Geoscience Reference
In-Depth Information
of electromagnetic waves, and its measurement resolution may be insucient.
Therefore, a highly maneuverable measurement method, which can follow
the local weather change, is desired. A lidar using the Stark effect has been
proposed,
1
although it has not yet achieved its ultimate goal.
As a tool for prediction of heavy rain and lightning strikes and pre-
vention of local disasters resulting from such phenomena, an in-line type
micropulse lidar (MPL) system has been developed.
2
−
4
This system can dis-
tinguish ice crystals from water particles, and the flow of ice crystals can be
used to derive meteorological parameters linked to heavy rain and lightning
strikes. The MPL system uses in-line optics, which allows a constant overlap
of the transmitting and receiving field-of-views (FOVs), and can receive the
lidar return signal with no obscuration distance (i.e. the return signal can
be obtained from the immediate front of the system) by using an outgoing
annular beam. Furthermore, an optical circulator is used to separate the
transmitted beam and the return signal. The lidar return signal of mutually
perpendicular polarizations (parallel and perpendicular to the polarization
of the outgoing beam) can be separately detected, which allows precise
measurement of the depolarization ratio. The FOV of 0.1 mrad is suciently
narrow to eliminate the depolarization effect caused by multiple scattering,
so the lidar system can distinguish ice crystals from depolarization. The
MPL system aims to derive the metrological parameters connected to heavy
rain and lightning strikes by monitoring the movement of ice crystals, and
is currently in year-round observation.
However, the movement of ice crystals is only indirectly associated
with local disasters such as heavy rain and lightning strikes. Furthermore,
the effects of seasonal change, regional difference, and variations in the
air current must be removed. If the metrological parameters connected to
lightning strikes could be obtained more directly, accurate prediction would
become possible.
The authors focused attention on the magneto-optical effect (Faraday
effect). In a partially ionized atmosphere, a propagating beam experiences
a rotation in the polarization plane due to the electromagnetic pulse
accompanying a lightning strike. In this study, the feasibility of optical
remote measurement of the change in the electromagnetic field or in the
ionized density distribution in a high-voltage discharge is examined. By
a laboratory-based experiment using an impulse voltage discharge, the
polarization extinction ratio and the receiver's dynamic range for the
detection of the polarization rotation angle are examined. The experimental
results are compared with the results of analytical calculations.
