Hironobu Iwabuchi

Efficient Monte Carlo Methods for Radiative Transfer Modeling

Abstract Demands for Monte Carlo radiative transfer modeling have grown with the increase in computational power in recent decades. This method provides realistic simulations of radiation processes for various types of application, including radiation budgets in cloudy conditions and remote measurements of clouds, aerosols, and gases. Despite many advantages, such as explicit treatment of three-dimensional radiative transfer, issues of numerical efficiency can make the method intractable, especially in radiance calculations. The commonly used local estimation method requires computationally intensive ray tracing at each collision. Furthermore, the realistic phase function of Mie scattering by cloud and aerosol particles has very sharp peaks in the forward direction. Radiance computations by Monte Carlo methods are inefficient for such spiky phase functions because of significant noise. Moreover, in optically thin regions, sampling of radiance contributions is so rare that long computing times are required...

Effects of Cloud Horizontal Inhomogeneity on the Optical Thickness Retrieved from Moderate-Resolution Satellite Data

Abstract Cloud remote sensing techniques are conventionally based on the independent pixel approximation (IPA). Here, three-dimensional (3D) radiative effects on IPA-based retrieved optical thickness from a visible-wavelength moderate-resolution (about 1 km) sensor are investigated. A Monte Carlo 3D radiative transfer model and a lognormal spectral cloud model are used to simulate monochromatic radiance reflected from overcast boundary layer cloud. A characterization of statistical properties of the optical thickness by the mean (M) and variance (S2) of the logarithm of the optical thickness is proposed, where S represents a degree of cloud inhomogeneity. Biases in retrieved values of the two parameters with the IPA are defined as ΔM and ΔS2 and attributed to neglect of net horizontal radiative transport in the IPA. Sensitivities of ΔM and ΔS2 are tested with respect to geometrical roughness, M, S, mean geometrical thickness, spectral exponent of optical thickness fluctuation, ground surface reflectance, ...

Physical and optical properties of persistent contrails: Climatology and interpretation

[1] The physical and optical properties of persistent contrails were studied with the measurements made by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar. MODIS data were used to determine the contrail locations on the basis of their artificial shapes easily distinguished from natural cirrus, and the so-identified contrails were analyzed with collocated CALIPSO lidar data. Statistics of the geography, geometry, meteorology, and optical properties are reported for approximately 3400 persistent contrails observed over North America, the North Atlantic Ocean, and Europe. The majority of the detected contrails appear in ice-supersaturated air with temperatures lower than � 40 � C. On average, contrails have significantly larger backscattering coefficients and slightly higher linear depolarization ratios (LDRs) than neighboring cirrus clouds. Depolarization tends to be strong when ice crystals are small, and LDR is approximately 0.4–0.45 for young contrails and contrail cores. The mean LDR for the detected contrails increases with decreasing temperature and is not strongly dependent on the lidar pointing angle. The backscattering properties suggest that contrails are primarily composed of small, randomly oriented ice crystals but may also contain a few horizontally oriented plates. Most contrails are optically thin with a mean (median) optical thickness of approximately 0.19 (0.14); however, optically thicker contrails do exist and tend to occur in warmer and more humid ambient air. The mean value and range of the observed LDR data are consistent with theoretical predictions based on a mixture of nonspherical ice crystals randomly oriented in the atmosphere. Citation: Iwabuchi, H., P. Yang, K. N. Liou, and P. Minnis (2012), Physical and optical properties of persistent contrails: Climatology and interpretation, J. Geophys. Res., 117, D06215, doi:10.1029/2011JD017020.

Radiative and microphysical properties of cirrus cloud inferred from infrared measurements made by the moderate resolution imaging spectroradiometer (MODIS). Part I: Retrieval method

AbstractAn optimal estimation–based algorithm is developed to infer the global-scale distribution of cirrus cloud radiative and microphysical properties from the measurements made by the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) at three infrared (IR) window bands centered at 8.5, 11, and 12 μm. Cloud-top and underlying surface temperatures, as a priori information, are obtained from the MODIS operational products. A fast-forward model based on semianalytical equations for the brightness temperature is used. The modeling errors in brightness temperature are mainly from the uncertainties in model parameters including surface emissivity, precipitable water, and cloud-base temperature. The total measurement–model errors are well correlated for the three bands, which are considered in the retrieval. The most important factors for the accurate retrieval of cloud optical thickness and the effective particle radius are cloud-top and surface temperatures, whereas model parameter uncertainties con...

Variational Iteration Method for Infrared Radiative Transfer in a Scattering Medium

AbstractA new scheme is proposed for using the variational iteration method (VIM) to solve the problem of infrared radiative transfer in a scattering medium. This scheme allows the zeroth-order solution to be identified as the absorption approximation and the scattering effect is included in the first-order iteration. The upward and downward intensities are calculated separately in VIM, which simplifies the calculation process. By applying VIM to two single-layer scattering media and a full radiation algorithm with gaseous transmission, it is found that VIM is generally more accurate than the discrete-ordinates method (DOM), especially for cirrostratus. Computationally, VIM is slightly faster than DOM in the two-stream case but more than twice as fast in the four-stream case. In view of its high overall accuracy and computational efficiency, VIM is well suited to solving infrared radiative transfer in climate models.

A New Radiative Transfer Method for Solar Radiation in a Vertically Internally Inhomogeneous Medium

AbstractThe problem of solar spectral radiation is considered in a layer-based model, with scattering and absorption parallel to the plane for each medium (cloud, ocean, or aerosol layer) and optical properties assumed to be vertically inhomogeneous. A new radiative transfer (RT) method is proposed to deal with the variation of vertically inhomogeneous optical properties in the layers of a model for solar spectral radiation. This method uses the standard perturbation method to include the vertically inhomogeneous RT effects of cloud and snow. The accuracy of the new inhomogeneous RT solution is investigated systematically for both an idealized medium and realistic media of cloud and snow. For the idealized medium, the relative errors in reflection and absorption calculated by applying the homogeneous solution increase with optical depth and can exceed 20%. However, the relative errors when applying the inhomogeneous RT solution are limited to 4% in most cases. Observations show that stratocumulus clouds a...

Multiple‐Scaling Method for Anisotropic Scattering And Its Applications to Radiance Calculations

Radiative transfer (RT) equation for anisotropic light scattering due to large particles is often solved using so‐called truncation approximation, which accelerates computation by approximating the phase function by a finite series truncated at a limited number of terms or by a geometrically‐truncated function. The approximation produces bias in computed radiance. This study devoted to reduce the bias. It is shown that different truncation approximations can be used for each order of scattering and that accurate radiance can be obtained by using exact phase function for the first‐order scattering and truncated, approximated function for multiple scattering. If the degree of approximation is small for the first three or four scattering events, highly accurate radiance can be calculated even if higher‐order scattering is treated with scaled properties with a smooth phase function. The multiple‐scaling method has been implemented in a new linearized radiative transfer model, which has been developed for the ...

Retrieval of cloud optical thickness and effective radius using multispectral remote sensing and accounting for 3D effects

Understanding spatial and temporal variations in cloud properties is crucial to determine the radiation balance on Earth. Remote sensing from satellites provides valuable information on cloud physical properties at global scales (e.g., Rossow and Schiffer, 1991). Recent Earth-observing sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Global Imager (GLI), have well-designed spectral channels and horizontal resolutions between 250m and 1000 m. Compared to earlier sensors, these sensors allow improved derivations of atmospheric and land surface properties. Operational products include the cloud optical thickness and effective particle radius, which are very useful for studying aerosols’ indirect effects (Radke et al., 1989; Rosenfeld, 2000).

Ice particle morphology and microphysical properties of cirrus clouds inferred from combined CALIOP–IIR measurements†

Ice particle morphology and microphysical properties of cirrus clouds are essential for assessing radiative forcing associated with these clouds. We develop an optimal estimation-based algorithm to infer cirrus cloud optical thickness (COT), cloud effective radius (CER), plate fraction including quasi-horizontally oriented plates (HOPs) and the degree of surface roughness from the Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) and the Infrared Imaging Radiometer (IIR) on the CALIPSO platform. A simple but realistic ice particle model is used, and the relevant bulk optical properties are computed using state-of-the-art light-scattering computational capabilities. Rigorous estimation of uncertainties related to surface properties, atmospheric gases and cloud heterogeneity is performed. The results based on the present method show that COTs are quite consistent with other satellite products, and CERs essentially agree with the other counterparts. A one-month global analysis for April 2007, in which CALIPSO off-nadir angle is 0.3∘, shows that the HOP has significant temperature-dependence and is critical to the lidar ratio when cloud temperature is warmer than −40∘C. The lidar ratio is calculated from the bulk optical properties based on the inferred parameters, showing robust temperature dependence. The median lidar ratio of cirrus clouds is 27–31 sr over the globe.

Cloud Discrimination from Sky Images Using a Clear-Sky Index

AbstractCloud fraction mainly affects the incoming solar irradiance at the surface and is measured with ground-based sky imagers. Although several methods enable cloud discrimination from digital sky images, cloud discrimination methods are still being developed to improve the classification accuracy. This study presents two methods for effective cloud discrimination from digital sky images using a newly defined clear-sky index (CSI). The CSI represents the degree of similarity of the spectral distribution to that expected for clear sky. In the advanced method (AM), the CSI is obtained from red–green–blue (RGB) signals in RAW format by linear transformation by taking into account the solar spectrum at the top of the atmosphere, the ozone transmittance, and the spectral response of the RGB channels. The simplified method (SM) uses digital signals in a JPEG format assuming prescribed color matching functions and atmospheric states. The AM can correctly classify broken gray clouds as cloud and aureole as cle...