PWS-Cu_Elec_Study-Final.pdf

Performance Work Statement For

Cumulus Electrification Study (Feb 2017)

Title: Cumulus Electrification Study

Required Work: Analyze locally developing cumulus clouds at Cape Canaveral Air Force Station (CCAFS) and Kennedy Space Center (KSC) to improve the Cumulus Rule in the Lightning Launch Commit Criteria (LLCC). The text of the Cumulus Rule is provided below. At least 150 cumulus clouds must be analyzed during one or more convective seasons (May-Sep) and excluding tropical cyclones. Locally developing cumulus clouds at CCAFS/KSC are most likely when there is an easterly component to the lightning flow regime (1000-700 mb), when the subtropical ridge is south of CCAFS/KSSC. Isolated cumulus clouds would be the ones most easy to analyze. Use the Weather Surveillance Radar (WSR) and the surface electric field mills in the Launch Pad Lightning Warning System (LPLWS) used by 45th Weather Squadron (45 WS) to do the following:

1) Determine how surface electric fields decrease with distance outside of cumulus clouds. One possible solution is a best-fit curve of the maximum surface electric field under or around the cloud based on the radial distance outside the cloud. One might expect the electric field to fall off as 1/r2 for shallow electrified cumulus clouds with cloud tops between about 0 °C and -10 °C. However, the electric field might begin to fall off as 1/r for deeper electrified cumulus clouds with cloud tops between about -10 °C and -20 °C. Therefore, a blended model of best-fit curves might be needed transitioning form one radial behavior to another based on the slant-path distance from the center or edge of the cumulus cloud.

- The isopleths of surface electric fields of fairly isolated cumulus clouds tend to be quasi-elliptical in shape. Analyzing the longer axis of those ellipses, where the fields decrease less rapidly with distance, will provide a conservatively safe estimate for the Cumulus Rule stand-off distances. Analyzing both the long and short axes will provide a representative estimate of the stand-off distances, which would indicate how much safety is bought by using the long axis distances alone.

2) Determine the distribution of maximum surface electric field under or around a cumulus cloud vs.

cloud top temperature, time rate of change of cloud top temperature, and both cloud top temperature and rate of change of cloud top temperature. One solution would be a regression of both predictor variables separately and a bivariate regression of both predictors together. Percentile regression for the maximum surface electric field versus these three groups of predictors may be a way to build conservative safety into the technique, e.g. 99% of maximum electric fields are below the regression equation using the three predictors.

- The cloud top temperature and time rate of change of cloud top temperature may both be factors in cumulus electrification. However, the time rate of change of cloud top temperature may not be available operationally to 45 WS, so the utility of just cloud top temperature in addition to the time rate of change needs to be determined.

- The onset of electrification may not occur until some temperature 0 °C

3) The number and type of hydrometeors in the cumulus cloud at the appropriate temperature level may be involved in the cloud electrification. Therefore, the volumetric radar reflectivity above the electrification cloud top temperature (0 °C or colder) should be explored. This may be a threshold that is required before electrification occurs and/or another predictor for the magnitude of maximum surface electric field.

4) Determine the distribution of times for cumulus clouds to grow between various temperature levels, e.g. +5°C to 0°C, -5°C, -10°C; 0°C to -5°C, -10°C; and -5°C to -10°C.

5) Recommend improvements to the Cumulus Rule of the Lightning Launch Commit Criteria based on combination of the results in 1) and 2) that can safely relax the cumulus cloud top temperature and stand-off distances. This may be new stand-off thresholds for the current cumulus cloud top temperatures of -5 °C, -10 °C and -20 °C.

6) Recommend improvements to the Cumulus Rule based on 3) that can safely relax the cumulus cloud top temperature thresholds for rapid cumulus growth. This may be a new colder temperature threshold than the current +5 °C threshold.

7) Provide the data in these studies in case post-analysis is desired.

Current Cumulus Rule in LLCC: This section applies to non-transparent cumulus clouds, except for cirrocumulus, altocumulus, or stratocumulus clouds. This section does not apply to an anvil cloud that is attached to a parent cumulus cloud.

1) A launch operator may not launch if the slant distance to the flight path is greater than 5 and less than or equal to 10 nautical miles from any cumulus cloud that has a top at an altitude where the temperature is colder than or equal to -20 degrees Celsius.

2) A launch operator may not launch if the slant distance to the flight path is greater than zero and less than or equal to 5 nautical miles from any cumulus cloud that has a top at an altitude where the temperature is colder than or equal to -10 degrees Celsius.

3) A launch operator may not launch if the flight path will carry the launch vehicle through any cumulus cloud with its top at an altitude where the temperature is colder than or equal to -5 degrees Celsius.

4) A launch operator may not launch if the flight path will carry the launch vehicle through any cumulus cloud that has its top at an altitude where the temperature is colder than or equal to +5, and warmer than -5 degrees Celsius unless:

a) The cloud is not producing precipitation;

b) The horizontal distance from the center of the cloud top to at least one working field mill is less than 2 nautical miles;

And

c) All electric field measurements at a horizontal distance of less than or equal to 5 nautical miles from the flight path, and at each field mill specified in paragraph d. (2) of this section, have been between -100 volts/meter and +500 volts/meter for at least 15 minutes.

Deliverables:

1) Final Report. Summarize the data used, quality control applied, methodology used, results, and recommendations for 45 WS applications. Other items of significance may be included.

2) Final Briefing. Same content as 'final report' but in briefing format. The final briefing will be scheduled in coordination with 45 WS.

3) Interim Progress Reports: Semi-annual progress reports will be provided to 45 WS. Content shall include progress to date, problems overcome, and especially any unresolved problems.

Performance Period: Initiation Date: 31 March 2017 Interim Milestones: Progress reports every 3 months, or more often as needed Completion Date: 30 March 2018

Points Of Contact:

Financial Contact: Name: Mr. William P. Roeder Organization: 45th Weather Squadron Position: Resource Advisor Phone: (321) 853-8410 E-mail: wiliam.roeder@us.af.mil

Submitter: Name: Mr. William Roeder Organization: 45th Weather Squadron Position: Resource Advisor Phone: (321) 853-8410 E-mail: william.roeder@us.af.mil

Technical Contact: Name: Mr. William Roeder Organization: 45th Weather Squadron Position: Resource Advisor Phone: (321) 853-8410 E-mail: william.roeder@us.af.mil

Alternate Technical Contact: Name: Mr. Todd McNamara Organization: 45th Weather Squadron Position: Meteorologist Phone: (321) 853-8663 E-mail: todd.mcnamara@us.af.mil