Light detection and ranging (lider), also called laser radar, is an active optical observation system in which a pulsed laser beam is emitted towards the sky above and collects laser beams backscattered by aerosol and other particles with a telescope, and then detects them with a photomultiplier tube. The lider observation system and an image of lider in operation are shown in Figures 22.214.171.124 and 126.96.36.199, respectively.
The principle specifications of the aerosol lider system at Ryori are as follows.
|Output wavelength||532 [nm]|
|Output energy||300 [mJ] / pulse|
|Pulse width||3.5 [nsec]|
|Pulse frequency||10 [Hz]|
|Beam divergence||0.12 [mrad]|
|Model||Continuum, Inc., Surelite I I-10|
|Aperture||28 [cm]||35.5 [cm]|
|Model||Celestron, LLC., SC-280L||Celestron, LLC., SC-355L|
|Detector||Hamamatsu Photonics K.K.
|Data collection||Analogue detection||Analogue detection /
|Signal processor||Licel GmbH, TR20-160|
The lider observation obtains the backscattering coefficient from the intensity of incoming scattering light and the aerosol extinction coefficient indicating the rate of light attenuation due to scattering and absorption by aerosol, and provides information such as the scattering ratio and depolarization ratio, which represent the concentration and asphericity of aerosol respectively.
The scattering ratio is obtained using the following equation by Fernald (1984):
The extinction coefficient is obtained from the detection signals parallel and perpendicular to the axis of polarization of the emitted laser, denoted as and respectively, using the equations:
Lider observation at Ryori is carried out 4 times daily at the times listed below.
|1st observation||2nd observation||3rd observation||4th observation|
|02h00m -- 04h00m||08h00m -- 09h00m||14h00m -- 15h00m||20h00m -- 22h00m|
The minute-basis values of output signals from the observation system undergo quality control by the observation staff, who remove the values which were affected by low level cloud and prepare 10-minute- and hourly- accumulated values. The accumulated values are sent to the JMA headquarters, where the vertical distribution of scattering and depolarization ratios in the low level is to be calculated using 10-minute accumulated values. Further, for observations with no upper or middle level clouds, the vertical distribution of scattering and depolarization ratios in the whole level is calculated for each session of observation described above based on the accumulated values.
Japan Meteorological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan
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