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The ER-2 Lightning Instrument Package (LIP), as it has flown since 1991, consists of two electric field mills and a Gerdien conductivity probe. The field mills are installed on the top and bottom of the aircraft just aft of the pilot's cockpit along the centerline of the plane (The bottom one is mounted on the aft section of the Advanced Microwave Precipitation Radiometer (AMPR) faring or on the lower E-bay hatch cover). The ER-2 electric field mills and the conductivity probe are compact sensors, each weighing less than 10 lbs. In this configuration, the field mills measure the vertical component of the electric field (Ez) over a wide dynamic range extending from fair weather electric fields (i.e., a few to tens of V/m) to large thunderstorm fields (i.e., tens of kV/m). The field mills also provide a measurement of the electric charge (Q) on the aircraft. Furthermore, we have recently established that the horizontal component of the electric field aligned with the direction of flight (Ex) can be derived from Q. This result arises from the particular charging characteristics of the ER-2 aircraft. Total lightning (i.e., cloud-to-ground, intracloud) is identified from the abrupt electric field changes in the data. Often it is possible to differentiate between intracloud and cloud-to-ground discharges.
The conductivity probe is installed on the right hand superpod nose cone. The conductivity probe measures the air conductivity at the aircraft flight altitude. The probe consists of a pair of Gerdien capacitor type sensors so that the contributions to the total conductivity due to positive and negative ions are obtained simultaneously throughout each flight. Storm electric currents can be derived using the electric field and air conductivity measurements.
Two important instrumentation enhancements will be made to LIP in support of the Texas Florida Underflight (TEFLUN), the Convection and Moisture Experiment (CAMEX-3), and subsequent missions. First, in the TEFLUN time frame, six state-of- the-art electric field mills will be added to LIP and installed on the superpods (three mills per pod). The existing Q-bay field mills will also be upgraded at this time. This enhancement will allow the vector components of the electric field (i.e., Ex, Ey, Ez ) to be readily obtained, and thus, greatly improve our knowledge of the electrical structure of storms overflown, particularly when the ER-2 passes storms off-center or encounters complex storm (and, thus, charge and generator) geometries. The new field mills will incorporate self-calibration capabilities that will reduce the time required to obtain a full aircraft calibration. By obtaining the full electric field we can validate, and improve upon if necessary, the earlier derivation of Ex from Q. With the new field mills, the electric field signals will be digitized at each mill and transmitted in a digital data stream, reducing signal noise and simplifying aircraft integration.
The second LIP instrumentation enhancement, implemented in the CAMEX-3 time frame, will be to install photometers (optical pulse sensors) and an electric field change sensor developed and flown on the National Center for Atmospheric Research (NCAR) high altitude WB-57 aircraft during Stratosphere Troposphere Experiment - Radiation, Atmosphere, Ozone (STERAO experiment in 1996. Earlier versions were flown in the 1980s on NASA's U-2 aircraft to provide baseline data for the design of the OTD and LIS (Christian and Goodman, 1987; Goodman et al., 1988; Christian et al., 1989; Christian et al., 1992). These sensors will measure the optical and electrical transient signals associated with lightning. These instruments can resolve individual strokes within a lightning discharge (i.e., 100 kHz sampling rate) and provide better identification of discharge type than possible from field mills. During the TRMM underflight missions, these sensors will be used to validate LIS performance parameters (i.e., radiometric validation, lightning detection efficiency, false alarm rate, and geo-location accuracy).
Figure 8 is a schematic diagram of the LIP instrumentation planned for TEFLUN and the subsequent field campaigns. The data products produced by ER-2 LIP system will include:
Christian, H. J., and S. J. Goodman, Optical Observations of Lightning From a High Altitude Airplane, J. Atmos. Oceanic Tech., 4, 701-711, 1987.
Christian, H. J., R. J. Blakeslee, and S. J. Goodman, The detection of lightning from geostationary orbit, J. Geophys. Res., 94 (D11), 13329-13337, 1989.
Christian, H.J., R.J. Blakeslee, and S.J. Goodman, Lightning Imaging Sensor (LIS) for the Earth observing system, NASA TM-4350, Available from Center for Aerospace Information, P.O. Box 8757, Baltimore Washington International Airport, Baltimore, MD 21240, 44 pp, 1992.
Goodman, S. J., H. J. Christian, and W. D. Rust, Optical pulse characteristics of intra- cloud and cloud-to-ground lightning observed from above clouds, J. Appl. Meteor., 27, 1369-1381, 1988.
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