Notes on satellite products using MODIS 250-m data

Mati Kahru, mkahru@ucsd.edu

· This document describes the process of generating enhanced true color product from MODIS data using Wimsoft (http://wimsoft.com) software. A more detailed tutorial is available here.

· At best we can have 2 passes per day: Terra descending node at approximately 10:30 local time and Aqua ascending node at approximately 13:30 local time. A good pass with the selected point near t center of the swath occurs approximately every second day. Of course, clouds further limit the availability of MODIS data. In a recent study using MODIS 250-m data (Kahru et al., MODIS detects a devastating algal bloom in Paracas Bay, Peru, EOS, Trans. AGU, Vol. 85, N45, p. 465-472, 2004) we were able to obtain 20 cloud-free, good-quality Terra and Aqua images of a small bay in Peru for a 25-day period including 3 days with both Terra and Aqua images in the same day. In clear-sky areas like the Gulf of California even more frequent coverage is possible.

· Both MODIS-Terra and MODIS-Aqua data can be used when using Level-1B data but only MODIS-Aqua data is currently being processed to Level-2 Ocean products (e.g. nLw and chlor_a). Using the procedures described below there is no difference in the quality between MODIS L1B data from Aqua and Terra. The advantage of using MODIS-Terra L1B 250-m data is available from 2000-02-24 whereas the MODIS-Aqua L1B 250-m data is available from 2002-07-04.

· The following MODIS bands are used of the L1B data:

Primary Use	Band	Bandwidth (nm)	Central Wavelength (nm)	Pixel Size (m)
Land/Cloud/Aerosols Boundaries	1	620 - 670	645.5	250
Land/Cloud/Aerosols Boundaries	2	841 - 876	856.5	250
Land/Cloud/Aerosols Properties	3	459 - 479	465.6	500
Land/Cloud/Aerosols Properties	4	545 - 565	553.6	500

· Currently there are 2 products that can be generated at 250-m resolution: turbidity and RGB. The RGB image is a combination of bands 1, 4, 3 and is sometimes called “true-color” but we are often stretching the colors to visualize certain features and the output is not even close to the “true color”. Other products are in the development.

· An example of the turbidity product for the San Francisco Bay area (left image) in grayscale is shown below. Darker color means higher turbidity, lighter colors means lower turbidity. The San Francisco Bay has the size and high turbidity contrast that makes it suitable for applying MODIS 250-m data. The turbidity product is currently a relative index of turbidity as no quantitative validation has been attempted. It would be interesting to attempt to create a quantitative algorithm using USGS in situ turbidity data from the San Francisco Bay area (http://sfbay.wr.usgs.gov/access/Fixed_sta/).

· The turbidity product is less useful for the San Diego area (right image) where 1) the San Diego Bay is much smaller; 2) the turbidity contrast is relatively small; 3) the kelp beds off La Jolla and off Point Loma seem to interfere with the algorithm (the turbidity index of the kelp bed area is artificially low). An example of turbidity off San Diego is shown below.

· The RGB product using MODIS bands 1, 4, 3 is very useful for detecting harmful algal blooms or other unusual phenomena (e.g. Kahru et al., EOS, Trans. AGU, Vol. 85, N45, 9 November 2004).

A harmful phytoplankton bloom dominated by the dinoflagellate Gymnodinium sanguineum in Paracas Bay, Peru in April, 2004 caused estimated economic damage of US$28.5 million. Standard ocean color products of SeaWiFS and MODIS were of little use due to insufficient resolution and problems in atmospheric correction and radiance inversion. MODIS medium-resolution bands were successfully used to monitor the bloom using empirical algorithms.

The left column shows the quasi true-color images using, MODIS bands 1 (red), 4 (green), and 3 (blue) that clearly identify the bloom in the Bay by its conspicuous bright color.

The right column shows the turbidity index, a semi-quantitative measure of the amount of particulate material in the near-surface water. Julian day is shown for left panel and date (month/day/year) for the right panel.

While turbidity is not specific to algal blooms, it is a quantitative estimate of the intensity of the bloom once its existence has been detected by the true-color images. Turbidity was inversely correlated with oxygen concentration during the bloom. Oxygen depletion caused most of the damage to the benthic communities.

Adobe Systems

During the 25-day period of the bloom we were able to obtain 20 cloud-free, good-quality Terra and Aqua images of a small bay, including 3 days with both Terra and Aqua images in the same day.

· An example RGB image of the San Francisco Bay area is shown below.

· The products generated from L1B data can be combined with standard Level-2 ocean products. The following is the standard chlor_a product from the same Aqua pass.

· The corresponding RGB product shows the high chlorophyll areas as dark (because of the strong absorption by phytoplankton and the associated pigments).

The following examples show the detection of sediment plumes during the exceptional California rainfall of winter 2005

Quasi true-color, turbidity and a combination of turbidity over ocean and quasi true-color over land are shown for various areas and satellite passes.

· Santa Barbara Channel, Terra-MODIS pass of January 12, 2005; extensive sediment plume from the Santa Clara river:

· Santa Barbara Channel, Terra-MODIS pass of January 30, 2005; extensive sediment plume from the Santa Clara river: