# Simultaneous Algebraic Reconstruction Technique

Large-scale observations of the three-dimensional interior of low clouds have been missing because physics-based tomography repeats expensive 3D radiative-transfer calculations, a cost that has kept it from large, realistic cloud scenes. HybridCT splits the problem: a U-Net estimates cloud optical thickness projections from multi-angle passive radiances, and SART then explicitly inverts those projections into a 3D extinction field. On a synthetic low-cloud scene it reconstructs extinction with RMSE 13.3 km⁻¹ in 3.5 s, against 22.7 km⁻¹ in about 30 min for the physics baseline AT3D.

Large-scale observations of the three-dimensional interior of low clouds have been missing because physics-based tomography repeats expensive 3D radiative-transfer calculations, a cost that has kept it from large, realistic cloud scenes. HybridCT splits the problem: a U-Net estimates cloud optical thickness projections from multi-angle passive radiances, and SART then explicitly inverts those projections into a 3D extinction field. On a synthetic low-cloud scene it reconstructs extinction with RMSE 13.3 km⁻¹ in 3.5 s, against 22.7 km⁻¹ in about 30 min for the physics baseline AT3D.