Retrieval And Analysis Of Cloud Optical Depth And Effective Particle Radius Based On Sky Radiometer

Cloud is an important component of the ground-atmosphere system,which plays an important role in regulating atmospheric heat balance,hydrological cycle and climate change.However,the optical depth and effective particle radius of clouds in global climate models are highly uncertain.Therefore,it is of great significance to use long-term continuous observation data to retrieval,accuracy verification and comparative analysis of cloud microphysical characteristics.At present,since satellites,aircraft and other space-based equipment cannot carry out long-term continuous monitoring of fixed observation points,this paper uses the continuous observation data of the cloud channel of the ground-based equipment sky radiometer of Xi ’an University of Technology,combined with the optimal estimation method and SBDART model to invert the cloud optical depth and effective particle radius.The retrieval results were compared and verified with the cloud parameter products of MODIS satellite and Japanese meteorological satellite Himawari-8,and then the variation characteristics of cloud microphysical parameters were statistically analyzed and the influence of aerosol on the variation of cloud microphysical parameters was preliminarily discussed.Firstly,the relationship between cloud transmittance and cloud optical depth and effective particle radius is analyzed.In this paper,the atmospheric radiative transfer model SBDART is used to simulate the relationship between water cloud and ice cloud transmittance and cloud microphysical parameters.In order to accurately determine the optical depth and effective particle radius of the cloud,a joint method of non-absorbent band and absorbent band is selected to establish the cloud transmittance calculation table.The sensitivity of cloud optical depth to transmittance is higher than that of effective particle radius by comparing the calculation tables of different bands,and the calculation tables of 1.02μm and 2.2μm can determine cloud optical depth and effective particle radius more accurately.According to the relationship between cloud transmittance and cloud microphysical parameters,the optical depth and effective particle radius of cloud are retrieved by using the optimal estimation method.Firstly,Langley calibration of the instrument is required,and then the radiation observed by the instrument is used to calculate the transmittance,and the measurement error matrix and prior error matrix are obtained by combining the cloud parameter products of the satellite.Finally,the SBDART forward model is continuously iteratively optimized to obtain the final cloud optical depth and effective particle radius.Statistical analysis of the retrieval results shows that the mean optical depth of clouds in spring and summer is 9.3 and 11.4,and that in autumn and winter is 13.3 and 14.5.The mean effective particle radius of the cloud has always been about 10μm.By comparing with the seasonal statistical results of satellite cloud parameter products,it is found that the difference of cloud optical depth and effective particle radius is less than 2.1,and the difference of effective particle radius is less than 1.5μm.The difference of multi-layer cloud is slightly larger,so it can be seen that multi-layer cloud is the main reason for the difference between ground-based equipment retrieval results and satellite cloud parameter products.By analyzing the correlation between the retrieval results of the sky radiometer and the cloud parameter products of MODIS satellite and Himawari-8 satellite,it is found that the correlation coefficients of optical depth of water cloud and ice cloud in MODIS satellite cloud products are 0.825 and 0.789,and the correlation coefficients of effective particle radius are 0.569 and 0.648.In order to investigate the correlation between the retrieval results of sky radiometer and the cloud parameter products of Himawari-8 satellite,the correlation of cloud optical depth and effective particle radius of single-layer cloud and multi-layer cloud is discussed first.The correlation coefficients of single-layer cloud are 0.998 and 0.891,respectively.The correlation coefficients of multi-layer cloud were 0.821 and 0.736,respectively.In addition,the correlation between different cloud types of single-layer cloud is analyzed,and the correlation coefficient of optical depth of deep convective cloud is 0.986.The correlation coefficient of effective particle radius of stratocumulus is 0.943.According to the analysis of different cloud types,the correlation coefficient of optical depth of nimbostratus cloud is 0.979.The correlation coefficient of effective particle radius of Stratocumulus is 0.967.The results show that the retrieval results of sky radiometer are in good agreement with satellite cloud products.Finally,the influence of aerosol optical parameters on cloud microphysical properties in Xi ’an area is analyzed.