Jiangnan Li

Radiative Forcing by Well-Mixed Greenhouse Gases: Estimates from Climate Models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4)

The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO2, CH4, N2O, CFC-11, CFC-12, and the increased H2O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH4 and N2O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.

Assessing 1D Atmospheric Solar Radiative Transfer Models: Interpretation and Handling of Unresolved Clouds

Abstract The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended? One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport ...

Two- and four-stream optical properties for water clouds and solar wavelengths

Presented is a parameterization of the liquid water droplet optical properties for the solar spectrum. Two models are provided: a 4-band model for use in general circulation models and a 31-band model for use in higher spectral resolution investigations. The form of the parameterization is a simple extension of Slingo [1989], and the subdivision of wavelengths into bands is almost identical. The parameterization scheme is for all of the optical properties needed to perform 2- and 4-stream calculations. The 4-band model has the same spectral divisions as Slingo [1989], whereas the 31-band model has finer resolution of the absorbing bands in the near infrared. The parameterized single-scattering optical properties are accurate to within 0.5% compared to exact Mie calculations for both band models over the reff range from 5 to 40 μm. The layer radiative properties, from both two- and four-stream methods, are almost always within 1% accuracy compared to calculations using exact optical properties as inputs. Also, it is shown that using the Henyey-Greenstein phase function to obtain the higher-order moments, used in four-stream calculations, results in substantial error in the layer radiative properties compared to using exact Mie inputs.

Ocean Surface Albedo and Its Impact on Radiation Balance in Climate Models

An analysis of several ocean surface albedo (OSA) schemes is undertaken through offline comparisons and through application in the Canadian Centre for Climate Modelling and Analysis (CCCma) fourthgeneration atmospheric general circulation model (AGCM4). In general, each scheme requires different input quantities to determine the OSA. Common to all schemes is a dependence on the solar zenith angle (SZA). A direct comparison of the SZA dependence of the schemes reveals significant differences in the predicted albedos. Other input quantities include wind speed and aerosol/cloud optical depth, which are also analyzed. An offline one-dimensional radiative transfer model is used to quantitatively study the impact of ocean surface albedo on the radiative transfer process. It is found that, as a function of SZA and wind speed, the difference in reflected solar flux at the top of the atmosphere is in general agreement between OSA schemes that depend on these quantities, with a difference 10 Wm 2 . However, for simpler schemes that depend only on SZA the difference in this flux can approach 10–20 W m 2 . The impact of the different OSA schemes is assessed through multiyear simulations of present-day climate in AGCM4. Five-year means of the reflected clear-sky flux at the top of the atmosphere reveal local differences of up to several watts per meters squared between any of the schemes. Globally, all schemes display a similar negative bias relative to the Earth Radiation Budget Experiment (ERBE) observations. This negative bias is largely reduced by comparison with the recently released Clouds and the Earth’s Radiant Energy System (CERES) data. It is shown that the local upward clear-sky flux at the surface is more sensitive to the OSA formulation than the clear-sky upward flux at the top of atmosphere. It is found that the global energy balance of the model at the top of the atmosphere and at the surface is surprisingly insensitive to which OSA scheme is employed.

Overlap of Solar and Infrared Spectra and the Shortwave Radiative Effect of Methane

Abstract This paper focuses on two shortcomings of radiative transfer codes commonly used in climate models. The first aspect concerns the partitioning of solar versus infrared spectral energy. In most climate models, the solar spectrum comprises wavelengths less than 4 μm with all incoming solar energy deposited in that range. In reality, however, the solar spectrum extends into the infrared, with about 12 W m−2 in the 4–1000-μm range. In this paper a simple method is proposed wherein the longwave radiative transfer equation with solar energy input is solved. In comparison with the traditional method, the new solution results in more solar energy absorbed in the atmosphere and less at the surface. As mentioned in a recent intercomparison of the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) and line-by-line (LBL) radiation models, most climate model radiation schemes neglect shortwave absorption by methane. However, the shortwave radiative forcing at the surface due to CH4 ...

Sea-salt optical properties and GCM forcing at solar wavelengths

The single-scattering optical properties of sea-salt solution particles are parameterised as functions of relative humidity for various dry size distributions at solar wavelengths. The accuracy of the parameterisation is typically within 10% as compared to exact Mie calculations. In addition to the optical properties, the growth of the droplet mass ratio and the effective radius of the size distribution are also parameterised in terms of the relative humidity. Two-band models are presented: a four-band model for use in GCMs for climate studies and a 23-band model for use in higher spectral resolution models. The parameterisation is implemented in the Canadian General Circulation Model GCMIII, and an estimate of the first-order globally and yearly averaged solar direct radiative forcing due to sea-salt is estimated to be −0.15 W/m2 (cooling). The northern hemisphere forcing is estimated to be −0.11 W/m2 and the southern hemisphere is −0.19 W/m2. The monthly trends in the forcing for the two hemispheres are presented and discussed. The sensitivity of the forcing to the treatment of the growth of aerosols in the hysteresis region, where aerosol particles are either dry or supersaturated, is investigated along with other sensitivities.

Estimation of Errors in Two-Stream Approximations of the Solar Radiative Transfer Equation for Cloudy-Sky Conditions

AbstractSolar flux densities and heating rates predicted by a broadband, multilayer δ-Eddington two-stream approximation are compared to estimates from a Monte Carlo model that uses detailed descriptions of cloud particle phase functions and facilitates locally nonzero net horizontal flux densities. Results are presented as domain averages for 256-km sections of cloudy atmospheres inferred from A-Train satellite data: 32 632 samples for January 2007 between 70°S and 70°N with total cloud fraction C > 0.05. The domains are meant to represent grid cells of a conventional global climate model and consist of columns of infinite width across track and Δx ≈ 1 km along track. The δ-Eddington was applied in independent column approximation (ICA) mode, while the Monte Carlo was applied using both Δx → ∞ (i.e., ICA) and Δx ≈ 1 km. Mean-bias errors due to the δ-Eddington’s neglect of phase function details and horizontal transfer, as functions of cosine of solar zenith angle μ0, are comparable in magnitude and have ...

The features of cloud overlapping in Eastern Asia and their effect on cloud radiative forcing

Characteristics of cloud overlap over Eastern Asia are analyzed using a three-year dataset (2007–2009) from the cloud observing satellite CloudSat. Decorrelation depth L*cf is retrieved, which represents cloud overlap characteristics in the simulation of cloud-radiation processes in global climate models. Results show that values of L*cf in six study regions are generally within the range 0–3 km. By categorizing L*cf according to cloud amount in subregions, peak L*cf appears near subregions with cloud amount between 0.6 and 0.8. Average L*cf is 2.5 km. L*cf at higher altitudes is generally larger than at lower latitudes. Seasonal variations of L*cf are also clearly demonstrated. The sensitivity of cloud radiative forcing (CRF) to L*cf in Community Atmosphere Model 3.0 of the National Center for Atmospheric Research (CAM3/NCAR) is analyzed. The result shows that L*cf can have a big impact on simulation of CRF, especially in major monsoon regions and the Mid-Eastern Pacific, where the difference in CRF can reach 40–50 W m−2. Therefore, accurate parameterization of cloud vertical overlap structure is important to CRF simulation and its feedback to climate.

Analytical Delta-Four-Stream Doubling–Adding Method for Radiative Transfer Parameterizations

Although single-layer solutions have been obtained for the d-four-stream discrete ordinates method (DOM) in radiative transfer, a four-stream doubling‐adding method (4DA) is lacking, which enables us to calculate the radiative transfer through a vertically inhomogeneous atmosphere with multiple layers. In this work, based on the Chandrasekhar invariance principle, an analytical method of d-4DA is proposed. When applying d-4DA to an idealized medium with specified optical properties, the reflection, transmission, and absorption are the same if the medium is treated as either a single layer or dividing it into multiple layers. This indicates that d-4DA is able to solve the multilayer connection properly in a radiative transfer process. In addition, the d-4DA method has been systematically compared with the d-two-stream doubling‐adding method (d-2DA) in the solar spectrum. For a realistic atmospheric profile with gaseous transmission considered, it is found that the accuracy of d-4DA is superior to that of d-2DA in most of cases, especially for the cloudy sky. The relative errors of d-4DA are generally less than 1% in both the heating rate and flux, while the relative errors of d-2DA can be as high as 6%.

Analytical Infrared Delta-Four-Stream Adding Method from Invariance Principle

AbstractThe single-layer solutions using a four-stream discrete ordinates method (DOM) in infrared radiative transfer (IRT) have been obtained. Two types of thermal source assumptions—Planck function exponential and linear dependence on optical depth—are considered. To calculate the IRT in multiple layers with a vertically inhomogeneous atmosphere, an analytical adding algorithm has been developed by applying the infrared invariance principle. The derived adding algorithm of the delta-four-stream DOM (δ-4DDA) can be simplified to work for the delta-two-stream DOM (δ-2DDA).The accuracy for monochromatic emissivity is investigated for both δ-2DDA and δ-4DDA. The relative error for the downward emissivity can be as high as 15% for δ-2DDA, while the error is bounded by 2% for δ-4DDA. By incorporating δ-4DDA into a radiation model with gaseous transmission, δ-4DDA is much more accurate than δ-2DDA. Also, δ-4DDA is much more efficient, since it is an analytical method. The computing time of δ-4DDA is about one-...