Measurement And Modeling Of Atmospheric Optical Turbulence In The South China Sea

Atmospheric optical turbulence is an irregular fluctuation caused by the fluctuations in atmospheric temperature or density.It is an important factor that restricts the photoelectric system,free space optical communication and laser atmospheric transmission.A thorough understanding of the characteristics of atmospheric optical turbulence and an accurate estimation of optical turbulence strength(C_n~2)have become a hot topic in optical turbulence research.However,the current research on atmospheric optical turbulence is mostly carried out on land,and the only ocean optical turbulence experiments are only conducted in the offshore area,which makes us lack sufficient understanding of atmospheric optical turbulence characteristics in the Marine environment.At same time,the lack of atmospheric turbulence data in the ocean directly makes it impossible to verify the effectiveness of existing turbulence models in the Marine environment,more can’t establish reliable ocean-atmosphere optical turbulence models.Based on three atmospheric optical turbulence measurement experiments conducted in the South China Sea,the characteristics of atmospheric turbulence near the sea surface and high altitude in the South China Sea are analyzed,and the estimation errors of several turbulence models in the South China Sea are calculated.At the same time,the influencing factors of estimation error are discussed.On this basis,we developed several available optical turbulence estimation models which conform to the turbulence characteristics in the South China Sea.The main contents and results of this paper are the following:1.The sea-land wind under the complex underlying surface has certain influence on the turbulent characteristics of the offshore atmospheric boundary layer.The isotropy of turbulence under sea wind is better than that under land wind.In addition,the study on the dissipation rate of turbulence kinetic energy and C_n~2 under the sea-land wind condition shows that the turbulence strength increases faster under the sea wind condition.2.It is found that C_n~2 near the sea surface has a typical "Mexican cap" diurnal variation in the South China Sea offshore,while C_n~2 in the open sea area decreases during the day,especially around noon,and increases at night.The variation trend of C_n~2 vertical profile in offshore and open sea areas is similar,showing a trend of"attenuation-enhancement-attenuation" with height.However,the C_n~2 at high altitude in the open sea is stronger than in the offshore as a whole,and it first increases and then decreases with time.There is strong atmospheric optical turbulence in the high altitude at noon.3.In the South China Sea,the C_n~2 profile estimated by HMNSP99 model is more accurate than that estimated by Dewan model.Moreover,when the wind shear exceeds 0.016 s-1 or the temperature gradient exceeds 0 K/m or the outer scale is less than 2.5 m,the errors of HMNSP99 model will increase significantly.Therefore,we obtain a new turbulence model by modifying the outer scale equation in the HMNSP99 model,and the statistical error of the new model is better than HMNSP99 model in reconstructing the C_n~2 profile of the South China Sea.In addition,we also improved the Sadot and Kopeika models using the stepwise regression method to better estimate C_n~2 near the sea surface.4.The results show that the backpropagation neural network model can obtain the boundary layer C_n~2 profile from the basic meteorological parameters through continuous learning,which can avoid the uncertainty and inaccuracy of the traditional optical turbulence model.Compared with the HMNSP99 model and HAP model,the backpropagation neural network model has smaller error in estimating C_n~2 in the South China Sea.5.ERA5 reanalysis data can estimate the variation characteristics of optical turbulence on large time and spatial scales over the South China Sea.The analysis results of the intermonthly variation characteristics of optical turbulence in the South China Sea showed that optical turbulence strength is strongest in summer and weakest in spring.The variation trend of wind shear and turbulence strength is roughly the same,with larger values near sea surface and about 15 km.The Richardson number profile decreases sharply near sea surface,then gradually increases to the tropopause,and then decreases with height above the tropopause.In this paper,the results showed that the optical turbulence near the sea surface and high altitude of the South China Sea shows different characteristics compared with that on land.If the existing optical turbulence model is directly used to estimate C_n~2 of the sea surface and high altitude of the South China Sea,there will be some deviation in the results,which requires us to improve the existing turbulence model.In addition,we verify the ability of the backpropagation neural network model to estimate the C_n~2 profile in the South China Sea.