Retrieved Optical Thickness for Analyzing The Aerosol and Cloud Properties Using Lidar Remote Sensing

The lidar remote sensing is the one important application to observe the aerosol and cloud of the atmosphere. The micropulse lidar (MPL) return signals were studied in the tropical area. In this investigation, the single scattering is analyzed by the physical properties of aerosol and cloud. The signal simulation of the single scattering predicts the maximum optical thickness by saturation. It was observed that saturation optical thickness from the lidar signal depends on the variation of extinction coefficient. This simulation is compared by the optical thickness estimation from the lidar data. The MPL data (at wavelength of 523 nm) was determined, and the sky radiometer (at wavelength 500 nm) was used as reference data. The maximum optical thickness of lidar was 2.6 at night time, and the maximum optical depth of lidar and sky radiometer data on the same day were 2.25 and 1.7, respectively.


INTRODUCTION
The aerosol particles affect the climate directly by interacting with solar and terrestrial radiation and indirectly by their affects on cloud microphysics, and precipitation.Biomas burning is a major source not only trace gas emission into the troposphere but also organic hygroscopic particles.These smoke particles deteriorate the local ground visibility and give rise to increase the aerosol with smaller geometric radius over a relatively long period.(Shiina, 2005).
Lidar (light detection and ranging) is one application of the optical technology.It is an important tool for detecting physical aspect of aerosol and cloud, and for directly providing their height and thickness.
The lidar sensing is affected by two physical parameters, i.e. extinction and backscattering coefficient.The application of lidar is used in environmental remote sensing to observe the air pollution, which is caused by the industry, and the forest fire.Another application is the observation of physical characteristic of aerosol and cloud such as height and size distribution, temperature, velocity of wind, extinction coefficient, and optical thickness.(Niranjan, 2007).This research investigates the single scattering process to analyze the optical properties of aerosol and cloud such as optical thickness at boundary layer and high altitude cloud.In the first section, the saturation of optical thickness from lidar signal is simulated to estimate the maximum optical thickness.
The saturation depends on both variation of extinction coefficient of the cloud, and cloud physical thickness.
The second part is the estimation of optical thickness using micro pulse lidar (MPL), and there were compared by sky radiometer as reference data.The optical thickness is derived by integration of the extinction coefficient obtained by the Fernald equation (Lottman, 2001).
Theoryote.The lidar system is one of the tools for detecting many aspects of atmosphere.In general, a lidar system consist of 3 parts, i.e, laser source, receiver (telescope) and signal processing, which is include the photo-multiplier-tube (PMT) and display.The   Where S 2 is the lidar ratio for molecular of atmosphere.
To determine the particle extinction coefficient á 1 (R), Fernald is derived in simple form as follows (Fernald, 1984) : It shows that the particle extinction coefficient á 1 (R) depends on S 1 value for particulate and S 2 for molecule.
In the calculation, the value of S 1 could be assumed in range of 1 to 100.Furthermore, the optical thickness can be derived by the integration of extinction coefficient in Eq. ( 6) such as follows: . . . . . . . . . . . . . . . ..( 7) Where is optical thickness, and is far-end boundary.

MATERIALS AND METHOD
In this section, the research methodology for

RESULTS AND DISCUSSION
The single scattering process was analyzed by means of Fernald method in Eq. ( 6) to determine the extinction coefficient of aerosol and cloud.The Optical Thickness calculated by Eq. ( 8) is called the retrieved  In Figure 5, the saturation of ô ret can be derived at about 2.3, at the extinction coefficien á 1 = 20 x 10 -3 m -1 .Furthermore, the optical thickness retrieved is around ô ret = 2, at á 1 = 30 x 10 -3 m -1 in figure 5.And ô ret is 1.6 at á 1 = 40 x 10 -3 m -1 in Figure 7.A maximum extinction coefficient is found at á 1 = 6 X 10 -4 m -1 , and at boundary layer of aerosol is á 1 = 1.9 X 10 -4 m -1 in 1 km high.Further more the optical thickness is calculated for several cases such as in figure 10.A maximum optical thickness (ô = 2.60) is found for the data 01:00:40 a.m, with an extinction coefficient of about á 1 = 6 X 10 -4 m -1 .A minimum value (ô = 0.50) is obtained at 01:11:17 a.m.Then, the maximum optical thickness of sky-radiometer as displayed on monitor was 2.55, as reference data.Our result that was displayed in Figure 11   thickness is bigger then sky-radiometer.We noted that receiver of sky-radiometer limited by the sun intensity and the weather condition.However, lidar is independent of the sun intensity.It may be operated on the time and at night.This is the limitation of the sky-radiometer does not apply to lidar.

CONCLUSIONS
The micro pulse lidar (MPL) was applied to observe the aerosol and cloud properties of the atmosphere in a tropical area.It was important and simple applications of remote sensing for monitoring the atmosphere.
The retrieved optical thickness was simulated by using Fernald inversion method.The results were indicated by the saturation, the retrieved optical thickness decreases whith the increase of the extinction coefficient.
The optical thickness properties of aerosol and cloud were analyzed by using MPL lidar operated at 523 nm.There were compared by the optical thickness from sky radiometer operated at 500 nm.This result shows that the radiometer optical thickness is a good estimation of the retrieved lidar optical thickness.
laser light source produces light shots directed to atmosphere.The transmitted signal induces the backscattering process from the aerosol and and clouds.Thus the backscattering signal can be detected by telescope.Furthermore, the geometrical form factor, G(r) includes the degree of overlap between the laser beam and the field of view (FOV) of telescope.The simple lidar system is shown in Figure 1.The power received by the telescope may be expressed by lidar equation (Dewang, 2000). . . . . ..(1)Where: P(R) : is the receiver power of the return signal from a scattering volume (Watt) R : is a slant range (Km).

Figure 1 .
Figure 1.The simple scheme blok diagram of Lidar System

Figure 7
Figure 7 displays an example of 24-hour, time-height indication of the MPL signal.It is seen that the height of planetary boundary layer stayed at an altitude about 2 km during the observation.Figure 8 shows the timeheight image lidar data at 0:00 a.m to 24:00 p.m of one day.The low level cloud is depicted below 3 km high at 0:00 to 13:00 p.m.The middle level cloud is about 3 to 9 km high during 16:00 to 23:40 p.m.And high level cloud is above 9 km, i.e a high altitude cloud about 9-14 km during 01:00 to 4:30 a.m, a boundary layer is also recognizable at a lower height of around 1 km.In the present analysis, however, we concentrate on highaltitude, thick cloud from 01:00 to 4:30 a.m.In this section, the MPL data is observed in the daytime (at 06:00 -18:00) and the night (at 18:00 -06:00).For the first, the MPL data is processed in the night to observe the lidar signal.For example, the simple of lidar signal can be seen in Figure9.A high altitude cloud is illustrated in 11 to 13 km high at 01:00:40 a.m.Maximum of original signal is 5.3 PhE/µsec km 2 .Here, PhE is photon energy in Joule.The simple estimation of extinction coefficient was calculated at high altitude cloud such as in Figure 10 at 01:00:40 a.m.
Figure 3 to Figure 7.The saturation behavior of the cloud optical thickness, in principle, brought about by the penetration and attenuation processes of the laser light in the cloud.Thus, it does not yield direct information on the optical thickness properties of tropical clouds.

Figure 11 .
Figure 11.The optical thickness in any cases at high altitude cloud

Table 1 .
Operational Syatem Parameters of MPL