Radiative transfer acceleration based on the Principal Component Analysis and Look-Up Table of corrections: Optimization and application to UV ozone profile retrievals

Abstract:

In this work, we apply the principal component analysis (PCA)-based approach combined with look-up table (LUT) of corrections to accelerate a radiative transfer (RT) model used in the retrieval of ozone profiles from backscattered ultraviolet (UV) measurements by the Ozone Monitoring Instrument (OMI). The binning scheme, which determines the accuracy and efficiency of the PCA–RT performance, is thoroughly optimized over the spectral range of 265 to 360 nm with the assumption of a Rayleigh-scattering atmosphere above a Lambertian surface. The high level of accuracy (~ 0.03 %) is achieved from fast-PCA approximations of full radiances in spite of limiting the expensive multiple scattering (MS) calculations only to a reduced set of PCA-derived optical states, with fast single scattering and 2-stream multiple scattering calculations and for every spectral points. The number of calls to the accurate MS model is only 51 in the application to OMI ozone profile retrievals from the spectral range 270-330 nm. We also developed the Look up table (LUT) to correct RT approximations due to using scalar RT model, four streams, and 24 layers, achieving the accuracy from simulations in vector with 12 streams and 72 layers; this speeds up the RT calculation by more than 2 orders of magnitude when ignoring other overheads. The saved computational cost allows us to increase the number of wavelengths in the simulation by a factor of three, which reduces the retrieval errors of the tropospheric ozone due to under-sampled RT simulations. In conclusion, the computational speedups of OMI retrieval performance are 3.3 orders of the magnitude. The better treatments for under-sampling errors and RT approximation errors improve spectral fittings (2- 5 %) and hence retrieval errors, especially for the tropospheric ozone (~5%).