The California Current merged satellite-derived 4-km dataset

Mati Kahru, mkahru@ucsd.edu                                                                 Updated : March 28, 2016

While ocean color satellites measure more or less the same variables, systematic differences exist between the products of different sensors and between satellite products and in situ measurements. Using methods described in Kahru et al. 2012 (Trends in the surface chlorophyll of the California Current: Merging data from multiple ocean color satellites, Deep-Sea Research. II, 77-80, 89-98; PDF) and Kahru et al. 2015 (Optimized merger of ocean chlorophyll algorithms, IEEE Geoscience and Remote Sensing Letters, 12, 11; PDF), a merged dataset using data from multiple satellite sensors has been created for the California Current. The method uses match-ups of each sensor with subsets of a large dataset of in situ measurements (e.g. over 12,000 for surface chlorophyll measurements) to minimize the differences between each sensor and in situ measurements AND between the overlapping satellite sensors. Modern ocean chlorophyll (Chla) datasets start with OCTS (1996) and are followed by SeaWiFS (1997-2010), MERIS (2002-2012), MODISA (2002-present) and VIIRS (2012-present). Daily spectral Rrs data at 4 km resolution (9 km for OCTS and SeaWiFS) are used to calculate daily products for each sensor using the regionally optimized algorithms, after which they are merged. Note that this is different from the 1-km data merger products that are available at http://www.wimsoft.com/CAL/. The 1-km products are merged between the standard products and without applying the optimized algorithms to the Rrs. The daily products are composited into 5-day and monthly products. In order to reduce the amount of missing data (due to clouds), interpolated 5-day datasets are created where the missing pixels are interpolated between the previous and the next 5-day product. The same procedure is applied again to produce 2x interpolated data. The following example shows the effect of temporal interpolation.

Net Primary Production (NPP) are calculated from the merged Chla, merged PAR and SST-OI data according to VGPM-CAL (Kahru et al. 2009, Trends in primary production in the California Current detected with satellite data, J. Geophys. Res., 114, C02004; PDF).

Export Flux of Carbon (EF, mgC m-2 day-1) and export fraction (EF/NPP) are calculated from NPP, SST and other variables using the Laws et al. (2011) model.

 

Fig. 1. An example of a 5-day Chla product of January 1-5, 2003 (left), the interpolated 5-day (middle) and the 2x interpolated (right) 5-day product.

The datasets are in Zipped files and can be downloaded using the following hyperlinks:

Chla Daily

Chla Daily

Chla Daily

Other

1996

2003

2010

Chla 5-day all years

1997

2004

2011

Chla 5-day interpolated

1998

2005

2012

Chla 5-day interpolated 2x

1999

2006

2013

 Chla monthly ;            Chla monthly PNG files

2000

2007

2014

 

2001

2008

2015

SST-OI 5-day 1981-2016, HDF4 files ; SST-OI 5-day 1981-2016, PNG files

2002

2009

2016

SST-OI monthly 1981-2017 ; SST monthly 2000-2017

2017

 

 

 

NPP Daily

NPP Daily

NPP Daily

Other

1996

2003

2010

Net Primary Production monthly

1997

2004

2011

 

1998

2005

2012

 

1999

2006

2013

 

2000

2007

2014

 

2001

2008

2015

 

2002

2009

2016

 
2017

EF, EFr Daily

EF, EFr Daily

EF, EFr Daily

Other

1996, 1996

2003, 2003

2010, 2010

EF = export flux of Carbon, same units as NPP

1997, 1997

2004, 2004

2011, 2011

EFr = export fraction, i.e. EF/NPP

1998, 1998

2005, 2005

2012, 2012

EF and EFr are calculated using daily NPP, SST and Laws et al (2011)

1999, 1999

2006, 2006

2013, 2013

 

2000, 2000

2007, 2007

2014, 2014

 

2001, 2001

2008, 2008

2015, 2015

 

2002, 2002

2009, 2009

2016, 2016

 
2017, 2017
       

 

The datasets on a grid of 540 (width) x 417 (height) with approximately 4000 m step are in HDF4 format.  The upper-left corner (lat, lon) is 45N; -140E; the lower-right corner is 30.03597N; -115.5454E. Chla values in each pixel of unsigned byte are log10-scaled and can be calculated from the pixel value (PV) as: Chl (mg m-3) = 10^(0.015 * PV - 2.0), i.e. 10 to the power of 0.015 * PV - 2.0. Pixel values 0, 1 (black in Fig. 1) and 255 (white in Fig. 1) are considered invalid and must be excluded from any statistics.  PV = 1 is used for coastline and has to be excluded too. The annotation (color bar) is written into the dataset. When reading with Matlab the unsigned byte variable is sometimes reported as signed byte (int8, values from -127 to 128) and not as unsigned byte (values from 0 to 255) and values over 128 become negative. A simple fix is to add 256 if the signed pixel value is negative. Net Primary Production (NPP) was calculated according to the modified VGPM algorithm (Kahru et al. 2009;  PDF). Pixel values are signed 2-byte integers in mgC m-2 day-1 with values -32767 or 32767 meaning no data.

The SST data are converted from the global AVHRR OI dataset (http://podaac.jpl.nasa.gov/dataset/NCDC-L4LRblend-GLOB-AVHRR_OI ) and use the same scaling as  in the full resolution dataset (http://www.wimsoft.com/CAL/Readme.htm).