Polarized discrete ordinate adding approximation for infrared and microwave radiative transfer

Abstract

A POLarized Discrete ordinate aDding Approximation (POLDDA) is proposed to solve the vector thermal radiative transfer equation in a vertically inhomogeneous plane-parallel atmosphere. The discrete ordinates method is employed to find the single-layer solution for vector thermal radiative transfer; this is an extension of the approach for scalar radiative transfer, with the I- and Q-components being dealt with in a parallel way by using a doubled dimension in the discrete ordinate space. Chandrasekhar’s invariance principle is used to drive the adding process for the connection of multiple layers. This adding process has no restriction on the layer optical properties. The accuracy and efficiency of POLDDA are evaluated under different atmospheric conditions for different number of computational quadrature angles (streams). It is shown that the relative differences between POLDDA (using 32 or 16 streams) and the benchmark calculations (PolRadtran/RT3 with 32 streams, a typical model based on the adding-doubling method) are negligible for both I- and Q-components. When 8 streams are used, the accuracy of POLDDA slightly decreases, but still higher than that of 8-stream RT3. The computational efficiency of POLDDA is much higher than RT3, especially for large optical depths and when a large number of streams (≥ 16) are used. Unlike RT3, the computational time of POLDDA is always the same regardless of the optical depth.