An Evaluation Of Radiometric Products From Fixed-depth And Continuous In-water Profile Data From Moderately Complex Waters

摘要

Radiometric products determined from fixed-depth and continuous in-water profile data collected at a coastal site characterized by moderately complex waters were compared to investigate differences and limitations between the two measurement methods. The analysis focused on measurements performed with the same radiometer system sequentially deployed at discrete depths (i.e., 1 and 3 m) and successively used to profile the water column. Within the 412-683-nm spectral interval, comparisons show uncertainties of 2% -4%, 3% -5%, and 2% for the subsurface values of upwelling radiance, L_(un), upward irradiance, E_(un), and downward irradiance, E_(dn), all normalized with respect to the above-water downward irradiance. The related spectral biases vary from -2% to 1 % for L_(un), are in the range of 2%-3% for E_(un), and are lower than 0.5% for E_(dn). Derived products like the irradiance reflectance, R, Q factor at nadir, Q, and normalized water leaving radiance, L_(WN), exhibit spectral uncertainties of 4%-6%, 2%-3%, and 2%-4%. The related spectral biases vary from 1% to 3%, 2% to 3%, and -2% to 1%, respectively. An analysis of these results indicates a general diminishing of uncertainties and biases with a decrease of the diffuse attenuation coefficient, K_d, determined at 490 nm, for most of the quantities investigated. Exceptions are E_(dn) and K_d because an increase of K_d reduces the perturbations due to wave effects on downward irradiance measurements. An evaluation of the perturbing effects due to the presence of optical stratifications, which lead to a nonlinear decrease with depth of log-transformed radiometric measurements, shows an expected increase in uncertainty and bias specifically evident for K_u, E_(un), K_(d), and L_(un), and derived quantities like R, Q, and L_(WN). Overall results, supported by a t-test analysis, indicate the possibility of using moorings in moderately complex coastal waters to determine L_(WN) with a slightly higher uncertainty with respect to that achievable with continuous profiling systems.