RADAR STUDIES OF
LIGHTNING PRODUCING
CLOUDS
Prof. Steven A. Rutledge
Department of Atmospheric Science
Colorado State University
ILMC 2014
Tucson, 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. Also
obvious is a link between large positive iCMC’s and Mesoscale
Convective Systems
Courtesy W. Lyons
Cummer et al. (2013)
Using the National Charge Moment Change Network
Detects ELF radiation from vertical channel segments of lightning.
Examine 5 year climatology of iCMC observations from 2007-2012.
Regional and seasonal evaluations.
iCMC (annual) Density; > 100 C km “Large”
> 300 C km “Sprite class”
+
_
+
_
10-2 km-2yr-1
X 0.1 for > 300 C km
Large +iCMC density, Seasonal, > 100 C km
10-2 km-2yr-1
Sprite-Class +iCMCs, Seasonal, > 300 C km
Sprite-class iCMC’s maximized in March-August time
period and occur in “MCS alley”
Ashley et al. 2003 Mon. Wea. Rev.
Zajac and Rutledge, 2001
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 for
positive iCMC’s. Negative iCMC’s are generated by non-MCS
precipitation, 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
Convective region
behavior
iCMC’s concurrent with active convective
precipitation and building stratiform region
Intense convection necessary
Builds stratiform region and contributes
charge via charge advection
Strong convection leads to mesoscale ascent in
stratiform 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
Regional
Environmental/Lightning
To examine relationship between
environmental parameters (CAPE, warm cloud
Studies
depth, LCL, etc) and charge structure / lightning characteristics
16
Normal charge structure
Temperature (°C)
Anomalous charge
structure
-40
-30
-20
-10
0
LMA mode
-40
-20
°C used to infer charge
Use LMA source°C
density profile
structure, how do storms develop mid-level or low level
dominant positive charge; why are these storms confined
17
to specific geographical locations?
Williams et al. demonstrated Flash Rate linked to cloud base height for tropical
locations. They also suggested that optimal intersection of sufficiently
large CAPE and significantly elevated cloud base heights may lead to superlative
electrification and storms producing dominant positive CG lightning. These storms have
inverted or anomalous charge structures (Wiens et al. 2005).
Williams et al. (2005)
18
Now examine environmental variables in these regions
• Colorado region;
highest flash rates
• DC region lowest
Flash rates via clustering algorithm developed by E. Bruning and others…
19
N
NCAPE: CAPE divided by the
height difference between the
LFC and Equilibrium Level.
J/kg/m. NCAPE is related to
parcel kinetic energy.
OK and CO are the winners in terms of NCAPE. Yet CO flash rates are larger.
20
Colorado median LCL
height ~ 3 times higher
Cloud base height
MSL = 1.4 km + AGL for CO
21
The final parameter: WCD---vertical distance between cloud base and the freezing level
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 phase
region due to reduced coalescence. Higher LCL/cloud
base heights likely reduce fractional entrainment by producing broader
updrafts.
Both processes lead to a higher adiabatic liquid water content
in mixed phase region. High liquid water contents linked to positive
charging of rimer via non inductive charging.
So where are the most “inverted” storms in our study region?
Plotting peak
LMA source
density as
function of T
AL/DC warm positive
charge layers
associated with
decaying, low flash
rate storms. EOSO
In Colorado, significant amount of active storms
have inverted or “anomalous” charge structures.
Recall, large NCAPE’s, high CBH’s and shallow WCD’s.
Role of shallow WCD’s and high LCL’s can be considered using recent framework by
Bruning et al. (2012). What can radar data can tell us about precipitation physics.
Charge reversal temperature
Sloping dashed lines represent
various liquid water depletion rates.
Depletion rates (via riming) decrease
from bottom to top. Depletion rates
affected by presence of ice
particles such as graupel and hail,
plus supply of supercooled liquid
water driven by storm updraft.
Hypothesis: Large WCD (low LCL) introduces
large drops immediately above the
freezing level which promotes rapid
depletion of SLW and negative charge
on rimer. Shallow WCD (high LCL) delays presence
of rimer, allowing SLW’s to increase at
colder temperatures, promoting positive
charge on rimer, and inverted (anomalous) charge structure.
A radar based case study using CSU-CHILL
Flash Rate
2152z
AGL
2152 UTC
ZDR column maps lofted supercooled drops
LDR cap indicates wet hail
Pulsing updraft produces this sequence
Positive charge descends as pulse weakens
A similar case where
enhanced mid-level
positive charge
develops after sharp
increase in GEV; ZDR
column evident in this
case too.
Zdr column indicates lofting of raindrops into
mixed phase region. These drops freeze and grow
rapidly 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
f = (5.8´10-2 )´GEV - 0.43
30-dBZ echo volume
f = (4.8´10-2 )´ 30-dBZ EV + 2.34
Precipitation ice mass (M vs. Z relationship)
f = (4.4 ´10-9 )´ p1.17
m
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 anticorrelation 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.
39
What are implications for NOx generated by lightning?
40
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 observations
Lang et al. 2014, Mon. Wea. Rev.