Diffuse attenuation coefficient of downwelling irradiance: An evaluation of remote sensing methods

Abstract

[1] The propagation of downwelling irradiance at wavelength l from surface to a depth (z) in the ocean is governed by the diffuse attenuation coefficient, K d (l). There are two standard methods for the derivation of K d (l) in remote sensing, which both are based on empirical relationships involving the blue-to-green ratio of ocean color. Recently, a semianalytical method to derive K d (l) from reflectance has also been developed. In this study, using K d (490) and K d (443) as examples, we compare the K d (l) values derived from the three methods using data collected in three different regions that cover oceanic and coastal waters, with K d (490) ranging from $0.04 to 4.0 m À1 . The derived values are compared with the data calculated from in situ measurements of the vertical profiles of downwelling irradiance. The comparisons show that the two standard methods produced satisfactory estimates of K d (l) in oceanic waters where attenuation is relatively low but resulted in significant errors in coastal waters. The newly developed semianalytical method appears to have no such limitation as it performed well for both oceanic and coastal waters. For all data in this study the average of absolute percentage difference between the in situ measured and the semianalytically derived K d is $14% for l = 490 nm and $11% for l = 443 nm.

An empirical algorithm for determining the diffuse attenuation coefficient Kd in clear and turbid waters from spectral remote sensing reflectance

“…Testing their method on a data set consisting of 875 observations taken in the Gulf of Mexico, the Arabian Sea, and the Baltic Sea, they obtained a significantly higher correlation between K d 490 estimates and measurements within the covered K d 490 range of 0.04 to 4.0 m -1 (R 2 = 0.911) as compared to the operational SeaWiFS algorithm (R 2 = 0.680). In the present study, we propose a method to determine K d 490 from the spectral R rs of comparable performance to that of Lee et al (2005). Similar to theirs, our method also uses information in the blue, green, and red part of the spectrum.…”

“…Lee et al (2005) recently proposed a "semi-analytical" method, i.e., a model-based approach combining theoretical considerations and empirical parameterizations to relate the remote sensing reflectance R rs to K d via the absorption coefficient a and the backscattering coefficient b b . In order to improve the performance of their method in turbid waters, they applied a wavelength in the red part of the spectrum.…”

“…The following Eq. is used to convert between these two parameters (Lee et al 1994): (9) where t is the transmittance of the air-sea interface, n is the real part of the complex refraction index of seawater, and Q designates the upwelling irradiance-to-radiance ratio. All three parameters t, n, and Q depend on a number of external factors such as observation geometry, optical properties of seawater, or wavelength.…”

Evaluation of a near-global eddy-resolving ocean model

“…The penetrating short-wave heat flux is applied over multiple model levels according to a single exponential decay law, with penetration depths based on SeaWiFS Kd-490 (e.g. Lee et al, 2005). The SeaWiFS Kd-490 is a measure of the turbidity of the water column that quantifies the depth over which short-wave radiation penetrates the ocean.…”

Water clarity patterns in South Florida coastal waters and their linkages to synoptic-scale wind forcing

“…Reflectance data from both sensors were used to derive Kd data products including the standard kd_lee (Lee et al, 2005;Lee et al, 2009) and a modification of the kd_lee algorithm validated for optically shallow water applications (Barnes et al, 2013). The entire time series of SeaWiFS and MODIS Level-2 ocean color data within the bounds of 24° to 31°N, 78° to 98°W were acquired from NASA Goddard Space Flight Center.…”