Downwelling Surface Spectral Irradiance Measurements with the Marine Optical BuoY

1. Data Acquisition MOBY: < 1 nm resolution, hyperspectral (340 nm to 960 nm), fiber-coupled, dual CCD spectrograph-based sensor system In this paper we only consider the Es(λ) data sets. • the scans are discrete and sequential (Es, Lu,MID, Es, Lu,TOP, Es, Lu,BOT, Es), • include dark scans, and an • optical multiplexer switches between fiber-optic inputs. Schematics of the Marine Optical BuoY, locator chart and an example of the variability in the physical location of the system caused by winds and currents. Downwelling Surface Spectral Irradiance Measurements with the Marine Optical BuoY B. Carol Johnson, Michael E. Feinholz, Stephanie J. Flora, Mark A. Yarbrough, Terrence Houlihan, Darryl Peters, and Dennis K. Clark National Institute of Standards and Technology, Gaithersburg, MD USA 20899; Moss Landing Marine Laboratories, Moss Landing, CA USA 95039; Marine Optical Consulting, Arnold, MD USA 21012 Abstract The Marine Optical BuoY (MOBY), off the coast of Lanai, Hawaii, has made in situ, in-water up-welling spectral radiance and downwelling surface spectral irradiance measurements since July 1997. The MOBY observatory concept was developed and implemented specifically to serve as the primary in-water oceanic observatory for the vicarious calibration of the U. S. satellite ocean color sensors SeaWiFS and MODIS. Vicarious calibration requires establishment of long-term radiometric stability, traceable to national standards, for all measurements. The downwelling surface irradiance measurements are primarily used for reliable determinations of the diffuse attenuation coefficients. In this poster, we present the preliminary results for the MOBY surface irradiance time series. The MOBY results feature hyperspectral, high resolution full spectral coverage (340 nm to 955 nm). We describe the calibration and characterization of this data set, and the present status of its investigation. The data set is of interest because the results can be spectrally averaged [e.g., photosynthetically active radiation (PAR) response] and studied for trends; a study of interest to the global dimming community because there are very few well-calibrated, long term measurement sites in open ocean environments. • Summary • For the first time: • A decadal time series of hyperspectral downwelling spectral irradiance for an open ocean site is presented; • Represents a unique data set, calibrated with full rigor to the SI; • A model to remove seasonal effects has been applied; • No statistically significant trend to these data as presented or analyzed; • Premature to rule on existence of trends exist or not; • Known instrumental effects are under investigation; • So, stay tuned. MOBY Observatory • Introduction • Global dimming and brightening (GDB): • The study of trends in the amount of solar radiation incident on the Earth’s surface; • Radiometric quantity is the spectrally-integrated (or total) global (direct beam plus diffuse sky radiation) irradiance, Eg↓. • Magnitude and slope of apparent decadal trends in Eg↓ is controversial: • Research on the role of natural and anthropogenic aerosols – • scatter, reflect, or absorb incident solar radiation; • influence cloud formation, and clouds have a major impact on the Earth’s radiation budget. • Interest arises from proposed links • Between anthropogenic aerosols and a cooler surface (cleaning up air would increase surface temperatures) • To the hydrologic cycle through cloud formation and evaporation rates (moist atmosphere but dryer soil) • The observational evidence for GDB measurements: • Derived primarily from broadband thermopile radiometers – • these pyranometers are components of land-based, surface radiation budget programs; • measurement results are traceable to Sun, not the International System of Units (SI). • Continuous decadal time series of Eg↓ measurements for the open ocean do not exist. • The Marine Optical BuoY (Moby) downwelling surface spectral irradiance, Es(λ) measurements: • Daily measurements with this radiometric buoy stationed 20 km off the coast of Lanai, Hawaii; • Up-welling spectral radiance, Lu(z,λ), and downwelling surface spectral irradiance, Es(λ), are acquired; • The MOBY Es(λ) data are the spectral counterpart to Eg↓ over a critical fraction of the Eg↓ 300 nm to 2800 nm range; • The MOBY Es(λ) data overlap with the PAR spectral region (400 nm to 700 nm); • The values are traceable to the NIST radiometric reference standards; • Operation of the MOBY observatory is being continued by NOAA in support of the upcoming VIIRS mission. • We report preliminary results of the MOBY decadal time series for Es(λ) as an investigation of this data set for GDB studies. Time series of the MOBY downwelling surface spectral irradiance response, MODIS band averaged and normalized to the means, for all deployments. The two groupings are for the even and odd buoys. An example of a typical clear atmosphere MOBY Es(λ) data set. Note that the Fraunhofer lines and the O2 A bands are clearly evident. Results Analysis Steps Every day, three complete sets of measurements are taken, timed to occur with the satellite overpasses. In each “hour” file, up to eight average Es(λ) scans (N=5) are acquired. We set a cutoff on the standard deviation of the five scans (taken sequentially) and the set of scans within each hour file that bracket the Lu measurements. This accounts for variability such as cloud contamination. To account for constant overcast conditions, we set a lower limit on solar normalized Es(490). The data were converted to physical units using the system responsivity, which includes a pre- and post-deployment radiometric and wavelength calibration. A correction for stray light and ambient temperature was applied. For this work, we incorporated for the first time the results of a laboratory determination of the cosine response of the MOBY Es(λ) optical sensor. The two panels on the left show the time series for the filtered MOBY Es(λ) values with the cosine response and solar normalization corrections applied. The hyperspectral data were weighted to SeaWiFS band 3 at 490 nm (top) and band 8 at 865 nm (bottom). The two panels on the right are the running weekly mean of these results. The green line is the result of a linear regression, and the red lines are ±5%. There is excellent continuity, as retrievals from sequential buoys typically match up. There are intervals with increased variability. The fitted slopes have opposite signs and this result may be related to instrument artifacts, may be statistically significant, or may be of great interest to the GDB community if it is valid. There are instrumental effects to be pursued, e.g., in the CIE color diagram the even and odd buoys are distinguishable (see below). Cosine Response Operational Corrections Laboratory Measurements The solar zenith angle, , with the buoy vertical (“no tilt”) and with the observed tilt added or subtracted to (top panel). The magnitude of the cosine response correction, based on the laboratory characterization data, for these values of  (lower panel). Photograph of the cosine response setup and results in terms of the ratio of measured to ideal. No spectral dependence was found for this correction. CIE Coordinates Seasonal Effects The CIE color diagram calculated using the 10 year time Es(λ) time series. The even and odd buoys are distinguishable; part of this difference could be from slight effects in the stray light corrections. a) The range of solar zenith angles and b) the magnitude of the cosine correction for the different hour files over the entire MOBY time series. a) b) The corrections to the Es(λ) values for the SeaWiFS band 3 at 490 nm for cosine response (“gonio”) and solar normalization (“Fn”). Solar normalization includes zenith angle, Earth-sun distance, Rayleigh scattering, and ozone absorption. The corrections are shown normalized by their respective uncorrected values. The authors acknowledge the current support of NOAA/NESDIS/STAR as well as previous support – NASA (NNG04HK33I). Dennis Clark (Marine Optical Consulting, Arnold, MD) is affiliated with Space Dynamics Laboratory as part of the Joint NIST/Utah State University Program in Optical Sensor Calibration.