Oceanographic Applications:
Ocean Color


This lecture includes the following topics:

1. Basic principles of satellite measurements of ocean color

2. Coastal Zone Color Scanner (CZCS)

3. Sea-viewing Wide Field-of-view Sensor (SeaWiFS)

4. MODerate resolution Imaging Spectroradiometer (MODIS)

5. Patterns of phytoplankton distribution in World Ocean obtained from ocean color


1. Basic principles of satellite measurements of ocean color

The measurements of ocean color are based on electromagnetic energy of 400-700 nm wavelength. This energy is emitted by the sun, transmitted through the atmosphere and reflected by the earth surface.

 

The sunlight is not merely reflected from the sea surface. The color of water surface results from sunlight that has entered the ocean, been selectively absorbed, scattered and reflected by phytoplankton and other suspended material in the upper layers, and then backscattered through the surface.

The color of water surface is regulated by the color of pure ocean water and the concentrations of different types of particles suspended in the upper water layer. At this image you see that in the coastal zone high concentrations of phytoplankton and suspended matter change water color.

 

The change of water color is also evident at satellite true-color images.

 

Clean ocean water absorbs red light, i.e., sun radiation of long wavelength and transmits and scatters the light of short wavelength. That is why ocean surface looks blue.

Phytoplankton cells contain chlorophyll that absorbs other wavelengths and contributes green color to ocean water.

In coastal area suspended inorganic matter backscatters sunlight, contributing green, yellow and brown to water color.

Thus, color (including water color) can be measured on the basis of the spectrum of visible light emitted from the study object.

Clean ocean water (A) has maximum in short (blue) wavelength and almost zero in yellow and red.


Higher is phytoplankton (i.e., chlorophyll and other plant pigments) concentration, more is contribution of green color (B).

In coastal zones with high concentration of dead organic and inorganic matter light spectrum has maximum in red (C).

For most regions of the world, the color of the ocean is determined primarily by the abundance of phytoplankton and associated photosynthetic pigments.

Measuring light reflected by ocean surface we can quantify its color measuring radiation at different wavelengths and relate it to phytoplankton concentration.

Chlorophyll a is a primary source of green color. Simple, semi-empirical equations can be used to estimate the concentration of chlorophyll-a and its degradation products from satellite measurements of backscattered sunlight at several (typically two or three) wavebands centered at blue and green regions of the spectrum.

The transparency of clean open ocean water is very high; the upper layer of tens of meters depth contribute to ocean color, this contribution decreasing with depth.


In turbid coastal waters the depth of the upper layer decreases to few meters.

 

Sunlight backscattered by the atmosphere contributes 80-90% of the radiance measured by a satellite sensor at these key wavelengths. Such scattering arises from dust particles and other aerosols, and from molecular (Rayleigh) scattering.

However, the atmospheric contribution can be calculated and removed if additional measurements are made in the red and near-infrared spectral regions (e.g., 670 and 750 nm). Since blue ocean water reflects very little radiation at these longer wavelengths, the radiance measured is due almost entirely to scattering by the atmosphere. Long-wavelength measurements, combined with the predictions of models of atmospheric properties, can therefore be used to remove the contribution to the signal from aerosol and molecular scattering.


Three water color sensors are of great significance for oceanography:

CZCS = Coastal Zone Color Scanner (1978 - 1986)

 

SeaWiFS = Sea-viewing Wide Field-of-view Sensor (since 1997)

 

MODIS = Moderate Resolution Imaging Spectroradiometer

Terra Satellite launched December 18th, 1999
Aqua satellite launched May 4th, 2002.


2. Coastal Zone Color Scanner (CZCS)

The Coastal Zone Color Scanner (CZCS), was a multi-spectral line scanner developed by NASA to measure ocean color as a means of determining chlorophyll concentrations and the distributions of particulate matter and dissolved substances.

 

CZCS was launched aboard Nimbus-7 satellite platform in October 1978. Due to the power demands of the various on-board experiments the CZCS operated on an intermittent schedule. The infra-red/temperature sensor (channel 6 - 10.5-12.5 microns) failed within the first year.

Nominal orbit parameters for the Nimbus-7 spacecraft are:

Launch date 10/24/1978
Orbit Sun-synchronous, near polar
Nominal Altitude 955 km
Inclination 104.9 deg
Nodal Period 104 min
Equator Crossing Time 12:00 noon (ascending)
Nodal Increment 26.1 deg

 

The following lists the sensor's channels and the primary purpose of each:
 
 Reflected solar energy was measured in 5 channels:

 

1
433-453 nm (blue)   chlorophyll absorption
2
510-530 nm (green)   chlorophyll concentration
3
540-560 nm (yellow)   Gelbstoffe concentration
4
660-680 nm (red)   aerosol absorption
5
700-800 nm (far red)   land and cloud detection


Infrared radiation was measured in one channel:

6
10.5-12.5 microns (infra-red) surface temperature

 

The CZCS had a scan width of 1556 km centered on nadir and the ground resolution was 0.825 km at nadir.  

The scenes (images) obtained by CZCS are partial orbital swaths.

In one two-minute data segment, the CZCS covers approximately 1.3 million square kilometers of the ocean surface. CZCS collected about 60,000 images.

Each scan of the CZCS viewed the Earth for approximately 27.5 microseconds. During this period, each channel of the analog data output was digitized to obtain a total of about 2000 samples. Successive scans occur at the rate of 8 per second.

The archive of CZCS data products began with November 2, 1978 and continued until June 22, l986. However, there are several periods of intermittent coverage. When operating full time, approximately 400 images were collected each month.

Level 1 data were processed to Level 2 – surface plant pigment concentrations.
These Level 2 data were compiled onto daily, weekly and monthly mosaics (Level 3 data). Each Level 3 file is a fixed, linear latitude-longitude (equal angle) grid of dimension 1024 (latitude) x 2048 (longitude) with ~18.5 km resolution at the equator.

Daily and weekly composites contain too few information to obtain a realistic global pattern of plant pigment distribution.

 

Monthly Level 3 composites provide realistic patterns of phytoplankton concentration during 7.5 years (from November 1978 to June 1986).

At this image monthly composites from November 1978 to October 1979 are shown.

 

This image shows total surface plant pigment concentration in the World Ocean averaged over the entire period of CZCS observations (November 1978 – June 1986).

CZCS data can be obtained from the website

ftp://daac.gsfc.nasa.gov/data/czcs/

It contains:

Level 1A GAC data;

 

Level 2
 

browse data (chlorophyll concentration);
GAC data

 

Level 3
  Monthly data


Level 1 data can be ordered from website
http://daac.gsfc.nasa.gov/data/dataset/CZCS/


3. Sea-viewing Wide Field-of-view Sensor (SeaWiFS)



The SeaWiFS program was started in 1980s, immediately after the end of the CZCS mission.

The launch of the satellite was first planned on 1993, but the spacecraft was launched in 1997.

The SeaWiFS radiometer and the OrbView-2 spacecraft were developed by ORBIMAGE Corporation in cooperation with NASA.

 

OrbView-2 satellite was launched to low Earth orbit on board an extended Pegasus launch vehicle on August 1, 1997.

Orbit Type

Sun Synchronous

 

Altitude

705 km

 

Equator Crossing

Noon +20 min
descending

 

Orbital Period

99 minutes

 

Swath Width 2,801 km LAC/HRPT
(58.3 degrees)
 

1,502 km GAC
(45 degrees)

 

Spatial Resolution 1.1 km LAC,
4.5 km GAC

 

The swaths overlap in high latitudes and are separated in low latitudes. Thus, each location of the ocean is observed every other day.

The OrbView-2 spacecraft navigation is a self-contained system, with complete information about the attitude of the spacecraft and its orbital position integrated into the downlinked data stream.

Orbit determination is provided by on-board Global Positioning System (GPS) receivers. The GPS receivers yield 100 m position accuracy (after ground postprocessing).

 

Instrument Bands

 

Band

Wavelength

 

1
402-422 nm
2
433-453 nm
3
480-500 nm
4
500-520 nm
5
545-565 nm
6
660-680 nm
7
745-785 nm
8
845-885 nm

 

 

>130 (at April 2004) receiving stations receive Local Area Coverage (LAC) data from SeaWiFS and provide high-resolution (1 km) images to NASA Goddard Space Flight Center (GSFC) in Maryland.

Global Area Coverage (GAC) data transmitted directly to GSFC have lower (4 km) resolution.

 

There is an embargo period of 2 weeks from collection for general distribution of data to research users to protect ORBIMAGE's commercial interest. Access to the data is permitted for research purposes by authorized users.

Level 1A data products contain raw radiance counts from all bands as well as spacecraft and instrument telemetry and calibration and navigation data. Each Level-1A product is stored as one physical HDF file.

Each data file have Browse file which contains a simplified image.

Browsing the files in GSFC Distributed Active Archive Center (DAAC) online database the user can select the files he/she needs, order them and download from FTP.

Level 2 data (geophysical parameters) can be obtained from Level 1 data using software SeaDAS.

 

SeaDAS (SeaWiFS Data Analysis System) is a free software for UNIX or LINUX machines.

To produce Level 2 data from Level 1 the user needs also ancillary (meteorological and ozone) data, which he/she can order at GSFC DAAC.

Meteorological data are produced by National Center for Environmental Prediction (NCEP) at 1-degree global grids every 6 hours. Each HDF file contains: meridional wind, zonal wind, atmospheric pressure, relative humidity and precipitable water.

Ozone data are produced at GSFC from satellite observations (EPTOMS and TOVS satellites).

In the absence of actual meteorological and ozone data the user can use climatological data provided with SeaDAS.

Using SeaDAS Level 1 data (raw radiances) can be processed to Level 2 (chlorophyll concentration, etc.).

 

Global Area Coverage (GAC) data are sub-sampled from full-resolution data with every fourth pixel of a scan line and every fourth scan line being recorded for each swath.

Level 1 GAC data are orbital swaths.

Most useful Level 2 GAC data are produced at GSFC and can be ordered by users.

 

Level 3 data produced from Level 2 data as daily, weekly and monthly global grids of 2048x4096 pixels size. Spatial resolution is about 9 km.

Normalized Digital Vegetation Index (NDVI) can be computed from SeaWiFS observations as (L(865)-L(670))/(L(865)+L(670)) where L is the total radiance less the Rayleigh radiance. At this image NDVI over land is combined with chlorophyll concentration over sea.


4. MODerate resolution Imaging Spectroradiometer (MODIS)

Two MODIS sensors are collecting information onboard satellites:

Terra (EOS AM-1) was launched by NASA December 18 1999
Aqua (EOS PM-1) was launched
May 4 2002

Both satellites have sun-synchronous near-polar orbit.

Terra's orbit around the Earth is timed so that it passes from north to south across the equator in the morning (10:30 a.m., descending node), while Aqua passes south to north over the equator in the afternoon (1:30 p.m., ascending node).

Both radiometers acquire data in 36 spectral bands from 0.4 µm to 14.4 µm. Two bands are imaged at a nominal resolution of 250 m at nadir, with five bands at 500 m, and the remaining 29 bands at 1 km. A ±55-degree scanning pattern at the orbit of 705 km achieves a 2,330-km swath and provides global coverage every one to two days.

MODIS data will improve our understanding of global dynamics and processes occurring on the land, in the oceans, and in the lower atmosphere.

The three basic types of MODIS ocean data products are ocean color, sea surface temperature, and ocean primary production.

In contrast to SeaWiFS, all MODIS data including high-resolution (1 km) data are processed to Level 2.

Each "granule" (I.e., data file) represents 5 minutes of the satellite viewing. The ocean color products are only collected during the day, resulting in 144 granules per day. The sea surface temperature products are collected both day and night, resulting in 288 granules per day. Pixels have 1 km spatial resolution. There are 1354 pixels across the granule, and 2030 pixels down the granule. Thirty-six ocean color parameters and four sea surface temperature parameters are available.

Level 2 granule MODIS Chlorophyll) - Florida, Bahamas, Cuba - October 29, 2000

There are also thirty-five quality control (QC) parameters which are inputs to the ocean algorithms and atmospheric correction (e.g., meteorological data, radiance data, brightness temperatures). The QC files also contain latitude and longitude for each pixel.

Typical QC is from 0 to 3, I.e. from “good” to “bad” quality.

At this image the chlorophyll data with QC >1 are shown black.

MODIS Level 2 data include:

Normalized Water-leaving Radiance on 412, 443, 488, 531, 551, 6617, and 678 nm;
Atmospheric parameters (e.g., Aerosol optical thickness at 865 nm); Chlorophyll concentration estimated by different algorithms;
Total photosynthetic pigment concentration;
Chlorophyll fluorescence;
Total suspended matter concentration;
Coccolithophore concentration;
Concentrations of photosynthetic pigments (phycoerythrobilin, phycourobilin);
Photosynthetically available radiation (PAR);
Gelbstoff absorption coefficient;
Sea Surface Temperature;
etc.

MODIS data also include Level 3 and Level 4 products, represented at global grids of 4096x8192 size, which corresponds to 4.5 km resolution.

Normalized Water-leaving Radiance at 443 nm - March 6 - 13, 2001

Semi-analytic Weekly Primary Productivity Index Behrenfeld-Falkowski Model March 6 - 13, 2001


5. Patterns of phytoplankton distribution in World Ocean obtained from ocean color

CZCS, SeaWiFS and MODIS data revealed general pattern of phytoplankton biomass distribution in the World Ocean.