Assessment of GOCI radiometric products using MERIS, MODIS and field measurements

The first Geostationary Ocean Color Imager (GOCI) launched by South Korea in June 2010 constitutes a major breakthrough in marine optics remote-sensing for its capabilities to observe the diurnal cycles of the ocean. The light signal recorded at eight wavelengths by the sensor allows, after correction for Solar illumination and atmospheric effects, the retrieval of coloured biogeochemical products such as the chlorophyll, suspended sediment and coloured dissolved organic matter concentrations every hour between 9:00 am and 4:00 pm local time around the Korean peninsula. However operational exploitation of the mission needs beforehand a sound validation of first the radiometric calibration, i.e. inspection of the top-of-atmosphere reflectance, and second atmospheric corrections for retrieval of the water-leaving reflectance at sea surface. This study constitutes a contribution to the quality assessment of the GOCI radiometric products generated by the Korea Ocean Satellite Center (KOSC) through comparison with concurrent data from the MODerate-resolution Imaging Spectroradiometer (MODIS, NASA) and MEdium Resolution Imaging Spectrometer (MERIS, ESA) sensors as well as in situ measurements. These comparisons are made with spatially and temporally collocated data. We focus on Rayleigh-corrected reflectance (?? _RC ) and normalized remote-sensing marine reflectance (nRrs). Although GOCI compares reasonably well with MERIS and MODIS, what demonstrates the success of Ocean Colour in geostationary orbit, we show that the current GOCI atmospheric correction systematically masks out data over very turbid waters and needs further examination and correction for future release of the GOCI products.     

Initial validation of GOCI water products against in situ data collected around Korean peninsula for 2010–2011

This paper provides initial validation results for GOCI-derived water products using match-ups between the satellite and ship-borne in situ data for the period of 2010?C2011, with a focus on remote-sensing reflectance (R _rs ). Match-up data were constructed through systematic quality control of both in situ and GOCI data, and a manual inspection of associated GOCI images to identify pixels contaminated by cloud, land and inter-slot radiometric discrepancy. Efforts were made to process and quality check the in situ R _rs data. This selection process yielded 32 optimal match-ups for the R _rs spectra, chlorophyll a concentration (Chl_a) and colored dissolved organic matter (CDOM), and with 20 match-ups for suspended particulate matter concentration (SPM). Most of the match-ups are located close to shore and thus the validation should be interpreted limiting to near-shore coastal waters. The R _rs match-ups showed the mean relative errors of 18?C33% for the visible bands with the lowest 18?C19% for the 490 nm and 555 nm bands and 33% for the 412 nm band. Correlation for the R _rs match-ups was high in the 490?C865 nm bands (R²=0.72?C0.84) and lower in the 412 nm band (R²=0.43) and 443 nm band (R²=0.66). The match-ups for Chl_a showed a low correlation (<0.41) although the mean absolute percentage error was 35% for the GOCI standard Chl_a. The CDOM match-ups showed an even worse comparison with R²<0.2. These match-up comparison for Chl_a and CDOM would imply the difficulty to estimate Chl_a and CDOM in near-shore waters where the variability in SPM would dominate the variability in R _rs . Clearly, the match-up statistics for SPM was better with R²=0.73 and 0.87 for two evaluated algorithms, although GOCI-derived SPM overestimated low concentration and underestimated high concentration. Based on this initial match-up analysis, we made several recommendations -1) to collect more offshore under-water measurements of the R _rs data, 2) to include quality flags in level-2 products, 3) to introduce an ISRD correction in the GOCI processing chain, 4) to investigate other types of in-water algorithms such as semianalytical ones, and 5) to investigate vicarious calibration for GOCI data and to maintain accurate and consistent calibration of field radiometric instruments.     

Development of atmospheric correction algorithm for Geostationary Ocean Color Imager (GOCI)

This paper describes an atmospheric correction algorithm for Geostationary Ocean Color Imager (GOCI) and its early phase evaluation. This algorithm was implemented in GOCI Data Processing System (GDPS) version 1.1. The algorithm is based on the standard SeaWiFS method, which accounts for multiple scattering effects and partially updated in terms of turbid case-2 water correction, optimized aerosol models, and solar angle correction per slot. For turbid water correction, we used a regional empirical relationship between water reflectance at the red (660 nm) and near infrared bands (745 nm and 865 nm). The relationship was derived from turbid pixels in satellite images after atmospheric correction, and processed using aerosol properties derived for neighboring non-turbid waters. For validation of the GOCI atmospheric correction, we compared our results with in situ measurements of normalized water leaving radiance (nL _w ) spectra that were obtained during several cruises in 2011 around Korean peninsula. The match up showed an acceptable result with mean ratio of the GOCI to in situ nL _w (??), 1.17, 1.24, 1.26, 1.15, 0.86 and 0.99 at 412 nm, 443 nm, 490 nm, 555 nm, 660 nm and 680 nm, respectively. It is speculated that part of the deviation arose from a lack of vicarious calibration and uncertainties in the above water nLw measurements.     

Seasonal variation of freshwater budget in the Yellow and East China Seas simulated from an ocean general circulation model

This study investigates a freshwater budget in the Yellow and East China Seas (YECS) using a global f general circulation model with a regional focus on the YECS. A freshwater budget analysis finds that major freshwater contributors over the YECS change seasonally. In summer, freshwater inflow from Changjiang and positive precipitation minus evaporation (P-E) dominates freshwater outflow across the boundaries around the YECS, resulting in net freshwater gain in the YECS. In winter, evaporation, intensified by strong winds, dominates freshwater inflow from Changjiang and precipitation, while net freshwater transport across the boundaries around the YECS is negligible, causing freshwater loss in total over the YECS. Although P-E has often been assumed to be negligible by supposing that the annual mean of precipitation is nearly equal to that of evaporation, this study suggests that P-E needs to be included in the seasonal freshwater budget in the YECS.     

Bed‐scale spatial patterns in dolomite abundance: Part II. Effect of varied fluid chemistry,flow rate,precursor mineralogy,temperature, textural heterogeneity,nucleation density and bed geometry

Reaction‐transport modelling shows that a lateral, metre‐scale pattern in dolomite abundance can form during replacive dolomitization of calcite and aragonite precursors by Mississippian, Triassic and modern seawaters advecting at >20 cm year^?1 in low temperature (≤60°C) hydrogeological systems. The modelled pattern develops best in beds >1 m thick with a relatively uniform nucleate density and a low variance in reactive surface area (for example, grainstones). These conditions suggest that a lateral pattern in dolomite abundance should be expected in dolomites formed by any shallow hydrodynamic system that advects dolomitizing fluid horizontally, including the down‐gradient portion of a reflux system, the oceanward reaches of geothermal (Kohout) convection systems, and the seaward reaches of a seawater entrainment zone below a freshwater aquifer. However, even in those systems, a pattern will not form in all dolomites. Pattern will be muted in thinner (≤ ca 50 cm thick) beds and non‐emergent where the precursor had a high variance in reactive surface area (for example, a skeletal wackestone) and/or large variation in nucleate density. Pattern also did not form at temperatures above ca 60°C, which implies that pattern should not be expected in dolomites formed at intermediate to deep burial depths. As the patterns are horizontal, they also should not be expected where dolomitizing fluids moved vertically (for example, reflux immediately below a brine source). Pattern metrics (short‐range correlation length, and wavelength and amplitude of the longer cyclic component) vary with flow rate, fluid chemistry, bed thickness, porosity, grain size, temperature and the flow complexity (one‐dimensional, two‐dimensional or three‐dimensional) within the bed. However, no modelled pattern produced metrics larger than those documented on dolomite outcrops. The results thus constrain the length scales of porosity and permeability variance to include in petrophysical models of dolomite reservoirs, as well as the geological scenarios in which to consider metre‐scale lateral petrophysical variability.     

Development of a seaweed species-selection index for successful culture in a seaweed-based integrated aquaculture system

Integrated multi-trophic aquaculture (IMTA) has been proposed as a concept that combines the cultivation of fed aquaculture species (e.g., finfish/shrimp) with extractive aquaculture species (e.g., shellfish/seaweed). In seaweed-based integrated aquaculture, seaweeds have the capacity to reduce the environmental impact of nitrogen-rich effluents on coastal ecosystems. Thus, selection of optimal species for such aquaculture is of great importance. The present study aimed to develop a seaweed species-selection index for selecting suitable species in seaweed-based integrated aquaculture system. The index was synthesized using available literature-based information, reference data, and physiological seaweed experiments to identify and prioritize the desired species. Undaria pinnatifida, Porphyra yezoensis and Ulva compressa scored the highest according to a seaweed-based integrated aquaculture suitability index (SASI). Seaweed species with the highest scores were adjudged to fit the integrated aquaculture systems. Despite the application of this model limited by local aquaculture environment, it is considered to be a useful tool for selecting seaweed species in IMTA.     

Evaluation of the seismic response of stone pagodas using centrifuge model tests

This study attempts to propose dynamic centrifuge model tests as a method of seismic risk assessment in order to discover how stone architectural heritages with masonry structures have endured seismic load, and whether there is any possibility of future earthquake damage. Dynamic centrifuge tests have been conducted for one fifteenth scale models of Seok-ga-tap and the five-storey stone pagoda of Jeongnimsa temple site, which are Korean representative stone pagodas. In order to make input motions of the earthquake simulator, site investigation and site-specific response analysis have been performed. The models of two stone pagodas, which have the same number of pieces with the real structures, have been produced and the dynamic centrifuge tests have been conducted for the model pagodas. Accelerometers were attached at different heights of the pagoda. The measured acceleration records and frequency responses were analysed during dynamic centrifuge test. Two real earthquake records, Hachinohe and Ofunato earthquakes and a sweeping signal with ranged frequency were utilised for input motions of dynamic centrifuge tests to evaluate the behaviour of the stone pagodas. For Seok-ga-tap models, it was observed that acceleration tends to be amplified with height. The third floor body shows at most 2.5 amplification of acceleration in comparison to the surface ground. The amplification was at a frequency of 3.83 Hz and it was considered as the natural frequency of the pagoda. For the five-storey stone pagoda, the seismic wave energy significantly reduced while it passed the first body floor, and then the peak acceleration was gradually amplified upwards. It was found that the pagodas did not collapse when the peak acceleration of ground surface was raised to 0.4 g. Given that the maximum design seismic acceleration specified in Korean seismic design guide is 0.22 g and the amplification ratio of peak acceleration in the supporting ground of the pagodas ranges from 1.45 to 1.74, it can be shown that the two pagodas are stable against 2400-year return period earthquake level, and have excellent seismic performance.     

Basin rotation method for analyzing the directional influence of moving storms on basin response

Hydrologic responses to variations in storm direction provide useful information for the analysis and prediction of floods and the development of watershed management strategies. However, the prediction of hydrologic responses to changes in storm direction is a difficult task that requires meteorological simulations and extensive computation. It is also difficult to identify the center of rotation of a storm affecting a basin of interest. Therefore, we propose a simple approach of rotating the basin position relative to the storm within the rainfall–runoff simulation model instead of changing the pathway of the storm, which we term the basin rotation method (BRM). The proposed BRM was tested on four major typhoon events in South Korea. The results illustrated that the original basin orientation (i.e., before it was rotated) exhibits earlier and higher peak discharge and earlier recession compared to the basin after rotation. We conclude that the proposed method (BRM) is a viable alternative for use in assessing the directional influence of moving storms on floods caused by historical rather than hypothetical storm events.