Improved Radiation Simulation in the Community Atmosphere Model with Implementation of Four-Stream Spherical Harmonic Expansion Approximation for Solar Radiation

Abstract:

The most important source of energy that drives atmospheric circulation in the climate system is solar radiation. The key of its simulation is the radiative transfer process which is prescribed by a differential-integral equation. A two-stream method is widely used in solving radiative transfer equation in Global Climate Models, but it may produce relative errors up to 20% under cloudy-sky condition. In order to improve the accuracy of the radiative simulation in CAM5 (Version 5.0 of Community Atmosphere Model), a four-stream Spherical Harmonic Expansion approximation for solar radiation is implemented. The more accurate method needs the higher moment of phase function. In this case, a parameterization of full four-stream cloud optical properties is developed. Exact Mie calculation is performed to get the extinction, scattering cross sections and phase function at numerous wavelengths across the solar spectrum. The band results of expansion coefficients parameterized by polynomials are obtained by averaging the wavelengths within each band. That involves the weighting factor of solar flux incident at 500hPa for solar zenith angle of 48.19° (Cronin 2014). By using column radiative model for different solar zenith angles, atmospheric profiles and aerosol conditions, it shows that the four-stream method can reduce the flux biases when aerosol is considered under clear sky. Implementation of the method in single column model indicates more accuracy than the original two-stream one when comparing with ARM data. Improvements are also inferred in validation of global climate model using ERBE and CERES data.