Dependence of spectral upwelling radiance distribution on optical properties and angular geometry in optically complex deep lakes

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The Q -factor is a key parameter characterizing the bidirectional structure of apparent optical properties, yet its variability in optically complex lakes has not been adequately characterized. This study quantitatively assesses and decouples the respective influences of inherent optical properties (IOPs) and angular parameters on the radiative regime, using data from 202 in situ stations with 208 b ~ bp in Lake Taihu and over 30 million radiative transfer simulations. The results show that Q values range from 0.3 to 6.0 and Q -nadir ( Q n ) exhibits spectral variations primarily driven by the particle backscattering ratio ( b ~ bp ), particularly in waters with an average number of collisions at 720 nm ( n ¯ 720 ) below 6.0 (Type 2 waters). In these waters, Q n displays an inverted Gaussian pattern in the red spectral region (670–720 nm), likely associated with solar-induced chlorophyll fluorescence (SICF), and increases with b ~ bp , opposite to the trends observed at other wavelengths and in extremely turbid waters ( n ¯ 720 >6.0). Q increases with the solar zenith angle and decreases with increasing viewing zenith angle ( θ v ). Coupled Q -factor models are proposed across the 400–750 nm spectral range at 5 nm intervals for optically deep lakes and yield R 2 values exceeding 0.85 and 0.71, with RMSE below 0.055 and 0.219 sr, respectively, when compared with Hydrolight simulated Q n and Q -factor ( θ v ≤60 ∘ ). Sensitivity analyses further reveal that wind speed and aerosol type have limited effects on Q n , whereas isotropic SICF emissions in Type 2 waters suppress angular variability and induce a maximum relative difference of approximately 6.0% in Q n at 680 nm. This study enhances the capability of multi-angle and hyperspectral remote sensing to model radiance distribution in optically complex deep lakes.

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