SeaWiFS JD149 vs. Sunphotometer Scans

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SeaWiFS JD141 vs. Sunphotometer Scans

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Characterizing Surface and Subsurface Thermal and Bio-Optical Fields in the Japan/East Sea During a Spring Bloom: Shipboard Measurements and Satellite ImageryRichard W. Gould, Jr1, Robert A. Arnone1

Craig M. Lee2

Burt H. Jones3

1Naval Research Laboratory, Code 7333, Stennis Space Center, MS 395292Applied Physics Laboratory, University of Washington, Seattle, WA 98105

3Dept. Of Biological Sciences, University of Southern California, Los Angeles, CA 90089In May/June 1999, we conducted an intensive shipboard survey in the Japan/East Sea, focusing on the thermal and optical variability across the basin and in the region of the Subpolar Front. In addition, a shipboard satellite receiving system for AVHRR and SeaWiFS imagery provided synoptic coverage and enabled us to track features of interest and optimize stations and cruise tracks. Underway flow-through measurements of temperature, salinity, absorption, and attenuation were collected to describe surface features; CTD profiles and 3D Seasoar grids provided information on the vertical structure of the optical and physical fields.

We examine the timing and spatial development of the spring bloom and the corresponding changes in the surface thermal field. We examine atmospheric correction and aerosol problems in the East Sea. We describe surface thermal and optical spatial patterns using shipboard and satellite measurements, and we link the surface expressions to the subsurface structure. We observed close agreement between shipboard-measured absorption and scattering coefficients and satellite-derived estimates, in terms of spatial pattern. Optical and thermal variability was clearly expressed along transects across meanders and eddies - sharp discontinuities were observed at the water mass boundaries in both the shipboard data and satellite imagery.

The Japan/East Sea is a semi-enclosed basin with restricted inflow and outflow and complex circulation. Warm, oligotrophic water enters the basin from the south through the Tsushima/Korea Strait and bifurcates into the Tsushima Current that flows eastward along the coast of Japan and the East Korean Warm Current (EKWC) that flows northward along the Korean Peninsula. A central branch through the central southern basin may also be present (Naganuma, 1977). The EKWC breaks away from the Korean coast near 37 N and meanders eastward, bisecting the basin and forming the Subpolar Front around 40 N (Figure 1). The front separates the cold, dense, weakly-stratified northern water from the warmer, stratified water to the south. The Subpolar Front is also a region of very active eddy formation and exhibits strong thermal and bio-optical gradients. Flow exits the basin through the Tsugaru and Soya Straits. By coupling AVHRR and SeaWiFS imagery we define the SST and bio-optical properties and begin to characterize and interpret the variability. Although SeaWiFS imagery can be used to derive surface optical properties (absorption, backscattering coefficients), additional validation of the atmospheric correction and bio-optical algorithms is required. CTD hydrosections and towed, undulating profiles (SeaSoar) enable us to link the subsurface distribution patterns with the surface expressions visible in the satellite imagery, to better understand the mixing processes.

ABSTRACT

INTRODUCTION

• Examine the spatial and temporal variability of the spring bloom.• Collect ground truth reflectance, aerosol optical thickness, absorption,

and scattering measurements to validate SeaWiFS atmospheric correction and optical algorithms.

• Compare spatial patterns from satellite and shipboard flow-through measurements.

• Examine relationships between surface and subsurface thermal and ocean color (bio-optical) features.

OBJECTIVES

1.) Spring Bloom: Chlorophyll and SST

METHODSWe have compiled a two-year climatology of daily AVHRR (SST) and

SeaWiFS (bio-optics) imagery of the Japan/East Sea, at 1 km spatial resolution. The imagery provides a long-term data base to characterize the spatial and temporal variability of the region as well as a broader context to aid interpretation of cruise data. Products from the SeaWiFS imagery include chlorophyll concentration and absorption and scattering coefficients at six wavelengths. In addition to the daily imagery, weekly and monthly composites have been created to reduce cloud-covered pixels.

Much of the spring cruise (May/June 1999) was dedicated to collecting highly-resolved undulating profiles using a towed SeaSoar instrument equipped with physical and bio-optical sensors. Upper-ocean velocities were measured with a shipboard acoustic Doppler current profiler (ADCP). During the cruise, we collected real-time satellite thermal and ocean color imagery (AVHRR and SeaWiFS) using a shipboard receiving system, enabling us to optimize station locations and cruise tracks and providing the large-scale context of the circulation. We collected along-track measurements of absorption (a) and scattering (b) at nine wavelengths using an AC9 meter in flow-through mode, for comparison with SeaWiFS-derived estimates. We measured remote sensing reflectance (Rrs) with an ASD Field Spectroradiometer and aerosol optical depth (AOD) with a sunphotometer for comparison with satellite estimates.CruiseTrack

Bathymetry

ADCP Currents(5/21, 0945 – 5/25, 0500)

overlaid onAVHRR SST

(5/20, 2136)

Chlorophyll SST

March MarchAprilApril

MayMayJune June

2.) SeaWiFS Atmospheric Correction: Standard vs NIR/412 Iteration

A

B

SeaWiFS vs. Shipboard Measurements: Aerosols and Reflectance

SeaWiFS JD141 vs. Sunphotometer Scans

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ASD vs. SeaWiFS Rrs ComparisonStations Correspond to Aerosol Comparison

ASD7, JD141 ASD24, JD149

Rrs

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3.) Shipboard Flow-Through: Physical vs. Optical Distributions

4.) Satellite vs. Shipboard Measurements: Surface and Subsurface Coupling

TRANSECT A(E/W Cross-Basin)

TRANSECT B(N/S Subpolar

Front)

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Scatterplot (AOT.STA 112v*13c)

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In the Japan/East Sea in 1999, the spring bloom started in late March – early April in the southern basin. By late April, the bloom progressed northward to cover the entire central and southern regions, including the Subpolar Front and even coastal areas along the Russian coast in the north. In May the bloom extended over the northern basin while chlorophyll values to the south decreased. Values north of the front remained high through May; by June, highest chlorophyll concentrations were localized in the Korean and Japanese coastal currents in the south with values north of the front declining. SST increased from south to north from March through May, with colder waters gradually retreating northward.

5/21/99 5/29/99

Standard AtmosphericCorrection

Standard AtmosphericCorrection

NIR/412Iteration

StandardAtmosphericCorrection

StandardAtmosphericCorrection

NIR/412Iteration

NIR/412Iteration

NIR/412Iteration

SeaWiFS Rrs(412) and Rrs(670) are negative in clear water?

Aerosol problem?

Calibration problem? (blue pixels indicate negative Rrs)

Rrs(412)

Rrs(670)

AOT(412)()

5/21/99 - AOT 5/29/99 - AOTblue lines from SeaWiFS, red lines from sunphotometer

SubpolarFront

Optical andthermal fields

are tightlycoupled

SeaWiFS aerosol models don’t match measured aerosol pectral shapes, yet SeaWiFS Rrs at blue wavelengths underestimate measured Rrs values ???

Rrs(412)

Rrs(670)

AOT(412)()

TRANSECT A(E/W Cross-Basin)

TRANSECT B(N/S Subpolar

Front)

SST

AbsorptionCoefficient

ScatteringCoefficient

SubpolarFront

Ship station(Rrs and AOT measurement)

Solid lines –shipboard RrsSymbols – SeaWiFS Rrs

red – 5/21/99blue – 529/99

red – satelliteblue - shipboard

circles- standardsquares - iteration

• Started in late March/early April in the southern basin, progressed northward through late May, and ended by early June.

• SST increased from south to north during the same period.• Standard SeaWiFS atmospheric correction yields negative Rrs(412) and Rrs(670), possibly due to strongly

absorbing aerosols in this region and/or a calibration problem.• A new iterative procedure adjusts the Rrs(412), and some of the Rrs(670), to positive values.

• Optical and physical fields (temperature and salinity) are tightly linked.• Highest absorption and scattering coefficients are coincident with physical gradients.

• Strong correlation was observed between satellite and shipboard measurements, in terms of spatial pattern (SST, absorption, scattering) and absolute magnitude (SST, absorption).

• Temperature and salinity isolines inside filaments can be displaced 225 m relative to ambient.• A chlorophyll maximum was observed at 35 m in the seasonal thermocline along both transects.

1001500cm/sec

Subsurface expression of surface features

(Seasoar)

Summary

Spring Bloom

SeaWiFS Atmospheric Correction

Optical/Physical Linkages

Satellite/Shipboard Comparisons and Surface/Subsurface Coupling

Strong correlation

between satellite (AVHRR,SeaWiFS)

and in situ measurements

SeaWiFSSeaWiFS

Flow-ThroughFlow-Through