7.2.12 Aerosol vertical distribution observation (dispersion and depolarization ratios)

   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 7.2.12.1 and 7.2.12.2, respectively.

   The principle specifications of the aerosol lider system at Ryori are as follows.

   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):

      (1)       (2) where, is the scattering ratio, is the received signal corrected with the square of the distance, is the backscattering coefficient, is the ratio of extinction coefficient to scattering coefficient, and the index notations aer and mol indicate that the parameters are related to aerosol and atmospheric molecules, respectively. Radiosonde observations at Akita, which is latitudinally the closest aerological observation station, are used to calculate the amount of scattering caused by atmospheric molecules (Rayleigh scattering). The matching height , in which only scattering by atmospheric molecules is taken into consideration in the assumption of no aerosol present, is 25km for nocturnal observations primarily entailing observation of aerosol in the stratosphere and the tropopause height obtained from the radiosonde observation for diurnal observations primarily entailing observation of aerosol in the troposphere. The value 35 is used for for both the stratosphere and troposphere.

   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:

      (3)       (4) where, is the reception sensitivity ratio of each polarized component, and is the depolarization ratio corrected using the sensitivity ratio . The value is a constant used to normalize the depolarization ratio, based on the hypothetical polarization ratio (assumed to be 0.45[%]) for atmospheric molecules in the stratosphere. Additionally, this constant is calculated from the theoretical value and the sensitivity-corrected depolarization ratio for the observations in the stratosphere using the following equation, where aerosol is assumed to be non-existing:       (5) where, and are the upper and lower limits, respectively, of the altitudes to be used in calculating the normalization constant .

   Lider observation at Ryori is carried out 4 times daily at the times listed below.

   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.