Satellite support for CCE-LTER

Mati Kahru, mkahru@ucsd.edu

 

Click here for JPEG images

 

Click here for image data in HDF

 

Abstract

 

This document describes the products and analysis of satellite data provided in support of the CalCOFI-LTER cruises. The chlorophyll maps are merged from SeaWiFS (1997-present), Aqua-MODIS (2002-present), and Terra-MODIS (2007-present) data and SST from Terra-MODIS and Aqua-MODIS data. Terra-MODIS Chl-a data is used with lower priority, i.e. only when data from other sensors is not available. Other products will be added in the future.

 

Methods

 

        Methods for creating the full resolution satellite time series are described in Full-resolution satellite time series of the California Current area. In short, chlorophyll-a (CHL-A) datasets are created from SeaWiFS (1997-present), Aqua-MODIS (2002-present) and Terra-MODIS (2007-present) level-2 data. For the overlapping years CHL-A is a merged product from all available sensors. SST is merged from Aqua-MODIS and Terra-MODIS level-2 data.

        For each CalCOFI-LTER cruise the corresponding dataset is downloaded from the CCE LTER Datazoo. A list of stations with respective longitude, latitude, date, time,   surface chlorophyll-a and other variables is created. Composite images of CHL-A and SST are created from daily mapped datasets (with a utility wam_composite_list.exe) for the time period of the cruise 1 day and saved in the HDF format. For color coding of Chl-a a logarithmic scale with the full range 0.05-10.0 mg m-3 (pixel values 48-200) and a lookup table chl1_white_end.lut are used. For color coding of SST a linear scale with the full range of 10.05-35.25 ˚C (pixel values 87-255) and a lookup table sst_kuring.lut are used. Locations of the standard CalCOFI stations are overlaid

        The following is an example of a cruise Chla image (for CAL0404). This image is 8 times reduced from the original full-size image of 3840 x 3805 pixels. Because of the reduced size the station locations and many details are not visible. The full-size JPEG can be seen by clicking on the image below. For 2006 and later the reduced images are in the PNG format and 4 times reduced from the full size image.

 

Fig. 1. SeaWiFS-Aqua-MODIS merged CHL for CalCOFI-LTER 0404.

 

        The following is an example of a cruise SST image (for CAL0404). This image is 8 times reduced from the original full-size image of 3840 x 3805 pixels. Because of the reduced size the station locations and many details are not visible. The full-size JPEG can be seen by clicking on the image below.

 

Fig. 2. Aqua-MODIS SST for CalCOFI-LTER 0404.

 

        Match-up analysis was performed between in situ near-surface (average of samples in the top 5 m) and daily mapped CHL images (merged between SeaWiFS and Aqua-MODIS). Using match-up criteria of less than 12 hours time difference and at least 3 valid pixels in the 3 x 3 pixel window centered at the nearest pixel to the in situ station produced 93 coincident measurements. A link to the match-up file is provided here. The match-up file is a comma separated (CSV) text file with the following columns: Longitude,Latitude,DateGMT,TimeGMT,Cruise,Station,AvgOfChla,Image,Timediff_Hr,VarName,PointValue,Nin,Nout,Min,Max,Mean,StDev,Median. 11 of the match-up points looked as obvious outliers (marked with # in the first character position) and were excluded from Fig. 3. Most column names should be obvious; less obvious are: Nin = number of valid pixels, Nout = number of invalid pixels. The statistics and the pixel counts are given for the 3 x 3 pixel window centered at the pixel nearest to the in situ station.

        Below is a plot of the satellite-detected CHL versus in situ chlorophyll. At in situ Chl-a < 0.5 mg m-3the two estimates agree very well but at in situ Chl-a > 1 mg m-3 it is obvious that satellite estimates are significantly below in situ values. Satellite estimates of the two highest in situ values less than 50% of the in situ values. A number of factors can contribute to this discrepancy. It is possible that the satellite algorithm is somewhat biased but such bias can also be produced by the different foot-prints of the two sampling methods.

 

 

Adobe Systems

 

 

Fig. 3. Satellite-detected CHL versus in situ CalCOFI-LTER chlorophyll (mean for the top 5 m samples). CalCOFI-LTER cruises 0401, 0404, 0407 and 0411 are included. The satellite CHL is estimated as the median in a 3 x 3 pixel window centered at the in situ station. The vertical bars show the min-max range of valid pixels in the 3 x 3 pixel window. The dashed lines show (from top to bottom) one-to-one agreement, 2 times below and 3 times below in situ values.