Efficient radiative transfer model for thermal infrared brightness temperature simulation in cloudy atmospheres.

An efficient radiative transfer model (ERTM) is developed to simulate thermal infrared brightness temperatures observed by the Advanced Himawari Imager (AHI) in this study. The ERTM contains an alternate mapping correlated k-distribution (AMCKD) scheme, a parameterization for cloud optical property, and a rapid infrared radiative transfer scheme. The AMCKD is employed to calculate the gaseous absorption in the inhomogeneous thermodynamic atmosphere. The optical properties of clouds are parameterized by the effective length for ice clouds based on the Voronoi model, and by the effective radius for water clouds based on the Lorenz-Mie theory. The adding method of four-stream discrete ordinates method (4DDA) is extended to be able to calculate the thermal infrared radiative intensity varying with the zenith angle in ERTM. The efficiency and accuracy of ERTM are evaluated by comparing with the benchmark model which is composed of discrete ordinate radiative transfer (DISORT) and line-by-line radiative transfer model (LBLRTM). Under the standard atmospheric profiles, the root mean square error (RMSE) of simulated brightness temperatures reaches a maximum of 0.21K at the B16 (13.28 µm) channel of AHI. The computational efficiency of ERTM is approximately five orders of magnitude higher than that of the benchmark model. Moreover, the simulated brightness temperatures by ERTM are highly consistent with the rigorous results and AHI observations in the application to the Typhoon Mujigae case.

Physical interpretation of gray cloud observed from airplanes.

When obliquely observed from an airplane, gray clouds near the horizon are sometimes observed to overlap with white clouds. Photographic observation from an airplane and simulations using a three-dimensional radiative transfer model are conducted to understand why such clouds appear gray. From observations, the brightness depression rate of gray clouds relative to surrounding whitish clouds is about 25%, whereas in simulations, it is as high as about 30%. Conditions necessary for the observation of gray clouds are as follows: (1) two clouds at different altitudes do not overlap, but the higher cloud overlaps with the lower cloud along the line of sight when these clouds are observed in near-horizontal direction, and (2) the higher cloud is optically thin in the vertical direction, but optically thick along the line of sight.

Estimation of spectral distribution of sky radiance using a commercial digital camera.

Methods for estimating spectral distribution of sky radiance from images captured by a digital camera and for accurately estimating spectral responses of the camera are proposed. Spectral distribution of sky radiance is represented as a polynomial of the wavelength, with coefficients obtained from digital RGB counts by linear transformation. The spectral distribution of radiance as measured is consistent with that obtained by spectrometer and radiative transfer simulation for wavelengths of 430-680 nm, with standard deviation below 1%. Preliminary applications suggest this method is useful for detecting clouds and studying the relation between irradiance at the ground and cloud distribution.

The spectral signature of cloud spatial structure in shortwave irradiance.

In this paper, we used cloud imagery from a NASA field experiment in conjunction with three-dimensional radiative transfer calculations to show that cloud spatial structure manifests itself as a spectral signature in shortwave irradiance fields - specifically in transmittance and net horizontal photon transport in the visible and near-ultraviolet wavelength range. We found a robust correlation between the magnitude of net horizontal photon transport (H) and its spectral dependence (slope), which is scale-invariant and holds for the entire pixel population of a domain. This was surprising at first given the large degree of spatial inhomogeneity. We prove that the underlying physical mechanism for this phenomenon is molecular scattering in conjunction with cloud spatial structure. On this basis, we developed a simple parameterization through a single parameter ε, which quantifies the characteristic spectral signature of spatial inhomogeneities. In the case we studied, neglecting net horizontal photon transport leads to a local transmittance bias of ±12-19 %, even at the relatively coarse spatial resolution of 20 km. Since three-dimensional effects depend on the spatial context of a given pixel in a nontrivial way, the spectral dimension of this problem may emerge as the starting point for future bias corrections.

Photographic observation and optical simulation of a pollen corona display in Japan.

Brightness and chromaticity profiles were extracted from a vivid solar corona image taken with a digital camera in Sendai, Japan, to compare with a radiative transfer simulation applying Lorenz-Mie theory and single-scattering approximation. The comparison revealed suspended particles having a narrow particle size distribution peaking at radius 14.5 µm. Presumably, pollen of an indigenous coniferous tree, the cryptomeria (Cryptomeria japonica), is responsible for the corona display. The extracted brightness and chromaticity profiles are reproduced well by assuming the presence of a water soluble aerosol and dust in addition to the pollen. We find that photographic analysis of corona displays, similar to that used to measure cloud particle size, is applicable to estimating pollen particle size distribution and column number density.