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The Havemann-Taylor Fast Radiative Transfer Code: A line-by-line sensor independent radiative transfer code

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) is based on Principal Components (PCs) and allows fast and exact radiance and/or transmittance calculations. It is ideally suited for the simulation of hyperspectral sensors with hundreds or thousands of channels. The HT-FRTC can simulate a full instrument spectrum for any atmosphere and surface within a few milliseconds. It works for satellite-based, airborne and ground-based sensors. The code has been applied in any part of the spectrum from the short-wave to the long-wave (i.e. infrared plus microwave). It includes the solar and the lunar source and can account for the spherical Earth. The HT-FRTC has been incorporated into a one-dimensional variational (1D-Var) retrieval system that also works solely in PC space. This keeps the dimensions of the matrices involved small. The solution of the full non-linear problem is achieved by an iterative Levenberg-Marquardt minimization procedure. The retrieval state vector includes the vertical profiles of atmospheric temperature, water vapour and ozone, and possibly other trace gases as well as the surface temperature and surface emissivity / reflectivity (the latter being represented by a set of PCs). For a scattering atmosphere, cloud parameters and aerosol parameters have been added to the state vector. The cloud part of the state vector for cirrus cloud includes cloud-top pressure, ice water content, cloud fraction and cloud geometrical thickness. For water cloud there is also an effective droplet size.