understanding anomalous storms
TRANSCRIPT
Understanding anomalous
storms
Brody Fuchs
23 Mar 2017
Atmospheric Electricity Lecture
Quick review of non-inductive
charging and lightning
• Polar nature of water molecule means that charge can be transferred during collisions
• Charged particles (primarily) vertically separated by convective motions in the storm– Gravity pulls larger particles downward
– Convection pushes small particles upward
– Large scale separation produces strong electric fields
– Lightning is results from electrical breakdown of air
– Acts to neutralize charge separation/electric fields
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Takahashi (1978); Saunders and Peck (1998);
Pereyra and Avila (2000)
Charge acquired by graupel particle a function of
temperature and supercooled cloud water content
• Particle growing faster by deposition acquires positive charge upon collision
• Fundamentally, supercooled cloud droplets are accreted by larger particles which then freeze and release latent heat
• Alters the fundamental heat balance
• Can alter growth rates from vapor deposition
• Could be affected by other things such as cloud droplet size
Takahashi et al. (2017)
Charge polarity/magnitude depend on
experiment setup
• Lots of different results from different studies
• Largely dependent on experimental setup
• A few common themes
– Graupel charges positively at warm temperatures
– At colder temps, graupel charge is dependent on supercooled water content
• In theory, should be able to back out supercooled water content if we can get graupel charge polarity
Lightning as an investigative tool
• Electric fields result from collisions between graupel and ice crystals
• Lightning flash rate linked to vertical air motions, graupel mass
• Various microphysical and dynamical processes control amount of SCLW in mixed-phase
• We should be able to work backwards from lightning flash rate and charge structure to learn stuff about microphysics/dynamics
Williams (1985); Baker et al. (1995) ; Lang et al. (2000); Williams and Stanfill (2002);Williams et al. (2005); Carey and Buffalo (2007)
Storm charge structure and lightning
• “Normal” dipole/tripole• Vertically separated,
oppositely charged regions• ICs between mid-level
negative/upper-level positive
• CGs initiate between lower positive and mid-level negative
• Lower negative charge to ground
• Majority of storms around the US and globe have this charge structure
• How do we know this???
• Lower atmosphere is a weak conductor
• Continuous current flow from ionosphere to
Earth surface (can be thought of as capacitor)
• Would not be continuous unless there was a
continuous charging mechanism
• Upper positive charge in thunderstorms is
transported to the ionosphere (or
electrosphere) and negative charge is
transported to the Earth’s surface through
cloud-to-ground flashes
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Rc
ABOUT 90% OF
CGS IN CONUS
ARE NEGATIVE
POLARITY
-+
-+
0 °C
-20 °C
-40 °C
Normal polarity, modest supercooled water
-CG flash
• Low level positive charge due to graupel charging at warm temps
• Mid level negative charge due to graupel charging at cooler temps and
modest supercooled water contents
• Upper positive from the ice crystals that collided with the mid level graupel
• Infer modest SCLW from –CGs, which likely from mid-level negative charge,
which is likely graupel -> therefore, SCLW must fall in the negative charging
zone
Cloud water content (g/m3)
Tem
per
atu
re (
°C)
Typical supercooled liquid water
contents (from our best estimates)
IC flash
Anomalous polarity, large supercooled water amounts
-+
? ? ?+CG flash
Cloud water content (g/m3)
Tem
per
atu
re (
°C)
• Mid level positive charge (likely on graupel)
• Corresponding negative ice crystals lofted to the top of the storm
• Not clear what charge (if any) resides near 0 °C, could be positive or
negative
• Higher fraction of +CG flashes, likely comes from mid-level positive, which
is likely comprised of graupel
• Additionally, these storms more likely to produce severe weather, also have
high IC:CG
Increase supercooled water content
over the threshold to get positive
graupel charging
IC flash
The problem
• Inference of supercooled water is very indirect
• We can’t get in situ measurements!
The payoff
• Lightning/charge structure/supercooled water are
functions of microphysics and dynamics
• Various lightning/charge quantities could inform us about
storm processes
– Entrainment
– Turbulence
– Updraft speeds?
– Mixed-phase processes
– Growth of graupel/hail
– Warm rain efficiency?
– Nowcasting
• So we have to try to understand lightning/charge
structures
Causal chain
Environment
Microphysics Dynamics
Charge transfer
Charge structures
Lightning
Boccippio et al. (2000); Williams et al. (2005); Fuchs et al. (2015)
Θw, Instability
CBH
IC:CG ratio
% +CGs
The thermodynamic argument
• 2 schools of thought about updraft strength
• Sun warms land more than oceans
– More instability
– Maybe not true? (Williams and Renno 1993)
– Higher cloud bases and shallow warm cloud
depths
• Greater land aerosol concentrations
– Aerosol invigoration of convection
• Simultaneous influences?
Thermodynamics vs. Aerosols
Lyons et al. (1998); Williams et al. (2002); Williams and Stanfill (2002); Andreae et al. (2004); Williams et al.
(2005); van den Heever et al. (2006); Rosenfeld et al. (2008); Stolz et al. (2015); Fuchs et al. (2015, 2017)
Freezing Height
CBH
WCD
CBH α surface dew point depression
My masters work• Investigate dependence of
storm characteristics on thermodynamics and aerosols
• ~ 4000 isolated convective storms from 4 distinct regions of US with NEXRAD and high-res lightning data
• Accurate total flash rates
• Infer gross charge structures
• Estimate supercooled water contents?
Boccippio et al. (2000); Williams et al. (2005); Fuchs et al. (2015)
Θw
Instability
CBH
CSU Lightning, Environmental, Aerosol and
Radar (CLEAR) Framework
• Designed by Lang and Rutledge (2011) to objectively analyze large amounts of data
• Objectively identifies cells based on reflectivity
• Used with – 3D mosaic (reflectivity only) data every 5 minutes
– Lightning mapping arrays (LMA) – flash counting
– Nat’l Lightning Detection Network (NLDN) CG data
– Environmental (RUC/RAP) analysis data
– Model CCN concentrations (GEOS-Chem)
• Combines data by attributing data to identified cells
Rison et al., (1999); Benjamin et al., (2004, 2006); King et al., (1999); Zhang et al., (2011); Lang and
Rutledge (2011); Pierce et al. (2013); Fuchs et al. (2015)
GLD 360/ WWLLN
NLDN
LMA
Different lightning processes
produce different
frequencies of radiation
Wavelength proportional to
characteristic size
Attenuation of radiation is
frequency-dependentCourtesy Walt Lyons
LMA operation
• Fancy system of radio antennas
• Detects sources within ~ 60-66 MHz band – Unused local analog television band (channel 3 or 4)
– Wavelength ~ 5 meters
• Time of arrival (TOA) from 6 or more stations– Redundant equations, higher accuracy
– Intersection of time-difference hyperbolas
• Decimated data – record most intense radiation within a time window (tens of ns)– Realtime output (smaller bandwidth)
• Full rate data- processed to provide all radiation sources
• VHF radiation produced by lightning channel propagation
Rison et al (1999); Thomas et al. (2002); Fuchs et al. (2015, 2017)
VHF Lightning MappersLMA LMA soon LDAR
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t1t2
t3
Ground
LMA time-of-arrival technique
VHF radiation α dI/dt
Cell
antenna
Radio antenna
Solar panel
Main
processor
IC flash
Lightning Mapping Arrays (LMAs)
• LMAs detect radiation produced by lightning propagation
• LMA sources grouped to make flashes
– Rates, areas and locations
• Higher detection efficiencies than LIS, OTD
– Implications for comparisons?
• Can be used to infer charge structures (LMA mode)
Temperature (°C)
0
-10
-20
-30
-40
Williams (1989); Rison et al (1999); Thomas et al. (2002); Fuchs et al. (2015)
LMA source
Use LMA mode as a crude way to represent charge structure
(or location of active positive charge)
Normal charge structure Anomalous charge structure
LMA mode-20 °C
Temperature (°C)
0
-10
-20
-30
-40
-40
°C
LMA charge inferenceLMA stations detect VHF radiation from propagation of lightning channel, more noisy in positive charge
Williams (1989)
(More common; typical LWCs) (Less common; large LWCs)
Upper positive
Mid-level negative
Time-Height
Looking north
Looking eastPlan view
Vertical distribution
Sample LMA lightning flash
Flash algorithm - DBSCAN• Most common clustering algorithm and most cited
in literature
• Finds regions of high density (in space and time)
• Groups clusters of sources into flashes
• No assumed distributions
• Free parameters
• Search radius
• Minimum points
• Relatively small sensitivities
• ~ 10% differences in flash rates
• < 15% difference with other algorithms
• Have to do subjective/manual analysis
~ 2
0 k
m
Fuchs et al. (2015)
Cell lightning flashes
Points are LMA sources
colored by time (in the 5
minute window)
Black ‘x’s are the
identified flashes by the
algorithm
Lots of data!!!
Flash products
• Flash rates
• More accurate than NLDN, IC:CG ratio– Satellite
– Field change
• Accurate measure of storm intensity
• Lat/lon/altitude of every flash
• Area of flash– Convex hull area
– Useful for NOx production studies
• With multiple flash products and charge structures, should be able to do some good investigating
Regional comparison
• Colorado has the highest flash rates
• And most anomalous storms (mid-level positive)
– Indicative of large SCLW and strong updrafts
Fuchs et al. (2015)
Environment and flash rates
Aerosol impacts• N40 = CCN proxy from GEOS-Chem
• Highest flash rates and mixed-phase
reflectivities for N40 < 1200 cm-3
• Drop-off at high N40
– Shortwave absorption?
– Decreased riming efficiency?
3 dB
Rosenfeld et al. (2008); Mansell and Ziegler (2013); Fuchs et al. (2015)
• No “clean” cases
• Hypothesize less aerosol impacts in
US
– Thermodynamics stronger
– CCN concentrations too high for
invigoration
• Does not rule out all aerosol impacts
– Charge structures? IN?
• What about CO?
Unique Colorado behavior
• Semi-arid
• High CBH
• Shallow WCD
• Unclear aerosol impacts
• Perhaps insufficient time for warm-phase aerosol impacts
• Aerosol impacts depend on warm cloud residence time?
??
?
Fuchs et al. (2015)
Summary
• Shorter warm cloud residence time results
in less warm phase precip fallout
• More in mixed-phase
• Results in more supercooled water,
graupel, collisions
• More likely to produce anomalous/high
flash rate storms
• Less CCN impacts
STILL LACKING THE
DIRECT MICROPHYSICAL
AND DYNAMICAL
EVIDENCE!!!
Fuchs et al. (2015)
Depletion of liquid water in a parcel
Let’s do something about this
• The lack of direct relationships is a problem
• Let’s use the automated case study system
on cases where we have polarimetric
radar/dual-Doppler coverage and LMA
coverage
• One of our PhD papers does just that
• Comparison of 2 populations
– 10 normal polarity cases from Alabama
– 6 anomalous polarity cases from Colorado
– Limited mainly to DC3 cases for now
Causal chain
Environment
Microphysics Dynamics
Charge transfer
Charge structures
Lightning
CLEAR updates
• Generalize to be able to take in any type of data
• Integrated with CSU-radartools (available on Github)
– Compiles years of CSU work on polarimetric retrievals
• Compute sophisticated storm metrics
• Keeps same simplicity to compile stats on many storm samples
• Also currently being used to compare WRF modeled storm to observed storm with radar
Fuchs et al. (2017)
Brief aside: polarimetric radar
• Transmits and receives in horizontal and vertical polarized radiation
• Information about size, shape, orientation, phase of hydrometeors in a volume
• Reflectivity (Z) – Measure of size/ concentration
• Differential reflectivity (ZDR) – Oblateness, sensitive to median drop size
• Specific differential phase (Kdp) – liquid water, drop oblateness, more sensitive to smaller drops than ZDR
• Correlation coefficient (ρhv) – Homogeneity of hydrometeors
• Linear depolarization ratio (LDR) – Indicative of canted particles and wet growth (only for alternating transmission mode)
Eumetcal.org; Bringi and Chandrasekar (2001); Doviak and Zrnic (1993)
RADAR
CLOUD
Polarimetric Retrievals
• Different hydrometeor species have different polarimetric signatures
– Raindrops: high ZDR, moderate-high Z, T > 0 C
– Pristine ice crystals: high ZDR, low Z, cold T
– Graupel: 0 ZDR, moderate-high Z, T < 0 C
– Hail: 0 ZDR, high Z
• Can use these differences to infer dominant species in a radar grid volume
• Can calculate other things like rain rates, graupel ice masses
Bringi and Chandrasekar (2001) and references therein
Charge structure/lightning characteristics
• Median flash rates ~ 10 flashes/min
• Mainly in line with larger sample from Fuchs et al. (2015)
• Limited by storms that were within DD lobes
More CAPE in AL
Similar NCAPE
Much higher surface T in CO
Similar adibaticwater content
Higher LCL heights in CO
Shallower WCDs in CO
Higher PW in AL Similar mid-level RH
Similar shear
Archetypal normal
AL caseMain updraft
LMA sources near
the top of the
updraft, graupel
HID
Hail, big drops in
main updraft
Modest flash rates
Robust upper
positive develops
after 2020Z
Vertical ice mass
mainly below 7 km
Modest updraft
speeds
Archetypal
anomalous CO case• Main updraft
• LMA sources on the
periphery of the strong
updraft
• Mostly LDG in the
updraft, also modest dBZ
• Higher flash rates, but
not super high
• Consistent mid-level
positive charge until
2243Z
• Shift in charge structure
after that
• Vertical ice mass
correlates with LMA
sources
• Much stronger updrafts
Quick case study summary
• Rain/hail/HDG in the updraft of the AL storm, also high dBZ, not the case in CO
• LMA sources (i.e. positive charge) near the top of the updraft in AL, not collocated with ice mass
• LMA sources (i.e. positive charge) on the periphery of strong updraft in CO, collocated with ice mass
• Stronger, larger updrafts in CO case
• Let’s see if these differences hold up in the larger sample
Bulk microphysical quantities
• Just picked a few typical
microphysical intensity
metrics, esp those related to
lightning
• In AL cases
• Larger graupel volume
• Larger mixed-phase 30
dBZ volume
• Larger hail volume
• Similar maximum graupel
heights
Updraft speed characteristicsMost vertical velocities near 0
Stronger updrafts more common in CO, also peak
updrafts higher as well
Updraft volumes
Larger UV5 and UV10 in CO anomalous cases
Most normal AL cases don’t have updrafts greater than 10 m/s
Updraft area with height
Picked -20 C because that’s
where most of the charging
is likely occurring
Much wider updrafts in CO
cases, gets wider with height,
peaks between -20 and -30 C
Location of LMA sources w.r.t. dynamics
• Collocate LMA sources
in a storm with w and
horizontal gradient of w
• w and wH CFAD
contours also overlaid
• AL cases have most
LMA sources near 0
vertical motion and near
0 wH
• LMA sources in CO
cases also occur in
relatively weak w, but
tend to occur in nonzero
wH
• Similar to the cases we
looked at
• CFAD wH contours in
CO suggest that the
strong updrafts are
broad
Collocation of LMA sources and ice mass
• Recall the cases, AL ice mass did not correlate with ice mass, but CO case did
• Nearly all of the ice mass due to graupel
• High correlation between ice mass/LMA would suggest that graupel carries positive
charge
• Colorado anomalous cases indicate that the graupel is the positive charge carrier
• In Alabama graupel is not the positive charge carrier
Warm cloud residence time
• Recall hypothesis from Fuchs et al. (2015)
• Short warm cloud residence time in CO cases > limited loss of water before
reaching mixed-phase > lots of SCLW > positive graupel charging
• Shorter times would likely result in smaller cloud drops, acts in same direction
• Using 95th percentile of updraft speed, CO time is about 3 minutes!, AL is about 12
minutes, if use 90th percentile, factor 4 remains constant
• Would likely talk to aerosol impacts
Summary of statistics
• Colorado anomalous cases had stronger, broader
updrafts than Alabama cases
• Similar instability and adibatic water
• Much higher CBH/shallower WCD
• Led to a much shorter warm cloud residence time
in CO cases, increased ability to transport cloud
water to mixed-phase region
• Ice mass correlations provide more direct
evidence of positive graupel charging, which is
likely due to large supercooled water content
• Raises some questions…
Questions/future work
• Why are Colorado updrafts much stronger
even though they live in similar
environments?
• Local generation of charge vs. advection?
• Vertical W gradients/LMA sources in AL
• Look into turbulence measures? Flash size
• What about normal polarity storms in
Colorado?
Lightning flash
characteristics
Lightning response to charge structure
• Don’t only study charge structures to learn about storm processes
• Lightning obviously responds to charge structure
• With multiple years of LMA data in a few regions, let’s create a climatology of flashes
• Colorado, Washington DC, and Alabama
• See what we see!
DC flash characteristics12 million flashes analyzed from 2007-2014
FLASH
DENSITY*
IC/CG RATIO MEDIAN FLASH AREA
Azimuthally integrated
Decreasing detection
Fuchs et al. (2016)
IC/CG RATIO* MEDIAN FLASH
AREA
AL flash characteristics40 million flashes analyzed from 2008-2014
Azimuthally integrated
FLASH
DENSITY*
Decreasing detection
Fuchs et al. (2016)
Alabama and DC summary• AL flash densities ~ 30-40 flash km-2 yr-1
– ~ 4-5 IC:CG
– Inside and near network
• DC flash densities ~ 18 flash km-2 yr-1
– 3-4 IC:CG
– Inside and near network
• Both within 50% of the satellite estimates from Cecil et al. (2014)
• Different detection and data collection methods
• Satellite estimate in Colorado around 20-25 flash km-2 yr-1…
Cecil et al. (2014); Boccippio et al. (2001)
??
Colorado flash characteristics10 million flashes analyzed from 2012 and 2014
Azimuthally integrated
FLASH
DENSITY*
IC/CG RATIO* MEDIAN FLASH
AREA
Decreasing detection
Fuchs et al. (2016)
How is Colorado lightning different?
Colorado Alabama
• Much larger IC:CG values (up to 25!)
• Similar calculated flash areas despite much higher points/flash
– Colorado is most sensitive network to date
– Physical flashes may actually be smaller than in AL/DC
• Flash height
• But this is only initiation, satellites can detect more than that!
Fuchs et al. (2016)
Lightning flash channels
Flash extent density/Flash channels
• Important quantity– What GLM will observe
– Spatial extent and location of the flash
– Amount of N2 and O2 exposed to channel heating
• Gives more information about the flash than just a single fractal area
• There are issues with this– Grid box size sensitivity
– 2D versus 3D
– Flashes with few points
– LMA detection
~ 1
0 k
m
~ 10 km
Flash channel calculation
• Impose a grid over
each flash identified
by the algorithm
• Any grid box/cube
that contains an LMA
source is considered
to have contained a
segment of the flash
• Binary = 1 or 0
• Add them up for all
flashes in a
volume/cell
Fuchs and Rutledge (2017)
Fuchs and Rutledge (2017)
Regional flash channel distributions• Most Alabama/DC channels
around 8-10 km
• Colorado much lower in the
storm, similar to Fuchs et al.
(2016)
• Flashes that start low in CO
tend to stay low
• Much larger values in
Colorado than other regions
• Flash channels at higher
reflectivities in Colorado
• Perhaps tougher for
satellites to detect
• Hopefully be able to test this
during GLM Cal/Val
• Could impact downstream
NOx and ozone production
Fuchs and Rutledge (2017)
Flash channels and charge structure
• Lowest median channel
heights in CO
• Lightning follows strong
positive charge (LMA mode)
• Lower altitude flashes
coincident with warm
positive charge
• Colorado has the largest
fraction of anomalous storms
– They also produce a lot
of lightning
• Implications for scattering of
photons??
Why is Colorado different?
• High cloud bases, shallow WCD
• Strong updrafts, high SCLW contents
– Positive charge to graupel/hail
• More turbulence, smaller flashes?
• More low-altitude IC flashes
– More difficult for optical detectors?
Temperature (°C)
0
-10
-20
-30
-40
Alabama storms Colorado storms
Fuchs et al. (2015, 2016, 2017)
WHERE ARE SIMILAR THERMODYNAMIC
ENVIRONMENTS TO COLORADO??
• May have implications on our understand of
global lightning distributions?