California merged satellite-derived 1-km dataset

Mati Kahru, mkahru@ucsd.edu    Updated : 5/07/2020

A merged 1-km dataset using data from multiple satellite sensors has been created for the California EEZ region. The methods are a combination of the methods described for the two related California Current datasets: the extended area 1-km dataset https://www.wimsoft.com/CAL/ and the optimally merged 4-km dataset https://www.wimsoft.com/CC4km.htm.

The datasets covers the state of California EEZ (Fig. 1A) on a grid of 1188 (width) x 1333 (height) with approximately 1 km step are in HDF4 format. The upper-left corner (lat, lon) is 42N; -129E; the lower-right corner is 30N; -115.791E. The longitude of each pixel can be calculated as Lon = -129 + 0.011119 * x where x is the column number from left to right (0 to 1187) and the latitude of each pixel can be calculated as Lat = 42 - 0.009002 * y where y is the row number from top to bottom (0 to 1332)

cid:image001.png@01D622D0.BAC2F850

Fig. 1. The EEZ boundaries (map in Albers Conic Equal Area projection) (left) and the ~1-km grid encompassing it (right).

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 (black in Fig. 2) and 255 (white in Fig. 2) are considered invalid and must be excluded from any statistics. PV = 1 may be used for annotating and for coastlines and has to be excluded too. The annotation color bar may be written over land into some of the products. 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.

Surface chlorophyll-a concentration (Chla, mg m-3) is derived by mapping available Level-2 data and merging valid pixel values of standard algorithms. Extensive validation has been done separately for each sensor and for the merged products (Fig. 3, e.g. Kahru et al., 2012). The merged Chla dataset starts with OCTS in November, 1996, and the continuously available data starts with SeaWiFS in September, 1997. Starting from 2000, data from more than one sensor have been merged.

A set of products are available for each variable. Various compositing operations are performed to fill the gaps caused by missing data (e.g. due to clouds). The following products are available:

(1) Daily merged dataset (Fig. 2, left) has typically many pixels with no data.

(2) Daily 5-day running mean. These are also daily estimates, centered at the middle day. The amount of missing data is reduced significantly(Fig. 2, right). The running mean operation is justifies as Chla usually does not change drastically from day to day.

(3) 5-day composites (made from the daily running means).

(4) 5-day composites interpolated in time to fill missing values. Note: only missing pixel values are affected.

(5) 5-day composites interpolated twice.  As Chla usually does not change rapidly over daily time scales (e.g. compared to Photosynthetically Available Radiation, PAR) these are used in daily Net Primary Production (NPP) estimates.

(6) Monthly composites in HDF4.

(7) Monthly composites in PNG.

 

Fig. 2. An example daily Chla image of 7-Jan-2020 (C2020007_chl_comp.remap_ca1km.hdf, left) and the corresponding 5-day running mean Chla (C20200052020009_chl_comp.hdf, right).

 

Fig. 3. Validation of the standard Chla algorithms for the sensors used in the merged Chla products. Match-ups with in situ Chla for MODIS-Aqua, MODIS-Terra, OLCI-A, OLCI-B, VIIRS-SNPP (VIIRSN) and VIIRS-NOAA20 (VIIRSJ).

 

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

Chla Daily

Chla Daily 5-day running mean

 

Chla 5-day and Monthly composites

1996, 1997, 1998

1996, 1997, 1998

 

Chla 5-day all years

1999, 2000, 2001

1999, 2000, 2001

 

Chla 5-day interpolated

2002, 2003, 2004

2002, 2003, 2004

 

Chla 5-day interpolated 2x

2005, 2006, 2007

2005, 2006, 2007

 

Chla monthly ;            Chla monthly PNG files

2008, 2009, 2010

2008, 2009, 2010

 

 

2011, 2012, 2013

2011, 2012, 2013

 

 

2014, 2015, 2016

2014, 2015, 2016

 

 

2017, 2018, 2019

2017, 2018, 2019

 

 

2020

2020

 

 

NPP Daily

NPP Daily

 

Other