太赫兹冰云辐射散射特性研究和探测参数设计

Terahertz radiation and scattering characteristics of ice cloud and sounding parameter design

星载被动太赫兹遥感是目前最具潜力的冰云探测手段,而理解冰云参数如何影响太赫兹辐射传输过程,掌握太赫兹频段冰云辐射散射特性是实现高精度太赫兹冰云定量探测的前提,对冰云探测仪器的设计和冰云参数反演也非常重要。基于离散纵标切线性辐射传输模式DOTLRT模拟计算太赫兹频段冰云辐射亮温及其雅可比矩阵,以气象再分析资料FNL为初值驱动中尺度气象研究与预报模式WRF预报得到冰云参数作为DOTLRT模式的输入。通过与星载微波探测仪ATMS的183 GHz频段观测亮温对比验证了模拟太赫兹冰云亮温具有较好的精度,并且表明太赫兹亮温同时受到冰云中的冰粒子和霰粒子的影响,且具有不同的特性。定量分析了冰云中冰、霰两种粒子的特性参数(等效粒径Dme、路径总量IWP、GWP)和观测几何对冰云太赫兹亮温的影响。在综合了太赫兹亮温的敏感性分析、雅可比矩阵的峰值高度分析和痕量气体吸收的基础上,给出了太赫兹冰云探测器的优化探测频段(183 GHz、243 GHz、325 GHz、448 GHz、664 GHz和874 GHz)和观测角度(53°±5°)。冰云散射和辐射特性表明太赫兹冰云探测需要同时考虑冰粒子和霰粒子。提供了冰、霰两种冰云粒子的辐射散射特性,突破现有研究仅仅计算单一冰粒子的局限,可为中国发展冰云太赫兹探测载荷提供技术支撑。

Satellite-borne passive terahertz remote sensing is currently most promising method for ice cloud sounding due to a number of potential advantages that complement existing visible and infrared techniques.Since the wavelength of terahertz radiation is comparable to the size of ice crystals,observed brightness temperature are well correlated to ice mass.The purpose of this article is to present the terahertz radiation and scattering characteristics of ice and graupel particles which constitute ice clouds and to understand how ice cloud parameters affect the terahertz brightness temperature,which guide the design of terahertz ice cloud sounding instruments.In this study,the terahertz brightness temperature and Jacobian matrix are simulated by applying the Discrete-Ordinate Tangent Linear Radiative Transfer(DOTLRT)model to the spatially and microphysically detailed output of ice clouds predicted by the Weather Research and Forecasting(WRF)model and Final(FNL)analysis data.The DOTLRT model uses the classical Mie scattering formula to calculate the scattering characteristics of liquid,rain,ice,snow,and graupel particles in clouds.The validity of the simulated 183 GHz brightness temperature is verified by comparison with the collocated observation from the Advanced Technology Microwave Sounder(ATMS).The simulation shows that the terahertz brightness temperature of the ice clouds is affected by ice and graupel particles with different properties,e.g.,for a cloud with IWP of 300 g/m2and GWP of 300 g/m2,the brightness temperature depression due to ice and graupel particles at 183.31+2.0 GHz are 2.28 K and12.26 K respectively,and at 183.31+7.0 GHz are 6.30 K and 62.37 K respectively.Then,the impacts of the observation geometry and the parameters of the ice and graupel particles,including mass equivalent spherical diameter(Dme),Ice Water Path(IWP),and Graupel Water Path(GWP),on the terahertz brightness temperature were quantitatively analyzed.Finally,the optimal sounding frequency bands(183 GHz,243 GHz,325 GHz,448 GHz,664 GHz,and 874 GHz)and observation angle(53°±5°)of the terahertz ice cloud sounding instrument were derived based on the sensitivity analysis of the terahertz brightness temperature,Jacobian matrix,and trace gas absorption.The calculated terahertz brightness temperature spectrums indicate that the ice and graupel particles need to be considered in the terahertz ice cloud remote sensing.The terahertz radiation and scattering characteristics of the ice and graupel particles studied in this work can provide technical support for the development of the future terahertz ice cloud sounding instrument.