Density and Temperature Profile in the Lower
Atmosphere at Kmitl, Thailand
Lidar system
In the experiment, the system is monostatic Rayleigh lidar that means the transmitter and receiver is located in the same place. The transmitter is the Nd: YAG laser that emit he second harmonic wavelength (532 nm) and the laser beam is transmitted in the vertical axis with respect to the earth. And, the received uses the telescope with 28 cm diameter that will collect the scattered laser light from molecule and any particle in the field of view of receiving telescope. The collected light by telescope will focus on a photomultiplier which converts the energy to be electrical signal. Consequently, in the optical remote sensing the photon counting techniques is helpful when the return signal is very weak. Finally, the photon counting signal will be transferred to a personal computer were the signal will be processed and analyzed. The detailed system is shown in Table 1.
Table 1. Characteristics of Lidar system
| Transmitter |
| Laser |
Nd. YAG, SHG (532 nm) |
| Output |
180/mJ/p |
| Repetition |
10 Hz |
| Beam div |
0.1 mrad |
| Receiver: |
| Telescope |
Sehmidt-Cassegrain 28 cm diameter |
| Detector |
PMT, S-1 R3236 Hamamatsu |
Signal and data analysis
As the telescope receives the scattered light in according to the transmitted laser light in the field of view of receiving telescope. The ground-based monostatic radar will be in this experiment. Usually, the optical remote sensing equation describes as follows,
N(z) = [hT2] [Plt/hc]
[lb(z)n(z)DZ]
[Ar/4pZ2]+ [Npt] (1)
N(z) is expected number of photons detected in the interval
DZ ,
n(z) is particles density of range Z, N
n are expected photon due to background noise,
DZ is receiver range bin length, A
r is the telescope aperture area,
l is optical wavelength, h is Plant's constant, c is light velocity,
P is laser power,
T = exp[-
Ss(z)DZ] is one-way transmittance where
s is the extinction cross section, R is laser pulse rate,
t is integration time,
h is system efficiency and
b(z) is effective particles backscattering cross section converts to be the number of density via the relation of average molecule with cross section at this wavelength. To determine the temperature from the lidar density profile is integrated upward or downward using the hydrostatic equation and the initial temperature may be obtained from a model or local temperature as integrate upward. The normally hydrostatic equation is
dP = -pg dz (2)
And the ideal gas law is
Finally, the integrated temperature equation is
T(z) is the atmospheric temperature profile, P(z) is atmospheric pressure profile, p(z) is density profile, g(z) is the gravitational acceleration, M is mean molecular weight of the atmosphere, R is universal gas constant is the altitude of temperature estimate.
