MODIS medium-resolution (250- and 500-m) bands were successfully used to detect and map the distribution of a harmful phytoplankton bloom (HAB) in the Paracas Bay, Peru, that caused economic losses estimated at about $28.5 million. A Red-Green-Blue combination of bands 1, 4 and 3 was used to visually distinguish the HAB while the turbidity index, a semi-quantitative measure of the amount of particulate material in the near-surface water, was used to estimate the intensity of the HAB. The turbidity index was inversely correlated with oxygen concentration in the bay. Temporary anoxia caused by the HAB was probably the main mechanism causing fish kills. The 250-m resolution provided by MODIS bands 1 and 2 is essential to detect localized HABs in coastal areas. While turbidity is not specific to algal blooms, it is a quantitative estimate of the intensity of the bloom once the existence of the bloom is detected by the RGB images.
The paper presents initial results of atmospherically corrected ocean color data from the Global Imager (GLI), a moderate resolution spectrometer launched in December 2002 aboard ADEOS-II satellite. The standard GLI atmospheric correction algorithm, which includes an iterative procedure based on in-water optical modeling is first described, followed by brief description of standard in-water algorithms for output geophysical parameters. Ship/buoy-observed and satellite-derived marine reflectances, or normalized water-leaving radiance, are then compared, under vicarious calibration correction factors based on global GLI-SeaWiFS data comparison. The results, over 15 water-leaving radiance match-up data collected mostly off California and off Baja California, show standard errors in GLI estimate of 0.1 to 0.36 μW/cm2/nm/sr for 412, 443, 490, and 565 nm bands, with improved standard errors of 0.09 to 0.14 μW/cm2/nm/sr if in situ data set is limited to those obtained by in-water radiance measurement. Under provisional de-striping procedure, satellite-derived chlorophyll a estimates compares well with 35 ship-measured data collected off California within one day difference from the satellite observation, showing standard error factor of 1.73 (+73% or -43% error).
Increased levels of biologically harmful Uv radiatonhave beenshown to affec aquatic ecosystems, marine photocynmetiry, and their imapct on carbon cycling. A quantiative assessment of UV effectw requires an estimate of the in-water raiationfield. An esitmate of underwater UV radiatonis porosed based on satellit meausrments fromthe TOMS and SeaWiFS and modesl fo radiatve transfer (RT). The Hydrolight code, modified toe xtnd it to the 290 - 400 nm wavleength range, is used for REt calucaitons in theocean. Solar direc tandidffuse radiances at the ocean surfce are calculated using a fulll RT code for clear-sky coditions, whicha re then modified for clouds and aerosols.Teh TOMS total column ozone and reflectivity productsa reinputs for RT calcuaitons in the atmosphere. An essential component of the in-water RT model is a model of seawater inherent optical properties (IOP). The IOP model is an extension of the Case-1 water model to the UV spectral region. Pure water absorption is interpolated between experimental datasets available in the literature. A new element of the IOP model is parameterization of particulate matter absorption in the UV based on recent in situ data. The SeaWiFS chlorophyll product is input for the IOP model. The in-water computational scheme is verified by comparing the calculated diffuse attenuation coefficient Kd, with one measured for a variety of seawater IOP. The calculated Kd is in a good agreement with the measured Kd. The relative RMS error for all of the cruise stations is about 20%. The error may be partially attributed to variability of solar illumination conditions not accounted for in calculations. The conclusion is that we are now able to model ocean UV irradiances and IOP properties with accuracies approaching those visible region, and in agreement with experimental in situ data.
Seasonal variability of solar UV radiation in ocean waters is estimated on a global scale by combining satellite measurements of scene reflectivity (TOMS), column ozone (TOMS) and chlorophyll concentration (SeaWiFS) with radiative transfer calculations for an ocean-atmosphere system. The new features are an extension of underwater radiative transfer (scattering and absorption) into the UV, inclusion of polarization in the above water diffuse radiances, the proper treatment of Fresnel reflection, and first order atmospheric backscatter of water-leaving radiance to the oceans. Maps of downwelling diffuse irradiances (Ed) at ocean surface and at different depths in the ocean, diffuse attenuation coefficient (Kd), and ten percent penetration depth (Z10) of solar irradiation are computed for open ocean waters. Results on spectral irradiances at 310 nm in UV-B and at 380 nm in UV-A part of the spectrum are presented with particular emphasis on the role of aerosols, clouds, and ozone in the atmosphere and chlorophyll concentrations in the ocean.
Satellite instruments currently provide global maps of surface UV irradiance by combining backscattered radiance data with radiative transfer models. The models are often limited by uncertainties in physical input parameters of the atmosphere and surface. Global mapping of the underwater UV irradiance creates further challenges for the models. The uncertainties in physical input parameters become more serious because of the presence of absorbing and scattering quantities affected by biological processes within the oceans. In this presentation we summarize the problems encountered in the assessment of the underwater UV irradiance from space-based measurements, and propose approaches to resolve the problems. We have developed a radiative transfer scheme for computation of the UV irradiance in the atmosphere-ocean system. The scheme makes use of input parameters derived from satellite instruments such as TOMS and SeaWiFS. The major problem in assessment of the surface UV irradiance is to accurately quantify the effects of clouds. Unlike the standard TOMS UV algorithm, we use the cloud fraction products available from SeaWiFS and MODIS to calculate instantaneous surface flux at the ocean surface. Daily UV doses can be calculated by assuming a model of constant daily cloudiness. Both SeaWiFS and MODIS provide some estimates of seawater optical properties in the visible.
A model of particle absorption coefficients is presented as a function of chlorophyll concentration. The model has been derived from remote sensing reflectance and chlorophyll concentration of CalCOFI bio-optical data set, using a radiance model. Variance in absorption coefficient for a given chlorophyll concentration can be reduced by introducing site- dependent particle backscattering coefficients, average of which is assumed to follow Morel's backscattering model. With an empirical algorithm for estimating the absorption by dissolved organic matter, we separate the absorption by particles from the total absorption. Through a simple quality control, statistical regression gives the parameterization of particle absorption. By applying the derived model to a semi- analytical inversion algorithm, we demonstrate the proposed model could be used to retrieve in-water parameters such as chlorophyll concentration, absorption by colored dissolved organic matter and particle backscattering coefficients.
We present a combined analysis of apparent optical properties and inherent optical properties of the California Current based on multi-instrument bio-optical measurements during the California Cooperative Oceanic Fisheries Investigation cruises. Detailed radiative transfer modeling is employed and radiance and irradiances for the model are derived and compared to measured values. Bio-optical parameterizations for the California Current are developed and compare to existing parameterizations for Case 1 waters. Discrepancies between absorption parameterizations are discussed.