Offshore tower shading effects on in-water optical measurements

A field campaign was performed to estimate the shading effect induced on in -water irradiance and radiance measurements taken in the immediate vicinity of the Acqua Alta Oceanographic Tower (AAOT), located in the northern Adri atic Sea, which is regularly used to support ocean color validation activit ies; Sequences of downwelling irradiance and upwelling radiance profiles we re collected at varying distances from the tower to evaluate the shading ef fects during clear-sky conditions as a function of the deployment distance. The experimental data, as well as Monte Carlo simulations, indicate that t he shading effect is negligible for both downwelling irradiances and upwell ing radiances at deployment distances greater than 15 and 20 m, respectivel y. At closer distances, for example, at the 7.5-m deployment distance regul arly used at the AAOT for the collection of underwater optical measurements , the shading effect is remarkable: both field and simulated data at a dept h of 7 m and a wavelength of 443 nm show that, with a relatively low sun ze nith angle of 22 degrees, the shading effect is within 3% for downwelling i rradiance and within 8% for upwelling radiance. Monte Carlo simulations at 443, 555, and 665 nm, computed at a depth of 0(-) m and with values of seaw ater inherent optical properties representative of the AAOT site, are used to extend considerations on shading effects to measurements taken during di fferent illumination conditions at the 7.5-m deployment distance. Simulatio ns for ideal clear-sky conditions (i.e., in the absence of atmospheric aero sols) show that errors induced by AAOT perturbations significantly vary as a function of wavelength and sun zenith angle. The highest values are obser ved at 443 nm where, with the sun zenith angle ranging from 20 degrees to 7 0 degrees, errors vary from 2.4% to approximately 6.2% for downwelling irra diance and from a minimum of 3.0% (occurring at 30 degrees) to almost 6.6% for upwelling radiance. Simulations also show that thr shading error can be as high as approximately 20% for both irradiance and radiance measurements taken during overcast sky conditions.