An ultrafast line-by-line algorithm for calculating spectral transmittance and radiance

- An ultrafast line-by-line model based on FFT and a binning technique is presented.
- Computationally expensive calculations are factored out into a pre-computation step.
- It is 103-108 times faster than LBL algorithms that do not employ speedup techniques.
- Good agreement with experimental data for the 2.7 μm band of H2O.

An ultrafast line-by-line algorithm for calculating spectral transmittance and radiance of gases is presented. The algorithm is based on fast convolution of the Voigt line profile using Fourier transform and a binning technique. The algorithm breaks a radiative transfer calculation into two steps: a one-time pre-computation step in which a set of pressure independent coefficients are computed using the spectral line information; a normal calculation step in which the Fourier transform coefficients of the optical depth are calculated using the line of sight information and the coefficients pre-computed in the first step, the optical depth is then calculated using an inverse Fourier transform and the spectral transmittance and radiance are calculated. The algorithm is significantly faster than line-by-line algorithms that do not employ special speedup techniques by a factor of 103-106. A case study of the 2.7 μm band of H2O vapor is presented.