Atmospheric Refractive Index Structure Parameter Research Articles

Research on integrated LiDAR and multi-parameter detection of atmospheric transmittance, turbulence, and wind

An integrated LiDAR system has been reported, which can be used for simultaneous detection of atmospheric transmittance, turbulence, and wind along the same path. Through the integrated design of optics and mechanics, the size and weight of the system were effectively reduced. The comprehensive detection distance of atmospheric transmittance, atmospheric coherence length, and radial wind had also been achieved at least 4 kilometers. Comparative experiments have demonstrated that the detection results of the integrated LiDAR system and the near-surface meteorological observation system have standard deviations of less than 0.03 km−1 for extinction coefficient, 0.2 m/s for wind velocity, and 7.15 × 10−14 m−2/3 for C 2 n (atmospheric refractive index structure parameter), thus verifying the accuracy of the system detection. The reliability of the integrated LiDAR system was validated through six consecutive nights of continuous detection experiments, measuring three parameters simultaneously. Through analysis, the influence laws among atmospheric transmittance, coherence length, and wind were preliminarily explored. Significant variations in wind velocity can induce substantial fluctuations in atmospheric coherence length, and it is positively correlated with atmospheric transmittance. The integrated LiDAR system can provide a variety of reliable real-time detection data for further research on the laser transmission process in the atmosphere.

Comparison of the imaginary parts of the atmospheric refractive index structure parameter and aerosol flux based on different measurement methods

Abstract. The complexity of aerosol particle properties and the diversity of characterizations make aerosol vertical transport flux measurements and analysis difficult. Although there are different methods, such as aerosol particle number concentration flux and aerosol mass flux based on the eddy covariance principle as well as aerosol mass flux measurements based on the light-propagated large-aperture scintillation principle, there is a lack of mutual validation among the different methods. In this paper, a comparison of aerosol mass flux measurements based on the eddy covariance principle and aerosol mass flux measurements based on the light-propagated large-aperture scintillation principle is carried out. The key idea of aerosol mass flux measurements based on the light-propagated large-aperture scintillation principle is the determination of the imaginary part of the atmospheric equivalent refractive index structure parameter (AERISP). In this paper, we first compare the AERISPs measured by two different methods and then compare the aerosol mass vertical transport fluxes obtained by different methods. The experiments were conducted on the campus of the University of Science and Technology of China (USTC). A light propagation experiment was carried out between two tall buildings to obtain the imaginary and real parts of the AERISPs for the whole path, which in turn can be used to obtain the aerosol vertical transport flux. An updated visibility meter is installed on the meteorological tower in the middle of the light path, which is utilized to obtain the single-point visibility, which is then converted to the imaginary part of the atmospheric equivalent refractive index (AERI). The imaginary parts of the AERISP were obtained via spectral analysis with visibility data. The results show that the imaginary parts of the AERISPs obtained by different methods are in good agreement. The imaginary part of the AERI measured by the visibility meter and the vertical wind speed obtained by the ultrasonic anemometer were used for covariance calculations to obtain the aerosol vertical transport flux. The trends in aerosol vertical transport fluxes obtained by the different methods are consistent, and there are differences in some details, which may be caused by the inhomogeneity in the vertical transport of aerosol fluxes. The experimental results also showed that urban green land is a sink area for aerosol particles.

Distinctive roles of elevated absorbing aerosol layers on free-space optical communication systems.

The impact of enhanced local heating due to absorption of solar radiation by elevated layers of aerosol black carbon (BC) in the lower troposphere in the performance of free-space optical (FSO) communication links is investigated. It is seen that a strong elevated BC layer at an altitude around 4.5km enhances the atmospheric stability locally and leads to a large reduction in the atmospheric refractive index structure parameter (Cn2), leading to improved performance of the FSO communication links. For layers in the tropical atmosphere with sufficiently high BC concentration, the signal attenuation due to BC absorption is alleviated by the large reduction in Cn2 due to BC-induced warming and brings down the link outage probability. Synergy between reduction in Cn2 and long wavelength transmission improves the link budget significantly by reducing the beam wander and number of adaptive optics units required.

Prediction model of atmospheric refractive index structure parameter in coastal area

In this paper, we focus on the prediction of atmospheric refractive index structure parameter () in coastal area using the routine meteorological parameters. Based on the micrometeorology, macrometeorology and Monin–Obukhov similarity theory, three modified prediction models of are presented in combination with the long-term observation data of and meteorological parameters in coastal city, respectively. For different weather, the applicable cases of three prediction models are comparatively analysed and their applicable effects are comprehensively evaluated. The results indicate that the modified micrometeorology model of shows better applicability for overcast sky, the offshore macrometeorology model of displays better predictability for sunny day and the offshore Thiermann model provides better availability for overcast sky, cloudy day, overcast to sunny or sunny to overcast day.

A new method for measuring the imaginary part of the atmospheric refractive index structure parameter in the urban surface layer

Abstract. The atmospheric refractive index consists of both real and imaginary parts. The intensity of refractive index fluctuations is generally expressed as the refractive index structure parameter, with the real part reflecting the strength of atmospheric turbulence and the imaginary part reflecting absorption in the light path. A large aperture scintillometer (LAS) is often used to measure the structure parameter of the real part of the atmospheric refractive index, from which the sensible and latent heat fluxes can further be obtained, whereas the influence of the imaginary part is ignored or considered noise. In this theoretical analysis study, the relationship between logarithmic light intensity variance and the atmospheric refractive index structure parameter (ARISP), as well as that between the logarithmic light intensity structure function and the ARISP, is derived. Additionally, a simple expression for the imaginary part of the ARISP is obtained which can be conveniently used to determine the imaginary part of the ARISP from LAS measurements. Moreover, these relationships provide a new method for estimating the outer scale of turbulence. Light propagation experiments were performed in the urban surface layer, from which the imaginary part of the ARISP was calculated. The experimental results showed good agreement with the presented theory. The results also suggest that the imaginary part of the ARISP exhibits a different diurnal variation from that of the real part. For the wavelength of light used (0.62 μm), the variation of the imaginary part of the ARISP is related to both the turbulent transport process and the spatial distribution characteristics of aerosols.

Some Results of Remote Sensing of Atmospheric Refractivity Near the Ground Using Laser Beam Scintillations

An experimental set-up developed for the measurement of path-averaged atmospheric refractive index structure parameter (C2n) is described followed by a brief summary of the theory involved in the generation and amplification of refractive index inhomogeneities. The results of the experiments conducted in the surface layer using a Helium-Neon laser source over an optical path length of 61.5 m at a height of 1.1 m above uniform surface are presented. The observed C2n ranges from 0.97 × 10−12 to 1.3 × 10−14 m−2/3. The diurnal variation of C2n shows rapid increase during morning hours, peaking around noon and slow decrease during evening hours. The observed relationship between the variations of C2n and temperature is explained and further studies to support and extend the present findings are indicated.

The contribution of atmospheric density to the drop-off rate of Cn2

Calculation and comparison of terms in the expression for Cn2, the atmospheric refractive index structure parameter, indicate that density is the prime factor in determining the drop-off rate of Cn2 with altitude in the lower stratosphere. We have then determined the drop-off rate in atmospheric density using rawinsonde data from sites spread out in latitude from approximately 70°N latitude to approximately 10°N latitude. These height drop-off values in density follow, very nicely, a linear relationship with latitude described by a value of 1.30 dB/km at 70°N latitude and 1.52 dB/km at 10°N latitude. No longitudinal variations were found. Density drop-off rates are then compared to drop-off rates of mean Cn2 measurements obtained using remote radars and also to in situ thermosonde mean Cn2 measurements. At high latitudes, radar Cn2 drop-off rates agree with the density drop-off rates but then diverge from the density drop-off rates as latitude decreases until the radar value is about a factor of two greater than the density drop-off rate at 10°N latitude. The thermosonde values of Cn2 drop-off are approximately equal to or no more than about 14% lower and 17% higher than the density drop-off rates over the latitudes investigated.

A Quantitative Comparison of the Refractive Index Structure Parameter Determined from Refractivity Measurements And Amplitude Scintillation Measurements at 36 GHz

Results are presented of the determination of the atmospheric refractive index structure parameter C2n, deduced from amplitude scintillation measurements made on a 36-GHz radio link and also from refractivity measurements using a turbulence probe. These measurements were made in a town environment (central London) and covered a range of atmospheric conditions. The two methods of measurement give good agreement, particularly in convective conditions, and provide further experimental evidence of the applicability of Tatar-ski's wave propagation theory in a turbulent medium to millimeter wavelengths.

Determination of the atmospheric refractive index structure parameter from refractivity measurements and amplitude scintillation measurements at 36 GHz

Results are presented of the determination of the atmospheric refractive index structure parameter C n 2, deduced from amplitude scintillation measurements made on a 36 GHz radio link and also from the refractive index frequency spectrum computed from refractivity measurements using a radio refractometer. The two methods of measurement give acceptable agreement and provide further experimental evidence of the applicability of Tatarski's wave propagation theory in a turbulent medium to millimetre wavelengths.