Joint ABFM Science Team/Lightning Advisory Panel Meeting

12 - 14 November 2002, Melbourne, FL

Summary Revised 10 February 2003

Introduction: On 12 - 14 November 2002, The ABFM Science Team and the Lightning Advisory Panel met at the Melbourne, Florida offices of ENSCO, Inc. to review the progress to date in data reduction and analysis and to discuss its applicability to modification of the Lightning Launch Commit Criteria. This meeting summary was compiled by Frank Merceret from materials submitted by the meeting participants, and especially by Dr. Jim Dye and Dr. John Willett.

Jim Dye's Meeting Summary (edited): After introductory remarks the workshop began with the PI, Jim Dye, showing a summary of the cases investigated during the field campaigns of June 2000, and May/June 2001. During these campaigns combined measurements of in-situ electric fields, particle concentrations and sizes, winds and state parameters closely coordinated with radar measurements from the WSR74C radar at Patrick Air Force Base and the 88D NEXRAD radar at Melborne and lightning measurements from the KSC LDAR and CGLSS lightning detection systems were made in anvils, regions of debris or disturbed weather for approximately 44 different storms on 33 different flights. About 30 of the cases had attached or detached anvils, 12 were stratiform in character (LLCC rules would classify these as disturbed weather and for some cases as thick clouds), and several were debris of once active thunderstorms that decayed in place with no anvil. A list of the different cases and flight days is appended at the end of this summary. In addition to the summer campaigns, a February campaign was conducted, that unfortunately was in the middle of the severe drought in Florida. The drought limited investigations within range of the radar to the one thick cloud case, Feb. 3, 2001. A NOAA ETL Technical Report was prepared for this case. Three other flights were flown in February, but out of range of the 74C radar.

This was followed by a brief overview of the different analysis tasks that have been pursued, most of which are ongoing:

* Examination of the different flights to categorize cases and identify general trends of Electric field in relationship to radar reflectivity and microphysics.

* Identification of some cases/periods for theoretically calculating decay times with a simplified model based on observed ice particle size distributions.

* For these selected cases/times, comparison of calculated decay times from the simple model with observed decay times.

* Surveying particle concentrations and sizes in regions with strong Electric fields to establish microphysical variability for decay time calculations.

* Development and exploration of radar parameters that might be useful as indicators of regions with strong Electric fields.

* Production of scatter plots for all flight days of the magnitude of the Electric field as a function of the 10 km box average (a derived parameter which is an average of dBZ values 10 km N, S, W, and E of the aircraft location from the freezing level to the highest level in the storm. This parameter seems promising as a possible indicator of strong Electric fields).

* Determination of correlation distances of Electric field magnitude, reflectivity at the aircraft position, and particle concentration. The correlation distance determines the effective sample size which is smaller than the number of measurements for correlated data since the measurements are not independent.

* Plotting calculated electric field decay times against reflectivity to see if reflectivity might be a proxy for possible decay times.

For purposes of discussion, the PI suggested a strawman candidate as a possible LLCC rule for anvils:

Use the 10 km box average radar reflectivity of <5 dBZ and other yet to be determined parameters such as electric fields from the surface network as a limiting value for low ambient electric fields. When the 10 km box average at the launch site and nearby area (to be quantified) is >5 dBZ, conditions are not assured to be safe.

Several members of the ABFM science team then presented cases to demonstrate and discuss different types of storms and anvils in which measurements were made. The cases discussed are shown in the workshop agenda that is included at the end of this summary. Graphical displays of many of the measurements presented at the workshop and analysis tools that have been developed can be viewed at the NCAR ABFM Web site. If you do not currently have access to this restricted website and wish to be granted permission, please contact the Principal Investigator.

FINDINGS TO DATE

The discussion of these cases was followed by talks summarizing the current findings including:

* We have gathered an excellent, unique data set with both electrical and microphysical measurements which hitherto was unavailable and with which possible new LLCC rules can be examined.

* Strong electric fields are associated with regions of higher reflectivity (~10 dBZ) above the freezing level but higher reflectivities do not necessarily indicate regions of strong electric field.

* When strong electric fields were measured, the particle concentrations in all size ranges from tens of microns to several millimeters were high, but higher particle concentrations do not necessarily indicate regions of strong electric field.

* In regions with strong electric fields even for different case types, there was a surprising degree of consistency of observed particle concentrations in all size ranges.

* The smaller ice particles in the anvils (<50 microns) are primarily spherical, while particles >100 microns are highly irregular showing at different times evidence of riming, diffusional growth and aggregation.

* For 22 cases examined to date, there was no evidence of supercooled liquid water being present in the anvils. This suggests that active charge separation and electrification is most likely not occurring in these anvils.

* The calculated electric field decay times in the anvils and ice cloud debris are primarily controlled by the particle size distribution, particularly in the size range 0.2 to a few millimeters.

* The optical extinction coefficient (as well as electrical decay time) is weighted toward mid-sized particles 200 microns and larger.

* Using the observed particle size distributions, calculated electric field decay times ranged from almost 3 hours (near the core of active storms) to only several minutes (toward the edge of anvils).

* Comparisons so far suggest that the calculated decay times are longer than the observed decay of electric field. Because of decay of particle concentrations and sizes with time, the model times are most likely upper limits.

* Reflectivity at the aircraft location or in the column of the aircraft is not a suitable parameter for comparing to electric field strengths, because of scan gaps in the radar sweeps of both the 74C and NEXRAD radars and possible refraction of the radar beam.

* A 10 km box average was examined for entire flights and showed that for most cases there was a well-behaved pattern. For the vast majority of data points, when the 10km box average was <5 dBZ, electric field strength was <3 kV/m. But there were outliers and these need further investigation. It is possible that the low dBZ readings in these cases may be due to attenuation of the radar by precipitation.

FURTHER WORK TO BE ADDRESSED

The following issues were discussed by the ABFM science team in their breakout session:

1. Determine which electric field solution (the M or K approach) should be used at what times.

2. Prepare a “filtered” data set that excludes time periods and cases that are within 10 nautical mi distance and 5 to 10 min time of any lightning or active convection.

3. Complete the analysis and writeup of cases not yet analyzed and provide summaries for those cases previously presented at conference calls.

4. Provide a very brief discussion of the meteorological context for each case.

5. Produce updated files of the aircraft measured winds.

6. Determine and add uncertainty bars to a generic particle size distribution to give an idea of uncertainties in different size ranges.

7. Try plotting the electric field magnitude on a log scale in the MER vertical section plots.

8. Examine all cases for any evidence of supercooled liquid water in anvils.

Recommendations of the LAP breakout session are presented next. Based on apparent agreement between the science team breakout session report and the LAP recommendations, it was decided that the scheduled Friday prioritization session was unnecessary. The meeting adjourned Thursday after everyone agreed to submit action items to Frank Merceret for inclusion in this meeting report. Drs Dye and Willett agreed to submit the summaries presented under their names here.

John Willett's Summary of LAP Recommendations (edited):

1) The ABFM Analysis Team (AAT) urgently needs a final conclusion from the MSFC group about which analysis algorithm, the Mach (M) algorithm, the Koshak (K) algorithm, or a combination of these algorithms (under what conditions), should be used to provide the best estimate of the ambient electric (E) field in clouds that were sampled during the ABFM campaign.

2) The AAT should "sanitize" the cloud E-field/radar-parameter dataset so as to minimize

the effects of the following:

a) Nearby lightning as a non-local source of the field -- use a 10 nm (18 km) standoff distance (the same as used in LCC Rule 1) and a standoff time TBD by the AAT;

b) Nearby cores of active thunderstorms that represent non-local sources of electric field – using altitude/radar thresholds and standoff distances TBD by Analysis Team;

c) Radar scan gaps, to the extent that they significantly influence the various radar parameters; and

d) Wet-radome and/or precipitation-attenuation effects, to the extent that

they significantly influence the various radar parameters.

3) Further explore the cloud reflectivity parameters (e.g., peak dBZ, average Z, average dBZ, box size) so as to optimize the potential relationships to ambient E-field in the "sanitized" dataset. Focus the analysis on thunderstorm anvils, thick clouds, and disturbed weather.

4) Further "sanitize" the cloud E-field/decay-time dataset so as to minimize, in addition to the effects listed in (2) above, any indications of a local electrical generator (i.e., radar indications of local development of the cloud, TBD by the AAT).

5) Pursue, to the greatest practical extent, testing of the E-field-decay model using the measured cloud microphysical parameters and observed field decays. For this, we also need better estimates of the ambient winds and the parcel trajectories in anvils.

6) Look for relationships between the predicted decay times and the associated radar reflectivity in the further "sanitized" dataset detailed in (4) above. Determine whether there are other types of radars (or ground-based sensors) that could serve as a proxy for the decay time of the cloud electric field.

7) Check ALL anvils for the presence of liquid water.

8) Archive all data obtained in the current ABFM campaign (ABFM II).

9) Retrieve and, to the extent possible, archive data obtained during the

previous ABFM campaign (ABFM I).

10) Integrate the ABFM I dataset with the ABFM II dataset in a coherent way. This may require re-calculating some of the radar parameters in ABFM II so that they match those previously calculated during ABFM I.

11) Re-examine in detail any specific cases that appear to be of concern in the ABFM I scatterplots.

After the meeting, these recommendations were circulated in draft written form for review among members of both the LAP and the AAT prior to inclusion here. Based on the ensuing email traffic, the editor of this report (Merceret) finds that there are some points of difference between the verbal understanding the group believed we had reached at the meeting and the wording in Dr. Willett's summary as interpreted and amplified in that correspondence. In particular, the LAP is still deliberating the wording of its recommendations 2C (dealing with radar scan gaps), 3 (exploration of various reflectivity parameters), 6 (seeking remote-sensing proxies for decay time) and 10 (merging ABFM I and ABFM II data sets). Rather than attempt to resolve these issues to produce a “clean” set of recommendations, the editor elected to present these recommendations as written and leave the resolution of open issues of interpretation to post-meeting discussions. These discussions may lead to some modification of the recommendations and related action items reported here.

ACTION ITEMS:

1. Merceret will edit and circulate a report of the meeting. Dye will submit a report of the science team and Willett will submit a report from the LAP for inclusion in the report. All members of the group will submit action items. (STATUS: This document)

2. Dye/Lewis will compile and post the meeting presentations on the NCAR website. All presenters will forward e-copies of their presentations to Dye.

3. MSFC will determine which E-field algorithm (M, K or combination) to use under various circumstances. (STATUS: complete)

4. Grainger will provide improved aircraft winds. (STATUS: winds recalculated, being verified)

5. Dye/Hall will provide particle spectra files for additional cases to Willett.

6. Grainger will provide Merceret with UND “Find Cloud” automated cloud detection algorithm. (STATUS: complete)

7. Merceret will prepare a draft manuscript of the scientific results of the ABFM program to date (Due 30 April 2003). All science team members will assist by providing a summary of their results.

8. Merceret will contact the NWS regarding the aging properties and maintenance of the hydrophobic coating on the WSR-88D. (STATUS: complete)

9. Merceret will summarize the synoptic situation for each mission day. (STATUS: Jennifer Ward is compiling the meteorological data for each day. Summaries will be written as soon as the data are all available.)

10. Merceret will examine issues relating to minimizing scan gaps over the launch pads for different situations or rules.

11. Willett will substantiate preliminary conclusion that computed field decay time depends primarily on particles in the size range 0.2 to a few mm.

12. Willett will study the relationship between computed decay time and radar return.

13. Willett and Grainger will collaborate on using the “drifty” program to extrapolate anvil parcel positions with time.

14. Willett will examine the following in more detail: 000628 (debris cloud decay); 010604 (lightning, graupel extending into anvil, small anvil); 010615 (decay time); 010624 (NEXRAD in lieu of 74C, decay times).

15. Willett and Dye will collaborate on radar box sizes for comparison to computed decay times.

16. Willett and Dye will collaborate on identifying additional cases for comparison of model and observed field decay.

17. Willett and Dye will collaborate on how to compare distributions of decay times of - model and observed fields.

Appendices

I. FLIGHT DAYS WITH BRIEF CATEGORIZATION OF STORM TYPES

Prepared by Jim Dye and Sharon Lewis, Nov. 7, 2002

June 2000

000604 complex storm with anvil hi E throughout

000607 limited anvil study, cabin press loss hi E throughout

000611 edge of decaying anvil wk E

000612 Decay of small storm with no anvil hi to wk E

000613 classic anvil decay study hi to wk E

000614

1 decayed anvil debris wk E

2 decay of E in anvil hi to wk E

000617 decayed mid-lvl maritime debris(thick cld?) wk E ???

000620 Isolated, tall cells w. no anvil mod to wk E

000623-1 many cells - no anvil; later stratiform hi to wk E

000623-2 stratiform - return to PAFB, no microphys hi E

000624-1 many cells grow to stratiform complex hi E throughout

000624-2 cont'd study of stratiform complex hi E throughout

000625 low-alt. below anvil, cabn pres loss mod. wk E

000628-1

1 decayed anvil from small cells mod. wk E

2 decayed anvil debris wk E

000628-2

1 decaying debris of anvil wk E

2 decay of small anvil hi to wk E

3 decay attach. anvil with active core hi to mod E

May/June 2001

010522 stratiform with embedded convection hi E

010525

1 attached anvil decaying to debris weak E

2 attached anvil to active convection hi Ei & ltng

010527

1 weak decaying anvil weak E

2 active convection hi E

010528 attached anvil and following decay hi E/ ltng at start

010529 small anvil attchd to persistent core hi to mod E & ltng

010602 attached anvil then stratiform hi E and ltng

010604

1 small attached anvil mod to wk E

2 long, long-lived anvil decay hi to wk E / cont. ltng

010605

1 maritime debris final decay mod to wk E

2 very active maritime, offshore storm hi E & ltng

3 end of debris decay wk E

4 axial pass, pseudo-anvil active storm hi to wk E

010606 decay of complex convection, no anvil hi to wk E

010607

1 passes in active region hi E

2 decaying convective debris wk E

010610 stratiform with some anvil at first hi to wk E

010615 decay of anvil-like outflow hi to wk E

010618 small convect line decay near strat. hi to wk E

010623 lg. convect line and stratiform hi to mod E

010624 anvil spatial decay of E; tornado case hi to wk E

010625 decaying anvil mod to wk E

010627 decaying anvils of complex system hi to mod

010628 thick cloud from lg. stratiform region wk E

II. INITIAL Agenda for ABFM Workshop

Prepared by Jim Dye 7 November 2002

TUESDAY MORNING, NOVEMBER 12

0830 - 0845 Workshop intro, logistics and goals - Merceret

0845 - 0915

OVERVIEW of ABFM 2000, 2001 -- Dye

* Brief discussion of the overall project and research flights for

June 2000, Feb 2001, June 2001

* Analysis approach

0915 - 1015

E FIELD SYSTEM on the Citation: Status of the calibration -- Mach

1015 - 1030 BREAK

1030 - 1045

Overview of MICROPHYSICAL products and any issues -- Dye/Willis

1045 - 1145

RADAR -- Merceret

* 74C -- scan strategy impacts on display and analysis;

attenuation --wet radome & intervening precip.

* NEXRAD -- scan strategy; effect of thresholding on analysis

1145 - 1200

REFLECTIVITY PARAMETERS -- possibilities considered -- Dye

1200 - 1315 LUNCH BREAK

1315 - 1335 SUPERCOOLED LIQUID WATER -- Grainger

Is there SLWC present in the anvils

1335 -1355 E FIELDS AT THE SURFACE COMPARED TO ALOFT -- Krider

1355 - 1415

MODEL FOR THEORETICALLY CALCULATING FIELD DECAY -- Willett

DISCUSSION OF SPECIFIC EXAMPLES

1. Anvil attached to active storm that decays

* June 14, 2000 Case 2

1415 - 1445 -- Case description --- Dye

1445 - 1500 -- Decay time: theory -- Willett

1515 - 1530 BREAK

* June 13, 2000

1530 - 1550 description; following to decay -- Dye

1550 - 1610 size distributions and images -- Willis

1610 - 1630 decay, comparing theory and observations -- Dye/Willett

1630 -1730

2. Detached, decaying weak anvils -- Dye

time from last lightning to E field at time of A/C arrival

* June 14, 2000

* June 25, 2001

* June 11, 2000

* June 17, 2000 (maritime case)

* June 5, 2001, case 1 (maritime case)

WEDNESDAY, NOVEMBER 13

0830 - 0915

3. Thick Clouds -- Bateman

* Feb. 3, 2001 with NOAA K band radar and Citation

* June 28, 2001

4. Severe Storms with attached anvils

* June 4, 2001 -- energetic storm with lightning far into anvil

0915 - 1000 Description of case -- Dye

1000 - 1015 -- Microphysical properties -- Willis

1000 - 1015 BREAK

1015 - 1045

Willett -- Model calculations for 4 June 2001 case

* June 24, 2001 (Tornadic storm, extreme example of spatial decay)

1045 - 1130 Description of case -- Merceret

1130 - 1145 Decay time calculations -- Willett

1145 - 1300 LUNCH BREAK

1300 - 1345

5. Debris clouds decaying in place -- Bateman

* May 25, 2001

* June 6, 2002

1345 - 1430

6. Stratiform

June 2, 2001 (anvil at start turning to stratiform, attenuation of 74C)

1430 - 1500 BREAK

SUMMARY FINDINGS

1500 - 1530

Particle Size Distrib. in anvils; control of decay times -- Dye

1530 - 1615

Comparison of observed decay times with model calculations -- Willett

1615 - 1700

Empirical findings E Field vs. reflectivity parameters -- Dye

1700 -1730

Lagged Correlation between E Field, Radar, and 1D Concent. -- Merceret

1730 -1800 Open Discussion

THURSDAY AM Discussion Leader -- Krider

0830 -- 0915

Overview of Results of ABFM 1990, 1991 -- Christian

0915 -- 1200 LAP Breakout session

Discuss possible LLCC Rules, guidance to ABFM TEAM, other business

0915 -- 1200 ABFM TEAM discussion (in parallel but without LAP members)

Discuss unresolved issues, cases of special interest and remaining cases

1200 - 1315 LUNCH BREAK

1315 - 1700 Discussion Leader -- Krider

Presentation of comments/suggestions/concerns from the LAP and

discussion of the issues

FRIDAY, November 15

PRIORITY SETTING Discussion Leader -- Merceret

0830 - 1200 Whole Group

Decide priorities for further analysis, and where we go from here

to come up with any LLCC revisions by May 1, 2003

1200 ADJOURN

III. List of Attendees

Bateman, Monte Madura, John

Bennardo, Phil (Tuesday AM only) Merceret, Frank

Christian, Hugh Roeder, Bill (Tuesday AM only)

Dye, Jim Rust, Dave

Grainger, Tony Walterscheid, Richard

Krider, Phil Willett, John

Mach, Doug Willis, Paul