Relationships between Q ‐factor and seawater optical properties in a coastal region

The Q ‐factor—the ratio between upwelling irradiance and upwelling radiance—describes the bidirectional structure of seawater apparent optical properties as a function of geometry and of marine and atmospheric optical characteristics. A 3‐yr time series of Q n (λ) measurements—the Q‐factor determined by nadir radiance—from the North Adriatic Sea coastal waters, has been analyzed. Scatter plots of Qn(λ) versus sun zenith, θ 0 , for different intervals of the diffuse attenuation coefficient Kd(λ) have shown a consistent exponential trend. In addition, leastsquares regressions of Q n (λ) versus Q n (490) have exhibited determination coefficients R 2 that vary from 0.77 to 0.94 in the spectral range between 412 and 555 nm and R 2 = 0.50 at 665 nm. To account for these findings, an empirical model for Q n (λ) as a function of θ 0 and K d (λ 0 ) at the reference wavelength λ 0 = 490 nm is proposed for North Adriatic coastal waters.