related toparcel kinetic energy.

OK and CO are the winners in terms of NCAPE. Yet CO flash rates are larger.

N

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Colorado median LCL height ~ 3 times higher

Cloud base height

MSL = 1.4 km + AGL for CO

Colorado storms have higher cloud bases and smaller Warm Cloud Depths compared to other regions.

Small warm cloud depth leads to higher SLW contents in mixed phaseregion due to reduced coalescence. Higher LCL/cloudbase heights likely reduce fractional entrainment by producing broaderupdrafts.

Both processes lead to a higher adiabatic liquid water content in mixed phase region. High liquid water contents linked to positivecharging of rimer via non inductive charging.

So where are the most “inverted” storms in our study region?

The final parameter: WCD---vertical distance between cloud base and the freezing level

AL/DC warm positivecharge layers associated with decaying, low flash rate storms. EOSO

In Colorado, significant amount of active stormshave inverted or “anomalous” charge structures.Recall, large NCAPE’s, high CBH’s and shallow WCD’s.

Plotting peakLMA sourcedensity as function of T

Sloping dashed lines representvarious liquid water depletion rates.Depletion rates (via riming) decreasefrom bottom to top. Depletion ratesaffected by presence of iceparticles such as graupel and hail,plus supply of supercooled liquidwater driven by storm updraft.

Charge reversal temperature

Role of shallow WCD’s and high LCL’s can be considered using recent framework byBruning et al. (2012). What can radar data can tell us about precipitation physics.

Hypothesis: Large WCD (low LCL) introduceslarge drops immediately above thefreezing level which promotes rapiddepletion of SLW and negative chargeon rimer. Shallow WCD (high LCL) delays presenceof rimer, allowing SLW’s to increase atcolder temperatures, promoting positivecharge on rimer, and inverted (anomalous) charge structure.

2152 UTC

2152zAGL

Flash Rate

ZDR column maps lofted supercooled dropsLDR cap indicates wet hail Pulsing updraft produces this sequencePositive charge descends as pulse weakens

A radar based case study using CSU-CHILL

A similar case whereenhanced mid-levelpositive charge develops after sharpincrease in GEV; ZDRcolumn evident in this case too.

Zdr column indicates lofting of raindrops intomixed phase region. These drops freeze and growrapidly into large graupel and hail, feeding on large SLW contents generated by updraft.

Lightning and the production of nitrogen oxides (NOx)

Goal: develop improved lightning parameterization schemes using the

DC3 dataset

Simple lightning parameterization schemes exist, relating flash rate to bulk storm parameters

• Useful for estimating total lightning and NOx production in numerical models• Necessary to rigorously test these schemes against observations (flash rates estimated from LMA data)

Existing schemes tested on four CO cases

The Test

Moving beyond the current parmaterizations: Graupel echo volume

30-dBZ echo volume

Precipitation ice mass (M vs. Z relationship)

Are these parameterizations just applicable in a specific region?

Results – new parameterizations

Still work to do…..Preliminary results suggest that microphysical processes modulating flash rate are not well represented by simple flash rate schemes; the “tuned” parameterizations are still not working well.

Do we use sounding data using NCAPE,WCD and LCL height?

Do we resort to storm echo top heights?

Much more work needed!

Preliminary considerations of flash size behavior

• DC3 investigators looking at the behavior of flash rate vs. flash size in order to consider the implications for parameterizations based on alternative lightning metrics• Qualitative assessment shows some anti-correlation between flash rate and average flash size, especially for 6 June late storm - as predicted by Bruning and MacGorman (2013). L. Carey and colleagues working along these same lines. •Important consequences for NOx production via lightning.

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What are implications for NOx generated by lightning?

2100-2200 27 June LMA density;Inverted storm distinctive with more sources and at a lower altitude than surrounding normal convection

~2130-2142 UTC 27 June charge identification; mid-level positive charge below upper-level negative charge. Storms ingesting smoke are inverted; have identical radar structures to normal storms

Electrified pyrocumulus, polarimetric radar observationsLang et al. 2014, Mon. Wea. Rev.