Electrification Options

This document is divided into THREE sections:

Eyewall Option:

To execute this option, the aircraft will fly radial legs out and back at constant radar altitude upon a reciprocal track through the eyewall at successively higher altitudes starting at the stratiform area melting level (~16,000 ft [4.8 km]) until the maximum operational altitude is reached. An ODW should be dropped outside the eyewall on the highest altitude leg to obtain a vertical sounding. Each successive radial pass (out and back) shall be 1,500 ft (500 m) higher than the previous one. Climbs and descents should occur in clear areas outside the eyewall (2 in Fig. 23), and leg lengths shall be altered as necessary to achieve this.

This out and back pattern (1-2-1 in Fig. 23) should be repeated until the aircraft reaches its maximum attainable altitude. The Doppler radar should be operated in a 360° scan mode during the radial passes. Upon completion of the radial legs, an equilateral triangle Doppler pattern will be executed, starting from inside the eye. The starting azimuth (Fig. 23) will be 60° upstream from the upstream edge of the strongest radar reflectivity feature in the eyewall or innermost convection. The legs should be ~43 nmi (80 km) long, with the inbound leg connected to the outbound leg by a downwind leg. The inbound leg should penetrate the convection at the downstream edge of the strong reflectivity area previously identified. Each triangle will require 10-20 min to complete, depending upon the leg length.

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Rainband Option:

If a convective outer rainband is available >80 nmi (150 km) from the eye, it should first be surveyed for evidence of electric fields. The survey consists of flying along the band until the field mills register a space charge or the Doppler radar reveals the presence of vigorous convection. When an interesting area is located, the aircraft should either seek a clear area and climb to maximum altitude or descend to the 0°C (~16,000 ft [4.8 km]) altitude, whichever is closer, and start making passes downwind (Fig. 24) through the middle of the band the feature.

Each downwind pass (Fig. 24, 1-2) should maintain a track along the axis of the band and be about 50 nmi (93 km) long and 1,500 ft (500 m) higher (lower) than the previous one. During this portion of the pattern, the Doppler radar should make 360? scans normal to the aircraft track. After the downwind pass is completed, the aircraft should exit the band on the outer side, climb (descend), and return (Fig. 24, 3-4) upwind to the start of the band. The Doppler data will be obtained on the upwind pass using the F/AST method. This pattern will require about 20 min to execute. Pass length may be altered as circumstances dictate. Repeat this pattern until the maximum altitude is reached, or seek a new area as desired. As an alternate, a zig-zag path downwind through the convective band may be flown if necessary for flight safety.

(NOTE: If the feature of interest is not translating, radial legs should be flown on a constant track instead of a constant heading. The length of the radial legs depends upon the diameter of the eye and the width of the rainband, respectively. Turns should be initiated into the wind.)

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Landfalling Storm Option:

The purpose of this option is to investigate the relationship between cloud physics, vertical velocity, and the occurrence and location of CG lightning. Outer convective rainbands are of primary interest since they are the most likely features to be electrified. Vertically pointing Doppler rays are used to estimate vertical air motions during passes through active convection in both tropical storms and hurricanes. Along with the vertical velocities, coincident microphysics and electric field measurements are made at heights above the melting level. Three-dimensional wind fields of the convective areas can be constructed from a Pseudo-dual Doppler technique and from the F/AST Doppler data. CG lightning data are available within 325 nmi (600 km) range of the NLDN (Fig. 26).

Together, these data sources and techniques should lead to a better understanding of the characteristics of the convective processes that lead to lightning in hurricanes and, possibly, to intensity changes of the storms.

For this option, the aircraft will initially fly a survey figure-4 pattern (Fig 25a) at ~18,000 ft (5.5 km) altitude.

The figure-4 pattern would be completed in 1.5-2.0 h with radial legs 80 nmi (150 km) in length. The second part of this option (Fig. 25b) concentrates on rainbands that are located within the useful range of the NLDN.

Upon exiting the eye at 4, the aircraft should climb as high as possible on the way to the rainband of interest (5). A sawtooth pattern is flown downwind (Doppler operating in standard mode) with repeated crossings of the rainband to 6. We prefer to fly directly down the band as noted in Fig. 24, but for reasons of safety, a sawtooth pattern may be flown. An upwind leg, flown outside of the band, is performed with the tail radar operating in the F/AST mode. The sawtooth pattern across the band is repeated with an exit toward the eye at 7. After entering the eye, the aircraft turns toward the second rainband at 8. The sawtooth crossings and the F/AST downwind leg are repeated as in the first rainband. A final center fix is made (time permitting) before returning to base from 10. About one hour should be spent in each of the rainbands. If only one rainband is present within the useful range of the NLDN, a second study of the same band can be performed after a circuit through the storm center.

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