Wayne D. Robinson

Calibration of SeaWiFS. II. Vicarious techniques.

We present an overview of the vicarious calibration of the Sea-Viewing Wide Field-of-view Sensor (SeaWiFS). This program has three components: the calibration of the near-infrared bands so that the atmospheric correction algorithm retrieves the optical properties of maritime aerosols in the open ocean; the calibration of the visible bands against in-water measurements from the Marine Optical Buoy (MOBY); and a calibration-verification program that uses comparisons between SeaWiFS retrievals and globally distributed in situ measurements of water-leaving radiances. This paper describes the procedures as implemented for the third reprocessing of the SeaWiFS global mission data set. The uncertainty in the near-infrared vicarious gain is 0.9%. The uncertainties in the visible-band vicarious gains are 0.3%, corresponding to uncertainties in the water-leaving radiances of approximately 3%. The means of the SeaWiFS/in situ matchup ratios for water-leaving radiances are typically within 5% of unity in Case 1 waters, while chlorophyll a ratios are within 1% of unity. SeaWiFS is the first ocean-color mission to use an extensive and ongoing prelaunch and postlaunch calibration program, and the matchup results demonstrate the benefits of a comprehensive approach.

Global mapping of underwater UV irradiances and DNA‐weighted exposures using Total Ozone Mapping Spectrometer and Sea‐viewing Wide Field‐of‐view Sensor data products

The global stratospheric ozone layer depletion results in an increase in biologically harmful ultraviolet (UV) radiation reaching the surface and penetrating to ecologically significant depths in natural waters. Such an increase can be estimated on a global scale by combining satellite estimates of UV irradiance at the ocean surface from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument with the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite ocean color measurements in the visible spectral region. In this paper we propose a model of seawater optical properties in the UV spectral region based on the case 1 water model in the visible range. The inputs to the model are standard monthly SeaWiFS products: chlorophyll concentration and the diffuse attenuation coefficient at 490 nm. Penetration of solar UV radiation to different depths in open ocean waters is calculated using the radiative transfer quasi-single scattering approximation (QSSA). The accuracy of the QSSA approximation in the water is tested using more accurate codes. Sensitivity studies of underwater UV irradiance to atmospheric and oceanic optical properties have shown that the main environmental parameters controlling absolute levels of UVB (280–320 nm) and DNA-weighted irradiance underwater are solar zenith angle, cloud transmittance, water optical properties, and total ozone. Monthly maps of underwater UV irradiance and DNA-weighted exposure are calculated using monthly mean SeaWiFS chlorophyll and diffuse attenuation coefficient, daily SeaWiFS cloud fraction data, and the TOMS-derived surface UV irradiance daily maps. The results include global maps of monthly average UVB irradiance and DNA-weighted daily exposures at 3 and 10 m and depths where the UVB irradiance and DNA-weighted dose rate at local noon are equal to 10% of their surface values.

Optimized retrievals of precipitable water from the VAS 'split window'

Abstract Precipitable water fields have been retrieved from the VISSR Atmospheric Sounder (VAS) using a radiation transfer model for the differential water vapor absorption between the 11 and 12 μm “split window” channels. Previous moisture retrievals using only the split window channels provided very good space-time continuity but poor absolute accuracy. This note describes how retrieval errors can be significantly reduced from ±0.9 to ±0.6 gm cm−2 by empirically optimizing the effective air temperature and absorption coefficients used in the two-channel model. The differential absorption between the VAS 11 and 12 μm channels, empirically estimated from 135 colocated VAS-RAOB observations, is found to be approximately 50% smaller than the theoretical estimates. Similar discrepancies have been noted previously between theoretical and empirical absorption coefficients applied to the retrieval of sea surface temperatures using radiances observed by VAS and polar-orbiting satellites. These discrepancies indi...

Atmospheric correction for NO 2 absorption in retrieving water-leaving reflectances from the SeaWiFS and MODIS measurements

The absorption by atmospheric nitrogen dioxide (NO2) gas in the visible has been traditionally neglected in the retrieval of oceanic parameters from satellite measurements. Recent measurements of NO2 from spaceborne sensors show that over the Eastern United States the NO2 column amount often exceeds 1 Dobson Unit (approximately 2.69x10(16) molecules/cm2). Our radiative transfer sensitivity calculations show that under high NO2 conditions (approximately 1x10(16) molecules/cm2) the error in top-of-atmosphere (TOA) reflectance in the blue channels of the sea-viewing wide field-of-view sensor (SeaWiFS) and moderate-resolution imaging spectroradiometer (MODIS) sensors is approximately 1%. This translates into approximately 10% error in water-leaving radiance for clear waters and to higher values (>20%) in the coastal areas. We have developed an atmospheric-correction algorithm that allows an accurate retrieval of normalized water-leaving radiances (nLws) in the presence of NO2 in the atmosphere. The application of the algorithm to 52 MODIS scenes over the Chesapeake Bay area show a decrease in the frequency of negative nLw estimates in the 412 nm band and an increase in the value of nLws in the same band. For the particular scene reported in this paper, the mean value of nLws in the 412 nm band increased by 17%, which is significant, because for the MODIS sensor the error in nLws attributable to the digitization error in the observed TOA reflectance over case 2 waters is approximately 2.5%.

Retrieving marine inherent optical properties from satellites using temperature and salinity-dependent backscattering by seawater.

Time-series of marine inherent optical properties (IOPs) from ocean color satellite instruments provide valuable data records for studying long-term time changes in ocean ecosystems. Semi-analytical algorithms (SAAs) provide a common method for estimating IOPs from radiometric measurements of the marine light field. Most SAAs assign constant spectral values for seawater absorption and backscattering, assume spectral shape functions of the remaining constituent absorption and scattering components (e.g., phytoplankton, non-algal particles, and colored dissolved organic matter), and retrieve the magnitudes of each remaining constituent required to match the spectral distribution of measured radiances. Here, we explore the use of temperature- and salinity-dependent values for seawater backscattering in lieu of the constant spectrum currently employed by most SAAs. Our results suggest that use of temperature- and salinity-dependent seawater spectra elevate the SAA-derived particle backscattering, reduce the non-algal particles plus colored dissolved organic matter absorption, and leave the derived absorption by phytoplankton unchanged.

Calibration of SeaWiFS on orbit

SeaWiFS was launched onboard the OrbView-2 satellite on 1 August 1997. On 4 September 1997, the day of first light for the instrument, SeaWiFS global images were processed automatically using the instruments prelaunch calibration and distributed on the World Wide Web. With the first reprocessing of SeaWiFS data in January 1998, the radiometric calibration coefficients for the SeaWiFS visible bands were linked to the water-leaving radiances measured by the Marine Optical Buoy (MOBY). In addition, the calibration coefficient for the 765 nm SeaWiFS infrared band was adjusted to give values consistent with those for an atmosphere with the maritime type of aerosol found in the vicinity of the MOBY buoy. Since the infrared bands were designed to allow the inference of aerosol type for the SeaWiFS atmospheric correction algorithm, this vicarious calibration forces their agreement with the conditions for a known aerosol type. With the second reprocessing in August 1998, temporal changes in the radiometric sensitivities of the SeaWiFS near infrared bands were corrected using lunar and solar measurements. The third SeaWiFS reprocessing in May 2000 introduced small time dependent calibration corrections to some visible bands. Future SeaWiFS reprocessings are scheduled to occur on an annual to biennial basis. With the third reprocessing, the emphasis of the instrument calibration program has shifted to the assessment of the surface truth comparisons used by SeaWiFS, principally those with MOBY.

SeaWiFS calibration: status after two years on orbit

The SeaWiFS Project has corrected the instrument calibration for drifts in the response of bands 1, 2, and 5-8 using lunar calibration data and has performed a vicarious calibration of SeaWiFS using in situ data from the Marine Optical Buoy deployed off of Lanai, Hawaii. This vicarious calibration has been validated by comparing SeaWiFS data with in situ data from various ship cruises. The calibration has also been validated through a global clear water analysis. Since the validation data come from regions of the ocean other than the MOBY site, the agreement between the SeaWiFS data in the validation data show that the vicarious calibration of SeaWiFS is a global calibration.

Vicarious calibration of SeaWiFS

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) has been providing daily global imagery of the worlds oceans since September 1997. SeaWiFS is an eight-band, visible and near-infrared satellite radiometer with a spatial resolution of 1.1 km at nadir. The goal of the SeaWiFS Project is to produce a five-year ocean color data set with a 5% absolute and 1% relative radiometric accuracy on the water-leaving radiances. The SeaWiFS atmospheric correction algorithm must remove the atmospheric signal to yield the water-leaving radiances. The atmospheric correction estimates the aerosol radiances at 765 nm and 865 nm and aerosol radiances to the other SeaWiFS bands. Uncertainties in the atmospheric correction algorithm and possible variations in the instrument calibration with time require a mission-long vicarious calibration program to monitor the performance of the sensor system and to meet the radiometric constraints on the ocean color data set. The Calibration/Validation (Cal/Val) group of the SeaWiFS Project is using the NASA/NOAA Marine Optical Buoy (MOBY), deployed off of Lanai, Hawaii, as the primary vicarious calibration site for SeaWiFS. The Cal/Val group has performed the initial vicarious calibration of the SeaWiFS data by comparing water-leaving radiances measured by MOBY with water-leaving radiances from contemporaneous SeaWiFS images of overflights of the buoy site. The group has derived a set of system gains and offsets which, when applied to the SeaWiFS instrument calibration, yields values for water leaving radiances measured by SeaWiFS and MOBY that agree to within 1%. The Cal/Val group has verified this vicarious calibration by comparing matchup data sets between SeaWiFS and various ship cruises.