Abstract
The Geometrical Optics (GO) approach and the FAST Emissivity Model (FASTEM) are widely used to estimate the surface radiative components in atmospheric radiative transfer simulations, but their applications are limited in specific conditions. In this study, a two-scale reflectivity model (TSRM) and a two-scale emissivity model (TSEM) are developed from the two-scale roughness theory. Unlike GO which only computes six non-zero elements in the reflectivity matrix, The TSRM includes 16 elements of Stokes reflectivity matrix which are important for improving radiative transfer simulation accuracy in a scattering atmosphere. It covers the frequency range from L- to W-bands. The dependences of all TSRM elements on zenith angle, wind speed, and frequency are derived and analyzed in details. For a set of downwelling radiances in microwave frequencies, the reflected upwelling brightness temperature (BTs) are calculated from both TSRM and GO and compared for analyzing their discrepancies. The TSRM not only includes the effects of GO but also accounts for the small-scale Bragg scattering effect in an order of several degrees in Kelvins in brightness temperature. Also, the third and fourth components of the Stokes vector can only be produced from the TSRM. For the emitted radiation, BT differences in vertical polarization between a TSEM and FASTEM are generally less than 5 K when the satellite zenith angle is less than 40°, whereas those for the horizontal component can be quite significant, greater than 20 K.
摘 要
几何光学(GO)方法和快速发射率模型(FASTEM)被广泛用于估算大气辐射传输模拟中的地表辐射,但其应用在特定条件下受到限制。在这项研究中,从海洋粗糙度理论出发,建立了一个双尺度反射率(TSRM)和发射率模型(TSEM)。与GO只计算反射率矩阵中的6个非零元素不同,TSRM包括所有16个有效元素, 用于提高大气辐射传输模拟精度。特别是,TSRM是首个覆盖从L到W波段频率范围的反射率矩阵模型。本文详细分析了所有TSRM元素对天顶角、风速和频率的依赖性。对于一组微波下行辐亮度,通过TSRM和GO计算出反射的上行亮温(BTs),并对其进行分析比较,以突显它们之间的差异。TSRM不仅包括了GO的影响,而且还考虑了小尺度的布拉格散射效应。在亮温幅度上,两种模式可导致几度的差异。另外,TSRM可以模拟生产斯托克斯矢量第三和第四分量。对于发射辐射,当卫星天顶角小于40°时,TSEM和FASTEM之间的垂直极化BT差异一般小于5K,而水平极化的差异较大,大于20K。
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Acknowledgements
This study is funded by the National Key Research and Development Program (Grant No. 2022YFC3004200), the National Key Research and Development Program of China (Grant No. 2021YFB3900400), Hunan Provincial Natural Science Foundation of China (Grant No. 2021JC0009), and the National Natural Science Foundation of China (Grant No. U2142212).
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Article Highlights
• TSRM is a generic microwave reflectivity matrix model that covers the frequency range from L-band to W-band.
• TSRM allows for computing the reflected upwelling radiances of four Stokes components.
• TSEM is the generic microwave emissivity model and allows for computing the emitted upwelling radiances of four Stokes components.
This paper is a contribution to the special topic on Applications of the New Generation of Fast Radiative Transfer Models (ARMS) in Satellite Data Assimilation and Remote Sensing.
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He, L., Weng, F. Improved Microwave Ocean Emissivity and Reflectivity Models Derived from Two-Scale Roughness Theory. Adv. Atmos. Sci. 40, 1923–1938 (2023). https://doi.org/10.1007/s00376-023-2247-y
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DOI: https://doi.org/10.1007/s00376-023-2247-y