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In the current operational four‐dimensional variational (4DVar) data assimilation (DA) system of the Global Forecast System developed by the China Meteorology Administration (CMA‐GFS), all microwave observation data in cloud and precipitation regions are discarded during pre‐processing. This study implemented a Pseudo All‐Sky DA (referred to as PAS‐DA hereafter) subsystem in the operational cycle version of the CMA‐GFS (CMA‐GFS v3.2). The term "pseudo" in this study indicates that the Jacobians of brightness temperature with respect to hydrometeors from the adjoint radiative transfer model were temporarily neglected. Specifically, a liquid hydrometeor sensitive channel (23.8 GHz V pol.) of the MicroWave Radiation Imager on the platform of FengYun‐3D (FY3D‐MWRI) was selected to assess the impact of the all‐sky assimilation approach using the PAS‐DA subsystem. In the observation error model, we proposed a new cumulative distribution function (CDF) bias correction method for the cloud proxy in consideration of large discrepancies between the probability density functions (PDFs) of the observed‐cloud‐proxy and simulated‐cloud‐proxy (known as "cloud bias"). Results of single‐observation experiments justified that the present PAS‐DA subsystem could extend analysis increments to cloud regions, meanwhile correcting the errors of humidity analysis according to the mislocation of cloud distributions. In addition, the forecast experiments that were run for 1 month of the 6 hr PAS‐DA cycle in July and August 2021 demonstrate obvious superiority of the PAS‐DA over the current operational clear‐sky DA cycle: (a) root‐mean‐square errors (RMSEs) of humidity analysis were reduced by about 10% in the tropics, (b) significant improvements in humidity forecasts could be sustained for 96 hr and (c) many other forecast scores in the tropics and Southern Hemisphere also benefit from the PAS‐DA approach. Therefore, the PAS‐DA could be used for all‐sky assimilation studies and particularly for understanding how the all‐sky assimilation approach works, although the PAS‐DA serves as a transition scheme from the clear‐sky to "true" all‐sky DA. [ABSTRACT FROM AUTHOR]

In this study, we attempted to address the frozen hydrometeor habit vertical inhomogeneity (FHHVI) effects on microwave radiative transfer simulations. For simplification, we focused on a "two habits and two layers" FHHVI scheme in which two different ice habits can be assumed to be above and below a fixed habit boundary line of 325 hPa as suggested by studies of tropical cyclone vertical structure. The aforementioned FHHVI scheme was implemented into a radiative transfer for the Television Infrared Observation Satellite (TIROS) operational vertical sounder (RTTOV), which allows for close examination of the FHHVI effect on the radiative transfer process. Four scenarios (thin plate, dendrite, thin plate over dendrite, and dendrite over thin plate) were considered in the simulations at frequencies ranging from 10 to 183 GHz. We found that FHHVI has an obvious impact on the brightness temperature (BT) simulations. For real case studies, we considered the 2018 tropical cyclone Jebi in the Western Pacific basin. The hydrometeor profiles forecasted by the Global/Regional Assimilation and Prediction System (GRAPES) regional numerical weather prediction (NWP) model were used, and the simulated results were compared with observations from the instruments aboard on the Fengyun‐3D satellite, which recorded 61 overpasses during the life span of typhoon Jebi. We found that the overall performance of the BT simulations over multichannels could be improved by considering FHHVI effects. Specifically, the total penalty values that quantify the spectral consistency were 63.05 (dendrite), 43.88 (thin plate), 46.23 (dendrite over thin plate), and 39.03 (thin plate over dendrite), respectively. Key Points: The frozen hydrometeor habit vertical inhomogeneity (FHHVI) has significant impacts on microwave radiative transfer simulationsIncorporating the FHHVI into the radiative transfer could improve the spectrum consistency when compared with Fengyun‐3D observationsFHHVI is believed promising in the way forward for improving radiative transfer simulations with relevant applications in data assimilation [ABSTRACT FROM AUTHOR]

Forward radiative transfer models (RTM) are an indispensable tool for quantitative applications of satellite radiometers, e.g., for data calibration, instrument development, retrieval, and so on. In this study, we develop an accurate and efficient RTM for radiometers onboard Fengyun satellites, namely FYRTM (RTM for Fengyun Radiometers). Correlated k-distribution models are developed to improve the computational efficiency for gas absorption, and the effects of cloud and aerosol multiple scattering and emission are accelerated with pre-computed look-up tables. FYRTM is evaluated with a rigorous simulation based on discrete ordinate radiative transfer model (DISORT) as well as a popular fast forward model, i.e., the Community Radiative Transfer Model (CRTM). Results indicate that FYRTM-based simulations are two to three orders of magnitudes faster than the DISORT-based simulations. Compared to the rigorous model, FYRTM relative errors are within 2% at solar channels, and brightness temperatures (BT) differences are within 1 K at infrared channels. Compared with CRTM, FYRTM is computationally similar at solar channels, but three times faster at infrared channels. Furthermore, simulated reflectances/BTs using FYRTM are in a good agreement with the satellite observations. Overall, FYRTM is capable to simulate satellite observations under different atmospheric conditions, and can be extended to other radiometers onboard the Fengyun satellites (both geostationary and polar-orbiting satellites). It is expected to play important roles in future applications with Fengyun observations. [ABSTRACT FROM AUTHOR]

Sea salt aerosols were assumed to be homogeneous spheres in most climate models. However, observations show that sea salt particles are inhomogeneous during the deliquesce and crystallization processes. Using a two‐layer sphere model, we found that backscattering of solar radiation associated with sea salts is underestimated in homogeneous sea salt models. The Community Earth System Model is used to assess the inhomogeneity effect on direct radiative forcing. For global climate model simulation, the inhomogeneity effect on radiative transfer is found to be small as high RHs over widespread oceans suppress the impact of inhomogeneity. On the other hand, in coastal regions, the inhomogeneity effect can cause up to 10% radiative forcing difference of sea salt aerosols. The inhomogeneity effect of sea salt aerosols has to be considered over coastal regions, especially in the Mediterranean, Australia, and the eastern coast of South America. Plain Language Summary: In a humid environment, solid sea salt particles can be coated with water. Sea salt aerosol has a cooling effect on the Earth, but coated sea salt has even stronger cooling effect. However, in most climate models, coated sea salt is not considered. This study confirms that coated sea salt aerosols may not significantly affect the global radiation budget, but they should definitely be considered in the coastal climate where large changes in relative humidity are evident. Key Points: For some range of relative humidity (50‐80%), sea salt is assumed to be an inhomogeneous two‐layer sphereThe inhomogeneity of sea salt decreases the asymmetry factor and increases backscattering of solar radiationInhomogeneity has a relatively large impact (up to 10%) on sea salt radiative forcing over coastal waters [ABSTRACT FROM AUTHOR]

To derive the bulk radiative properties of ice clouds, aircraft contrails and snow grains, which are fundamental to atmospheric radiative transfer calculations in downstream applications, it is necessary to accurately simulate the scattering of light by individual ice particles. An ice particle optical property database reported in 2013 (hereafter, TAMUice2013) is updated (hereafter, TAMUice2016) to incorporate recent advances in computation of the optical properties of nonspherical particles. Specifically, we employ the invariant imbedding T-matrix (II-TM) method to compute the optical properties of particles with small to moderate size parameters. Both versions use the Improved Geometric Optics Method (IGOM) to compute the optical properties of large ice crystals, but TAMUice2016 improves the treatment of inhomogeneous waves inside the scattering particles in the case where ice is absorptive such as at infrared wavelengths. To bridge the gap between the extinction efficiencies computed from the II-TM and the IGOM, TAMUice2016 includes spectrally dependent higher order terms of the edge effect in addition to the first order counterpart considered in TAMUice2013. Furthermore, the differences between TAMUice2013 and TAMUice2016 are quantified with respect to the computation of the bulk optical properties of ice clouds. [ABSTRACT FROM AUTHOR]

Understanding the inherent optical properties (IOPs) of coccoliths and coccolithophores is important in oceanic radiative transfer simulations and remote sensing implementations. In this study, the invariant imbedding T-matrix method (II-TM) is employed to investigate the IOPs of coccoliths and coccolithophores. The Emiliania huxleyi ( Ehux ) coccolith and coccolithophore models are built based on observed biometric parameters including the eccentricity, the number of slits, and the rim width of detached coccoliths. The calcification state that specifies the amount of calcium of a single coccolith is critical in the determination of the size–volume/mass relationship (note, the volume/mass of coccoltihs at different calcification states are different although the diameters are the same). The present results show that the calcification state, namely, under-calcification, normal-calcification, or over-calcification, significantly influences the backscattering cross section and the phase matrix. Furthermore, the linear depolarization ratio of the light scattered by coccoliths is sensitive to the degree of calcification, and provides a potentially valuable parameter for interpreting oceanic remote sensing data. The phase function of an ensemble of randomly oriented coccolithophores has a similar pattern to that of individual coccoliths, but the forward scattering is dominant in the coccolithophores due to the large geometric cross sections. The linear depolarization ratio associated with coccolithophores is found to be larger than that for coccoliths as polarization is more sensitive to multiple scattering than the phase function. The simulated coccolithophore phase matrix numerical results are compared with laboratory measurements. For scattering angles larger than 100°, an increase of the phase function with respect to the scattering angle is confirmed based on the present coccolithophore model while the spherical approximation fails. [ABSTRACT FROM AUTHOR]

Presented is a review of the radiative properties of ice clouds from three perspectives: light scattering simulations, remote sensing applications, and broadband radiation parameterizations appropriate for numerical models. On the subject of light scattering simulations, several classical computational approaches are reviewed, including the conventional geometric-optics method and its improved forms, the finite-difference time domain technique, the pseudo-spectral time domain technique, the discrete dipole approximation method, and the T-matrix method, with specific applications to the computation of the single-scattering properties of individual ice crystals. The strengths and weaknesses associated with each approach are discussed. With reference to remote sensing, operational retrieval algorithms are reviewed for retrieving cloud optical depth and effective particle size based on solar or thermal infrared (IR) bands. To illustrate the performance of the current solar- and IR-based retrievals, two case studies are presented based on spaceborne observations. The need for a more realistic ice cloud optical model to obtain spectrally consistent retrievals is demonstrated. Furthermore, to complement ice cloud property studies based on passive radiometric measurements, the advantage of incorporating lidar and/or polarimetric measurements is discussed. The performance of ice cloud models based on the use of different ice habits to represent ice particles is illustrated by comparing model results with satellite observations. A summary is provided of a number of parameterization schemes for ice cloud radiative properties that were developed for application to broadband radiative transfer submodels within general circulation models (GCMs). The availability of the single-scattering properties of complex ice habits has led to more accurate radiation parameterizations. In conclusion, the importance of using nonspherical ice particle models in GCM simulations for climate studies is proven. [ABSTRACT FROM AUTHOR]

Abstract: A fundamental problem in remote sensing and radiative transfer simulations involving ice clouds is the ability to compute accurate optical properties for individual ice particles. While relatively simple and intuitively appealing, the conventional geometric-optics method (CGOM) is used frequently for the solution of light scattering by ice crystals. Due to the approximations in the ray-tracing technique, the CGOM accuracy is not well quantified. The result is that the uncertainties are introduced that can impact many applications. Improvements in the Invariant Imbedding T-matrix method (II-TM) and the Improved Geometric-Optics Method (IGOM) provide a mechanism to assess the aforementioned uncertainties. The results computed by the II-TM+IGOM are considered as a benchmark because the II-TM solves Maxwell׳s equations from first principles and is applicable to particle size parameters ranging into the domain at which the IGOM has reasonable accuracy. To assess the uncertainties with the CGOM in remote sensing and radiative transfer simulations, two independent optical property datasets of hexagonal columns are developed for sensitivity studies by using the CGOM and the II-TM+IGOM, respectively. Ice cloud bulk optical properties obtained from the two datasets are compared and subsequently applied to retrieve the optical thickness and effective diameter from Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. Additionally, the bulk optical properties are tested in broadband radiative transfer (RT) simulations using the general circulation model (GCM) version of the Rapid Radiative Transfer Model (RRTMG) that is adopted in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM, version 5.1). For MODIS retrievals, the mean bias of uncertainties of applying the CGOM in shortwave bands (0.86 and 2.13μm) can be up to 5% in the optical thickness and as high as 20% in the effective diameter, depending on cloud optical thickness and effective diameter. In the MODIS infrared window bands centered at 8.5, 11, and 12μm, biases in the optical thickness and effective diameter are up to 12% and 10%, respectively. The CGOM-based simulation errors in ice cloud radiative forcing calculations are on the order of 10Wm−2. [Copyright &y& Elsevier]

Abstract: The extended boundary condition method (EBCM) and invariant imbedding method (IIM) are two fundamentally different T-matrix methods for the solution of light scattering by nonspherical particles. The standard EBCM is very efficient but encounters a loss of precision when the particle size is large, the maximum size being sensitive to the particle aspect ratio. The IIM can be applied to particles in a relatively large size parameter range but requires extensive computational time due to the number of spherical layers in the particle volume discretization. A numerical combination of the EBCM and the IIM (hereafter, the EBCM+IIM) is proposed to overcome the aforementioned disadvantages of each method. Even though the EBCM can fail to obtain the T-matrix of a considered particle, it is valuable for decreasing the computational domain (i.e., the number of spherical layers) of the IIM by providing the initial T-matrix associated with an iterative procedure in the IIM. The EBCM+IIM is demonstrated to be more efficient than the IIM in obtaining the optical properties of large size parameter particles beyond the convergence limit of the EBCM. The numerical performance of the EBCM+IIM is illustrated through representative calculations in spheroidal and cylindrical particle cases. [Copyright &y& Elsevier]

Nonspherical ice crystal optical properties are of fundamental importance to atmospheric radiative transfer through an ice cloud and the remote sensing of its properties. In practice, the optical properties of individual ice crystals need to be integrated over particle size distributions to derive the bulk optical properties of ice clouds. Given a particle size distribution represented in terms of size bins, the conventional approach uses the microphysical and optical properties of ice crystals at the bin centers as approximations to the bin-averaged values. However, errors are incurred when the size bins are large. To reduce the potential errors, a kernel technique is utilized to calculate the bulk optical properties of ice clouds by computing the bin-averaged values instead of using the bin-center values. Comparisons between the solutions based on the conventional method and the kernel technique for different numbers of size bins from in situ measurements demonstrate that the results computed from the kernel technique are more accurate. The present study illustrates that, for a given size distribution, 40 or more size bins should be used to calculate the bulk optical properties of ice clouds by the conventional method. Although the accuracy of bulk-scattering properties can be improved by using fine bin resolutions in the single-scattering property computation, the advantage of using a precomputed database of scattering kernels allows efficient computation of ice cloud bulk optical properties without losing the accuracy. [ABSTRACT FROM AUTHOR]

• The optical properties of dust particles associated with three sets of refractive indices were compared in a spectrum region (650–1135 cm−1). • The refractive indices have a clear impact on the spectrum slope of brightness temperature. • The refractive index impacts on bright temperature are essentially associated with the height of dust layer and the aerosol optical thickness. In this paper, we modeled dust impacts on atmospheric radiative transfer in the thermal infrared region of 650–1135 cm−1 by using the radiative transfer for TOVs model (RTTOV). We assessed three sets of Asian dust refractive indices (the Volz refractive index implemented in the RTTOV as default, and the Gobi and Taklamakan refractive indices) for brightness temperature simulations. The US 76 standard profile was employed to represent the atmospheric status. Three scenarios of high (532.6~575.2 hPa), middle (661~703.6 hPa) and low (786.3~825.8 hPa) dust layers and four cases of aerosol optical thickness (AOT) (0.1, 0.5, 1.0, and 1.5 at 1000 cm−1) were considered in the simulations. It was found that the refractive indices have a clear impact on simulating the values and the spectral slope of brightness temperature in the mid-thermal infrared band. Brightness temperature difference (BTD) for different refractive indices (Gobi/Taklamakan–Volz) increases with the height of dust layer and the AOT. The BTD reaches ~4 K for the high dust layer (532.6~575.2 hPa) and the AOT of 1.5. Most BTDs (0.5–4 K) are much larger than the noise-equivalent temperature difference of high-spectral resolution infrared sounders. Even for the AOT of 0.1, BTDs for different refractive indices could be close to or larger than 0.2 K. This study emphasizes the requirement of accurate refractive indices according to dust origins for improving brightness temperature simulations and provides valuable references for potential improvement of dust retrievals. [ABSTRACT FROM AUTHOR]

• Impacts of cloud scattering properties on FY-3D HIRAS simulations are assessed in ARMS and CRTM. • ARMS and CRTM simulated brightness temperatures are compared with HIRAS observations to provide quantitative evaluation. • Spherical ice cloud approximation has been found to be not appropriate for ARMS model simulations. Cloudy sky spectral radiance at the top of the atmosphere has always been an important while difficult variable to simulate for fast radiative transfer models. In this paper, we focus on examining the impacts of cloud scattering properties on the spectral radiance signature of the High-spectral Resolution Infrared Atmospheric Sounder (HIRAS) onboard the Fengyun-3D (FY-3D) satellite by using the Advanced Radiative transfer Modeling System (ARMS) and the Community Radiative Transfer Model (CRTM). Cloud scattering properties used in the radiative transfer models are critical for modeling the spectral radiance under cloudy sky, which involves choices of appropriate cloud particle models and particle size distributions, etc. Multiple FY-3D HIRAS observations over Southern China and Southeast Asia with ice or liquid water cloud cover on 6 May 2018 are examined, respectively. Vertical atmospheric profiles are derived from the Modern-Era Retrospective analysis for Research and Applications, Version 2 reanalysis product. Cloud property retrievals from the Moderate Resolution Imaging Spectroradiometer are used. Cloud scattering property parameterization schemes based on spherical and nonspherical cloud particle shapes are implemented for liquid water and ice clouds in ARMS and CRTM, respectively. Results show that both ARMS and CRTM can well simulate the radiance at the HIRAS spectral ranges under liquid water cloud condition as compared with the HIRAS observation with mean absolute error (MAE) of brightness temperature of less than 1 K. However, for ice cloud conditions, ARMS model using assumed spherical ice properties exhibits large biases between simulation and observation. CRTM with nonspherical ice properties using 16-stream approximation shows MAE less than 1 K and MAE of about 1 K using 2-stream approximation. [ABSTRACT FROM AUTHOR]

A discrete random medium is an object in the form of a finite volume of a vacuum or a homogeneous material medium filled with quasi-randomly and quasi-uniformly distributed discrete macroscopic impurities called small particles. Such objects are ubiquitous in natural and artificial environments. They are often characterized by analyzing theoretically the results of laboratory, in situ , or remote-sensing measurements of the scattering of light and other electromagnetic radiation. Electromagnetic scattering and absorption by particles can also affect the energy budget of a discrete random medium and hence various ambient physical and chemical processes. In either case electromagnetic scattering must be modeled in terms of appropriate optical observables, i.e., quadratic or bilinear forms in the field that quantify the reading of a relevant optical instrument or the electromagnetic energy budget. It is generally believed that time-harmonic Maxwell’s equations can accurately describe elastic electromagnetic scattering by macroscopic particulate media that change in time much more slowly than the incident electromagnetic field. However, direct solutions of these equations for discrete random media had been impracticable until quite recently. This has led to a widespread use of various phenomenological approaches in situations when their very applicability can be questioned. Recently, however, a new branch of physical optics has emerged wherein electromagnetic scattering by discrete and discretely heterogeneous random media is modeled directly by using analytical or numerically exact computer solutions of the Maxwell equations. Therefore, the main objective of this Report is to formulate the general theoretical framework of electromagnetic scattering by discrete random media rooted in the Maxwell–Lorentz electromagnetics and discuss its immediate analytical and numerical consequences. Starting from the microscopic Maxwell–Lorentz equations, we trace the development of the first-principles formalism enabling accurate calculations of monochromatic and quasi-monochromatic scattering by static and randomly varying multiparticle groups. We illustrate how this general framework can be coupled with state-of-the-art computer solvers of the Maxwell equations and applied to direct modeling of electromagnetic scattering by representative random multi-particle groups with arbitrary packing densities. This first-principles modeling yields general physical insights unavailable with phenomenological approaches. We discuss how the first-order-scattering approximation, the radiative transfer theory, and the theory of weak localization of electromagnetic waves can be derived as immediate corollaries of the Maxwell equations for very specific and well-defined kinds of particulate medium. These recent developments confirm the mesoscopic origin of the radiative transfer, weak localization, and effective-medium regimes and help evaluate the numerical accuracy of widely used approximate modeling methodologies. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a user-friendly database software package of the single-scattering properties of individual dust-like aerosol particles for application to radiative transfer calculations in a spectral region from ultraviolet (UV) to far-infrared (far-IR). To expand the degree of morphological freedom of the commonly used spheroidal and spherical models, tri-axial ellipsoids were assumed to be the overall shape of dust-like aerosol particles. A combination of four computational methods, including the Lorenz–Mie theory, the T-matrix method, the discrete dipole approximation, and an improved geometric optics method, was employed to compute the phase matrix, extinction efficiency and single-scattering albedo of ellipsoids with various aspect ratios and sizes. The scattering property database was developed for 42 particle shapes specified in terms of two aspect ratios, 69 refractive indices and 471 size parameters. Additionally, accompanying software, based on interpolation, was developed to provide the single-scattering properties for user-specified aspect ratios, refractive indices and size parameters. The software package allows for the derivation of the bulk optical properties for a given distribution of particle microphysical parameters (i.e., refractive index, size parameter and two aspect ratios). The array-oriented single-scattering property data sets are stored in the NetCDF format. [Copyright &y& Elsevier]

• An advanced radiative transfer modeling system has been developed for satellite data assimilation and remote sensing applications. • It focuses on the radiative transfer components for applications to Fengyun satellites. • The accuracy is validated against other model results and the measurements. An advanced radiative transfer modeling system (ARMS) has been developed for satellite data assimilation and remote sensing applications. It inherits the capability of other fast radiative transfer models developed mainly for US and European satellite programs but focuses on the radiative transfer components that are specific for the assimilation of the Fengyun satellites and those sensors not included in existing models. The ARMS forward operator includes a fast transmittance module, a new particle absorption and scattering look-up table, surface emissivity and polarimetric radiative transfer solver. The particle optical properties of aerosols and ice clouds are calculated based on a super-spheroidal T-matrix model. The polarimetric two-stream approximation is adopted as the preferred solver to simulate the Stokes vector and their Jacobians. The simulations of ARMS are compared with those of other models and observations of Fengyun satellites, respectively. The comparison shows close agreement between ARMS and other existing fast radiative transfer models. Therefore, ARMS has robust simulation capabilities for existing satellite sensors. [ABSTRACT FROM AUTHOR]