Post on 20-Jan-2016
SeaWiFS JD149 vs. Sunphotometer Scans
Wavelength (nm)
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ical
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SeaWiFS mean
SeaWiFS ± std. dev.SCAN47
SCAN48
SeaWiFS JD141 vs. Sunphotometer Scans
Wavelength (nm)
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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
Wavelength (nm)
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SeaWiFS mean
SeaWifs ± std. dev.SCAN16
SCAN17
400 450 500 550 600 650 700 750-0.002
-0.001
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circles - SeaWiFS extracted, Gordon standard atmospheric correctionsquares- SeaWiFS extracted, NRL NIR and 412 iterations
ASD vs. SeaWiFS Rrs ComparisonStations Correspond to Aerosol Comparison
ASD7, JD141 ASD24, JD149
Rrs
(sr
-1)
Wavelength (nm)
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)
129 130 131 132 133 1340.0
0.1
0.2
0.3
temp (C)sal (PSU)b440 (m-1) a440 (m-1)
a440 b440 salinity temperature
Longitude
1.0
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33.8
34.0
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34.4
34.6
8
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37.5 38.0 38.5 39.0 39.5 40.0 40.50.0
0.1
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0.3temp (C)
a440 b440 salinity temperature
Latitude
1.0
1.5
2.0
2.5
3.0
3.5
33.8
34.0
34.2
34.4
34.6
sal (PSU)b440 (m-1) a440 (m
-1)
8
10
12
14
16
18
129 130 131 132 133 13412
13
14
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18
= 1.2 C
overpass
AVHRR SST (NOAA-12, 5/21/99, 0636 local) Flow-Through SST (5/20/99, 1653 - 5/21/99, 2059)
SS
T (
C)
Longitude37.5 38.0 38.5 39.0 39.5 40.0 40.59
10
11
12
13
14
15
16
17
18
overpass
AVHRR SST (NOAA-14, 5/22/99, 0419 local) Flow-Through SST (5/22/99, 0113 - 5/22/99, 2133)
SS
T (
C)
Latitude
129 130 131 132 133 1340.0
0.1
0.2
0.3
overpass
SeaWiFS a443 (5/21/99, 1248 local) Flow-Through a440 (5/20/99, 1653 - 5/21/99, 2059)
Ab
sorp
tion
Coe
ffici
en
t (m
-1)
Longitude
129 130 131 132 133 1340.0
0.1
0.2
0.3
0.4
0.5Flow-ThroughSeaWiFS
SeaWiFS b443 (5/21/99, 1248 local) Flow-Through b440 (5/20/99, 1653 - 5/21/99, 2059)
Sca
tterin
g C
oeffi
cien
t (m
-1)
Longitude
1.0
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3.5
overpass
37.5 38.0 38.5 39.0 39.5 40.0 40.50.0
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SeaWiFS a443 (5/21/99, 1248 local) Flow-Through a440 (5/22/99, 0113 - 5/22/99, 2133)
Abs
orpt
ion
Coe
ffici
ent (
m-1)
Latitude
37.5 38.0 38.5 39.0 39.5 40.0 40.50.0
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Flow-ThroughSeaWiFS
SeaWiFS b443 (5/21/99, 1248 local) Flow-Through b440 (5/22/99, 0113 - 5/22/99, 2133)
Sca
tterin
g C
oeffi
cien
t (m
-1)
Latitude
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50
Scatterplot (AOT.STA 112v*13c)
WAVELNTH
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SeaWiFS mean
SeaWiFS ± std. dev.SCAN47
SCAN48
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