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William O’Hirok and Catherine Gautier

Abstract

A new Monte Carlo–based three-dimensional (3D) radiative transfer model of high spectral and spatial resolution is presented. It is used to investigate the difference in broadband solar radiation absorption, top-of-the-atmosphere upwelling, and surface downwelling solar radiation in a cloudy atmosphere between 3D and 1D calculations. Spatial variations of these same radiation components (absorption, upwelling, and downwelling), together with pathlength distributions, are analyzed for different wavelengths to describe the main physical mechanisms at work.

The model contains all of the important atmospheric and surface radiative constituents. It includes Rayleigh scattering, absorption and scattering by both cloud droplets and aerosols, and absorption by the major atmospheric gases. Inputs include 3D liquid water fields, aerosols, and gas distribution, type, and concentrations.

Using satellite imagery of a tropical cloud field as input, model results demonstrate that various plane-parallel (1D) assumptions can underestimate atmospheric absorption when compared to 3D computations. This discrepancy is caused by a complex interaction of gaseous absorption, cloud droplet absorption, and the solar zenith angle. Through a sensitivity analysis, the authors demonstrate that the most important factor is the morphology of the cloud field, followed by the vertical stratification of water vapor.

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William O’Hirok and Catherine Gautier

Abstract

In this second part of a two-part paper, the spectral response of the interaction between gases, cloud droplets, and solar radiation is investigated using a Monte Carlo-based three-dimensional (3D) radiative transfer model with a spectral resolution of 0.005 μm. Spectrally resolved albedo at the top of the atmosphere, transmission to the surface, and absorption throughout the atmospheric column between 0.25 and 4.0 μm are computed and compared for 3D and independent pixel approximations at various solar zenith angles.

Analysis of a tropical cloud field shows that for overhead sun, the effects of cloud morphology reduce absorption in the UV and increase absorption in the water-vapor absorption bands. At steep solar zenith angles, the enhanced absorption shifts spectrally to the near-infrared atmospheric windows, where increases up to 50% in absorption by cloud droplets are obtained. While there is no change in absorption in the visible, the 3D effect is shown to produce false estimates of cloudy-column absorption based on residual net fluxes in this spectral region. Alterations to the albedo and enhanced absorption in the 3D computations generate reduced estimates in remotely sensed cloud optical thickness and overestimates in cloud-droplet size distribution. Finally, a band ratio of 0.94 to 1.53 μm downwelling irradiance at the surface is proposed as a possible measure of 3D enhanced atmospheric absorption.

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William O’Hirok and Catherine Gautier

Abstract

Within general circulation models (GCMs), domain average radiative fluxes are computed using plane-parallel radiative transfer algorithms that rely on cloud overlap schemes to account for clouds not resolved at the horizontal resolution of a grid cell. These parameterizations have a strong statistical approach and have difficulty being applied well to all cloudy conditions. A more physically based superparameterization has been developed that captures subgrid cloud variability using an embedded cloud system resolving model (CSRM) within each GCM grid cell. While plane-parallel radiative transfer computations are generally appropriate at the scale of a GCM grid cell, their suitability for the much higher spatially resolved CSRMs (2–4 km) is unknown because they ignore photon horizontal transport effects. The purpose of this study is to examine the relationship between model horizontal resolution and 3D radiative effects by computing the differences between independent column approximations (ICA) and 3D Monte Carlo estimates of shortwave surface irradiance and atmospheric heating rate.

Shortwave radiative transfer computations are performed on a set of six 2D fields composed of stratiform and convective liquid water and ice clouds. To establish how 3D effects vary with the size of a grid cell, this process is repeated as the model resolution is progressively degraded from 200 to 20 km. For shortwave surface irradiance, the differences between the 3D and ICA results can reach 500 W m−2. At model resolutions of between 2.0 and 5.0 km the difference for almost all columns is reduced to a maximum of ±100 W m−2. For atmospheric heating rates assessed at the level of individual model cells, 3D radiative effects can approach a maximum value of ±1.2 K h−1 when the horizontal column size is 200 m. However, between model resolutions of 2.0 and 5.0 km, 3D radiative effects are reduced to well below ±0.1 K h−1 for a large majority of the cloudy cells. While this finding seems to bode well for the CSRM, the results ultimately need to be understood within the context of how 3D radiative effects impact not only heating rates but also cloud dynamics.

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