The SDSMT armored T-28 conducted 8 research flights during our 4-week deployment to Norman, OK. Six of these missions included storm penetrations and two were conducted to test and calibrate instrumentation. They were conducted in collaboration with the ongoing Joint Polarization Experiment (JPOLE) and the Thunderstorm Electrification and Lightning Experiment (TELEX). Our observations are a small part of the total volume of operations collected during this deployment, including polarimateric (KOUN) and standard (KTLX) Doppler radar observations, mobile electrical balloon soundings, National Lightning Detection Network (NLDN) lightning ground strike locations, and Oklahoma lightning mapping array (LMA) 3-dimensional lightning channel mapping. The number of missions was less than expected. Near-normal precipitation occurred in the region during this time interval, but the large-scale circulation pattern favored development of MCS's in the western part of Oklahoma during the afternoon which did not reach central Oklahoma until after dark. T-28 storm missions are conducted only during daylight hours and it was not possible to sample these nocturnal storms.
Missions were conducted on the following days (all times UTC):
16 May 17:56-19:20
19 May 23:40-01:00 (approx)
22 May 23:25-00:45 (clear air instrument tests)
23 May 15:32-16:43
1 June 20:01-21:06
2 June 24:10-00:56 (clear air instrument tests)
4 June 16:06-17:20
10 June 22:37-23:25
Our primary scientific objectives were to monitor electric fields, nitric oxide (NO), and hydrometeors in electrified storms. We also carried an X-ray detector developed at New Mexico Institute of Mining and Technology by Ken Eack. Instrumentation worked well, except for our optical probe for monitoring hydrometeors with sizes between tenths of mm and tens of mm (the SPEC HVPS). Although the HVPS data will be qualitatively useful, quantitative analysis of the data will be difficult.
The TEI 42C-TL NO/NOx analyzer, which is not designed for aircraft work, worked well in the T-28 once we developed a scheme to feed pure oxygen into the ozone generator in the instrument. In the instrument ozone reacts with NO in the air to produce excited NO2 molecules. Decay of these excited molecules results in emission of light which is detected by a photomultiplier. The number of photons detected is proportional to the NO concentration in the air. If insufficient ozone is generated, the instrument works improperly. With only the oxygen naturally present in ambient air available to the ozone generator, the instrument works only up to 16,000 ft. MSL. With pure oxygen feeding the ozone generator, the instrument worked well to 21,000 ft MSL, the highest altitude at which we tested it.
Tests showed that the TEI instrument combined with our sample inlet system has a quasi-first-order time response, with a time constant of 3 to 4 seconds. Relationships have been developed to estimate true NO concentration when the sample is ingested into the instrument for only one or a few seconds as is the case when the aircraft passes through a narrow region of NO recently generated by a lightning discharge. A number of instances of narrow peaks in NO concentration were observed that we attribute to cylindrical plumes of NO created by recent lightning discharges that passed through the sampled region. Peak values were typically from a few to several ppbv. In one instance lightning attached to the propeller. NO concentration rose to ~180 ppbv in one second, then decayed. Depending on the actual time during which NO generated during this discharge entered the inlet (probably less than one second), and subject to a more refined calibration of the raw instrument reading, we estimate the peak concentration exceeded 800 ppbv as the aircraft penetrated through the lightning channel region..
A variety of hydrometeor types were observed, including hail on several occasions. All research flights were conducted while KOUN, the WSR-88D radar modified for polarimetric capabilities, was scanning the region in which the aircraft was sampling although these observations were only available post-flight. Real-time access to KTLX WSR-88D operational radar observations facilitated flight operations and data from both radars will facilitate analysis of storms observed during these operations.
Although the aircraft was struck by lightning during one flight, and several flights were through thunderstorms with active lightning occurring, there were no detectable X-ray events during any of these missions.
We are grateful to our hosts and collaborators at the NSSL and OU/CIMMS for making our operations possible. These include Dave Rust, Don MacGorman, Terry Schuur, Dave Priegnitz, and the NSSL facilities, phone and IT crews. Further thanks go to Prof. Patrick McCann of the Dept. of Electrical Engineering at OU, and his associates, for help in verifying the concentration in our calibration mixture of NO, and for assisting in other aspects of calibration activities with the TEI 42C-TL.
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