| The first spaceborne Doppler wind lidar?DWL?designed by ESA?European Space Agency?was launched successfully on 22 August 2018,which indicates that people made a big step forward in the field of global wind observations.However,the solar background radiation?SBR?has a significant impact on the accuracy of the day-time observations of spaceborne lidar.And the accuracy determines whether the observations of spaceborne DWL can be used to improve the accuracy of numerical weather prediction.To study the impact of SBR on the observations of spaceborne DWL,we simulated the spaceborne DWL and calculated the TOA?top-of-atmosphere?radiance.The solar background noise?SBN?was converted from the TOA radiance based on the optical parameters of ALADIN?Atmospheric Laser Doppler Instrument?.The impact of SBN on accuracy of the observations of spaceborne lidar was assessed.The difference of the global distribution of SBR on different orbits parameters and its impact on the accuracy of retrieved wind by spaceborne DWL was also assessed.The main researches and conclusions were shown as follows:?1?The simulation method of SBR was studied and the feasibility of this method was validated.In this paper,the TOA radiance was simulated using radiative transfer model?RTM?libRadtran,and the upper estimates of SBR on six typical land cover types and three weather conditions were made.The SBR converted from TOA radiance was compared with the measurements of CALIOP?Cloud–Aerosol Lidar with Orthogonal Polarization?and MODIS?Moderate Resolution Imaging Spectroradiometer?.The results show the good consistency in trend and high correlation coefficients among the three,which demonstrate the feasibility of the simulation.?2?The simulation of spaceborne DWL was established based on the operation principle and systematic structure of spaceborne DWL,and the impact of SBR on the accuracy of retrieved wind was assessed.The theory equations used to calculate the uncertainties of retrieved wind on Rayleigh channel was also proposed in this paper.The comparison between the uncertainties calculated by theory equations and the simulation esults shows good agreements in tendency,which demonstrate the accuracy of the simulation of spaceborne DWL and the theory equations.?3?We assessed the impact of SBR on the observations of spaceborne DWL based on the instrument parameters of ALADIN.The sensitivities study shows that,once 20mW m-2 sr-1 nm-1 on SBR on 355 nm was added,the additional uncertainties on Rayleigh channel were 0.38 m s-1,0.18 m s-1,and 0.69 m s-1 in the boundary layer,troposphere,and stratosphere,respectively.And the additional uncertainties on Mie channel are 0.07 m s-1 and 0.21 m s-1 in the boundary layer and troposphere,respectively.Combined the wind retrieved by Rayleigh channel and Mie channel,the results show that the SBR has less impact in boundary layer and has more impact in the stratosphere.We also test the sensitivities of the accuracy of retrieved wind to atmospheric climatology and aerosol types.The results illustrate that the differences of the uncertainties are between 0.02 m s-1 and 0.43 m s-11 due to the differences in atmospheric climatology.And the maximum difference is 1.89 m s-1,which is caused by the differences in aerosol types.The differences of uncertainties caused by seasons are small,and the differences caused by surface types are also small except the surface of snow and ice.And the differences in atmospheric climatology are the main reasons that account for the differences of uncertainties in the troposphere and stratosphere,the differences in aerosol types are the main reasons that account for the differences of uncertainties in the boundary layer.In this paper,we defined the quality coefficient for the profiles of retrieved wind according to the minimum uncertainties demand of ESA.The results reveal that more than 70%of the profiles of retrieved wind by spaceborne DWL were in high quality.?4?We assessed the impact of the parameters of orbits on SBR.As to the sun-synchronous orbit,the main parameters are orbits heights and local times of ascending node?LTAN?.The differences in global distributions of SBR with orbit heights of 320km and 396 km show that the orbit height has less impact on the global distributions of SBR.However,the SNR?signal noise ratio?would substantially increase in lower orbits heights which lead to lower uncertainty.The differences of the uncertainties between the two orbit heights are 0.45 m s-1,0.81 m s-1,and 2.01 m s-1 in the boundary layer,troposphere,and stratosphere,respectively.The increase in the intensity of laser shot can make up these differences.The differences of global distributions of SBR in the orbits with LTANs of 18:00,15:00 and 12:00 are relatively large.The closer the LTANs are to the noon,the greater the received SBR.Based on reasonable assumptions,the relationship between the uncertainty of wind observations and the signal-to-noise ratio?SNR?of Rayleigh channel is established in this paper.Therefore,it provides a theoretical basis for improving the uncertainty by increasing the laser energy to reduce uncertainty.Finally,the relationship between laser energy,SBR,and wind observation uncertainty is established.According to the relationship,it is found that when the laser energy reaches 80mJ,the accuracy of the wind observations of the spaceborne DWLs operates on the sun-synchronous orbits with LTANs of 15:00 and 12:00 can reach the accuracy level of the wind observations of Aeolus. |
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