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The Rayleigh lidar system at Zhongshan station consists of three units: laser transmitter, optical receiver, and time control & signal acquisition unit. Figure 1 shows lidar schematic diagram and the system configurations are listed in Tab.1.
Table 1. System configurations of Zhongshan station Rayleigh lidar
Configuration Parameters Laser transmitter Wavelength/nm 532 Pulse energy/mJ 400 Repetition rate/Hz 30 Pulse width/ns 8 Lidar receiver Telescope diameter/m 0.8 Telescope f number 1.8 Fiber diameter/mm 1.5 Fiber NA 0.39 PMT quantum efficiency ~40% @ 532 nm Time control & signal acquisition Time generator DG645 Photon counting card P7882 The lidar transmitter unit mainly consists of a neodymium-doped yttrium aluminum garnet (Nd:YAG) pulsed laser. The output laser is at the wavelength of 532 nm with single pulse energy ~400 mJ and repetition rate of 30 Hz. A high-precision beam controller is used to steer the laser beam to the zenith direction.
A prime focus telescope with the diameter of 0.8 m is used as lidar receiver. In order to perform well under the cold weather condition in Antarctica, the telescope's primary mirror was made of devitrified glass, which has relatively smaller thermal expansion coefficient. A multi-mode optical fiber with 1.5 mm core diameter and 0.39 numerical aperture (NA) is used to couple the telescope to the subsequent optical path. A mechanical chopper with the rotation rate of 5400 rpm is used to block the photon signal at lower altitudes for protecting the photon multiplier tube (PMT) from saturation. After passing through the chopper and an optical filter, received photons are finally detected by a PMT (Hamamatsu H7421-40, with the quantum efficiency ~40% at 532 nm).
The PMT converts optical photons to electrical signals being recorded by a digitizer (Fast ComTec MCA-3 Series/P7882). The timing control of the lidar is designed as follows. The primary timing is generated by the chopper in the receiving channel at the frequency of 180 Hz. Then the chopper triggers a digital delay/pulse generator (DG645) so that for every six pulses from the chopper, the DG645 is triggered once. Therefore a timing signal with the repetition rate of 30 Hz is produced to trigger the Nd:YAG laser's flash lamp. The same timing is also synchronized to the data acquisition unit.
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The atmospheric density and temperature retrieval of Rayleigh lidar we used here follows a standard method[6], which is based on the lidar equation, ideal gas law, and hydrostatic equilibrium equation.
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The Rayleigh scattering lidar equation can be expressed as:
$$ N\left({\textit{z}}\right)=A\left(\eta {T}_{{\rm{A}}}^{2}\right)\left({\sigma }_{{\rm{ray}}}\rho \right({\textit{z}}\left){\Delta }{\textit{z}}\right)\dfrac{1}{{{\textit{z}}}^{2}}+{N}_{{\rm{B}}} $$ (1) where A is a parameter dependent on both laser pulse energy and telescope diameter,
$ \eta $ is system efficient,${T}_{{\rm{A}}}$ is atmospheric transmittance,${\sigma }_{{\rm{ray}}}$ is Rayleigh backscatter cross section,$ \rho \left({\textit{z}}\right) $ is atmospheric density at altitude z, and${N}_{{\rm{B}}}$ is background noise. If we choose$ {{\textit{z}}}_{0} $ as the reference altitude, we can calculate atmospheric density at altitude z as:$$ \rho \left({\textit{z}}\right)=\dfrac{{{\textit{z}}}^{2}\left(N\left({\textit{z}}\right)-{N}_{{\rm{B}}}\right)}{{{\textit{z}}}_{0}^{2}\left(N\left({{\textit{z}}}_{0}\right)-{N}_{{\rm{B}}}\right)}\rho \left({{\textit{z}}}_{0}\right) $$ (2) where
$ \rho \left({\textit{z}}\right) $ and$ N\left({\textit{z}}\right) $ are the air density and lidar photon count at altitude z, respectively,$ \rho \left({{\textit{z}}}_{0}\right) $ is the reference density from an atmospheric model. In our case, we used the MSIS (Mass Spectrometer and Incoherent Scatter) model[19-20].The uncertainty of Rayleigh lidar density is caused by the random statistical uncertainty of PMT detectedphoton counts, which obeys the Poisson distribution, that is,
$ \Delta {N\left({\textit{z}}\right)}^{2}=N\left({\textit{z}}\right) $ . Therefore, the uncertainty of Rayleigh lidar density can be expressed as:$$ \frac{\Delta \rho \left({\textit{z}}\right)}{\rho \left({\textit{z}}\right)}\approx \frac{\sqrt{N\left({\textit{z}}\right)}}{N\left({\textit{z}}\right)-{N}_{{\rm{B}}}} $$ (3) -
The ideal gas law is expressed as:
$$ PV=nRT $$ (4) where
$ P $ is atmospheric pressure, V is volume,$ n $ is the amount of gas,$ R $ is the ideal gas constant, and$ T $ is temperature. The relation between mass and density can be expressed as:$$ \rho =\frac{m}{V} $$ (5) And hydrostatic equilibrium equation is shown as:
$$ {\rm{d}}P=-\rho g{\rm{d}}{\textit{z}} $$ (6) By combining Eq. (4)-(6), we can get the atmospheric temperature as:
$$ T\left({\textit{z}}\right)=T\left({{\textit{z}}}_{{\rm{seed}}}\right)\frac{\rho \left({{\textit{z}}}_{{\rm{seed}}}\right)}{\rho \left({\textit{z}}\right)}+\frac{1}{R}{\int }_{{\textit{z}}}^{{{\textit{z}}}_{{\rm{seed}}}}g\left(r\right){\rm{d}}r\frac{\rho \left(r\right)}{\rho \left({\textit{z}}\right)} $$ (7) where
$T\left({{\textit{z}}}_{{\rm{seed}}}\right)$ and$\rho \left({{\textit{z}}}_{{\rm{seed}}}\right)$ are temperature and density at the seeding altitude zseed, respectively. They are usually adopted from atmospheric model, such as the MSIS model.To estimate the Rayleigh temperature uncertainty, Eq. (7) can be approximately expressed as:
$$ T\left({\textit{z}}\right)\approx T\left({{\textit{z}}}_{0}\right){\left(\frac{{\textit{z}}}{{{\textit{z}}}_{0}}\right)}^{2}\frac{{N}_{{\rm{R}}}\left({{\textit{z}}}_{0}\right)}{{N}_{{\rm{R}}}\left({\textit{z}}\right)}+\frac{\Delta {\textit{z}}}{{{R}}}\sum\nolimits_{r={\textit{z}}}^{{{\textit{z}}}_{0}}\frac{{N}_{{\rm{R}}}\left(r\right)}{{N}_{{\rm{R}}}\left({\textit{z}}\right)}\frac{g\left(r\right){{\textit{z}}}^{2}}{{r}^{2}} $$ (8) where
${N}_{{\rm{R}}}\left({\textit{z}}\right)=N\left({\textit{z}}\right)-{N}_{{\rm{B}}}$ is the Rayleigh scattering signal.And from Eq. (8), the temperature uncertainty can be calculated as:
$$\begin{split} & \Delta {{T}}\left( {{{\textit{z}}}} \right){{ = }}\sum\nolimits_{{{{{\textit{z}}}}_{{i}}}{{ = {\textit{z}} + }}\Delta {{{\textit{z}}}}}^{{{{{\textit{z}}}}_{{0}}}} {k(} {{{{\textit{z}}}}_{{i}}}{{)N(}}{{{{\textit{z}}}}_{{i}}})/\\ & N({{{\textit{z}}}})\sqrt {\frac{{ \displaystyle\sum\nolimits_{{{{{\textit{z}}}}_{{i}}}} {{k}} {{\left( {{{{{\textit{z}}}}_{{i}}}} \right)}^{{2}}}{{\left( {\overline { \Delta {{N}}\left( {{{{{\textit{z}}}}_{{i}}}} \right)}} \right)}^{{2}}}}}{{{{\left[{\displaystyle\sum _{{{{{\textit{z}}}}_{{i}}}}}{{k(}}{{{{\textit{z}}}}_{{i}}}{{)N(}}{{{{\textit{z}}}}_{{i}}}{{)}}\right]}^{{2}}}}}{{ + }}\frac{{{{\left( {\overline {\Delta {{N}}\left( {{{{{\textit{z}}}}_{{i}}}} \right)}} \right)}^{{2}}}}}{{{{N}}{{\left( {{{{{\textit{z}}}}_{{i}}}} \right)}^{{2}}}}}} \end{split} $$ (9) where
$ k\left({\textit{z}}_{i}\right)=\dfrac{\Delta {\textit{z}}}{R}g\left({\textit{z}}_{i}\right){\left(\dfrac{{\textit{z}}_{i}}{\textit{z}}\right)}^{2} $ for$ {\textit{z}}_{i}={\textit{z}}+\Delta {\textit{z}}, {\textit{z}}+2\Delta {\textit{z}},\cdots ,{\textit{z}}_{0} $ . Since the air density decreases exponentially with increasing altitude, the maximum temperature uncertainty is around the seeding altitude where the lidar signal is the weakest. From Eq. (9), the Rayleigh temperature uncertainty can be estimated by:$$ \Delta T\left({\textit{z}}\right)\leqslant T\left({\textit{z}}\right)*\sqrt{\frac{1+\dfrac{1}{S\!BR\left({\textit{z}}\right)}}{2{N}_{{\rm{R}}}\left({\textit{z}}\right)}} $$ (10) where
$S\!\!BR\left({\textit{z}}\right)={N}_{{\rm{R}}}\left({\textit{z}}\right)/{N}_{{\rm{B}}}$ is the signal to background ratio.
Initial results of Rayleigh scattering lidar observations at Zhongshan station, Antarctica
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摘要: 一套瑞利散射激光雷达已部署在南极中山站(69.4° S, 76.4° E)用于探测大气密度和温度。该激光雷达的光源为二倍频Nd:YAG脉冲激光器,重复频率30 Hz,单脉冲能量约400 mJ,同时使用一台0.8 m口径的垂直指向望远镜作为接收望远镜,可以探测平流层上层及中间层下层(USLM)区域的大气密度及温度廓线。在垂直分辨率为300 m,时间分辨率为30 min的情况下,由光子噪声引起的大气密度和温度测量不确定性分别小于1.5%和1 K。该激光雷达自2020年3月开始在中山站开展常规观测,有助于研究极区USLM区域的大气密度、温度的变化特征以及大气波动的传播特性。Abstract: A Rayleigh scattering lidar for measuring the atmospheric density and temperature has been deployed at Zhongshan Station (69.4° S, 76.4° E), Antarctica. Lidar transmitter was a frequency doubled Nd:YAG laser with ~400 mJ pulse energy and 30 Hz repetition rate. A telescope with 0.8 m diameter pointing to the zenith direction served as the lidar receiver. This lidar was capable of profiling the density and temperature in the Upper Stratosphere and Lower Mesosphere (USLM) region. At the vertical resolution of 300 m and the temporal resolution of 30 min, the lidar measurement uncertainties, mainly due to the photon noise, were calculated to be within 1.5% and 1 K for density and temperature, respectively. Since March 2020, this lidar has been routinely operated at Zhongshan station for exploring the atmospheric density and temperature variations and wave propagation characteristics in the polar USLM region.
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Figure 3. Comparison of atmospheric density (a) and temperature (c) between lidar (red solid), MSIS model (blue dash), and SABER satellite (green dash) data near Zhongshan station on March 6, 2020. The vertical resolution is 300 m and the temporal resolution is 30 min for lidar data. Lidar measured density uncertainty (b) and temperature uncertainty (d) were also plotted
Table 1. System configurations of Zhongshan station Rayleigh lidar
Configuration Parameters Laser transmitter Wavelength/nm 532 Pulse energy/mJ 400 Repetition rate/Hz 30 Pulse width/ns 8 Lidar receiver Telescope diameter/m 0.8 Telescope f number 1.8 Fiber diameter/mm 1.5 Fiber NA 0.39 PMT quantum efficiency ~40% @ 532 nm Time control & signal acquisition Time generator DG645 Photon counting card P7882 -
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