Comparisons of Argo profiles and nearby high resolution CTD stations

Megan Scanderbeg

Motivation• Investigate how much recent data is in the reference

database used for DMQC and/ or how much recent CTD data is available

• Find out how paired Argo and CTD profiles compare• Increasingly important to have recent, high quality CTD

data for delayed mode quality control considering recent pressure offset concerns and increasing use of Argo data in global change analyses

• Comes from an Action item from AST-10: M. Scanderbeg will work with S. Diggs to find number of collocated shipboard CTD from 2004-2008 and Argo profiles

Where to get CTD data?• Started with current

reference db (CTD_for_DMQC2008V01), but very few stations in 2004 or later (~200)

• Worked with S. Diggs at CCHDO to get access to most recent CTD data for Argo DMQC use only

• Downloaded all publicly available CTD data on CCHDO website from 12/2003-12/2008

• Combined for a total of 1543 stations

Graph from C. Coatanoan’s presentation on the ref db from DMQC3 showing temporal distribution of CTD data in ref db

CCHDO CTD Stations

Other sources of CTD data• Requested WOD data from NODC’s Search and Select website (

http://www.nodc.noaa.gov/OC5/SELECT/dbsearch/dbsearch.html) on 7/23/09 using the following criteria:– Ocean station data and high resolution CTD/XCTD– 01/01/2004 through 04/31/2009– Temperature and salinity data– All casts deeper than 1000 meters

• Received 17,849 stations, refined to 15,169 CTD only stations• Several of the stations are repeats of data from CCHDO• Ignored overall qc variable assigned to temperature by WOD as

several (1125) profiles failed monthly, seasonal, and/ or annual checks resulting in “bad” overall qc variables even though individual flags on individual measurements were good

WOD CTD Stations

CCHDO stations not in WOD• Searched for duplicate CCHDO data in WOD database by latitude +/- 0.1, longitude +/- 0.1 and day +/- 1• at least 236 stations from CCHDO only

Comparing Argo profile to CTD profile

• Looked for matches within “scaled distance” of 100 km and 10 days• Found 2361 matches• Comparison method:

– Moved Argo profile up and down by 40 dbars compared to CTD profile – For each shift of the Argo profile:

• For each Argo temperature measurement, interpolated to corresponding smoothed CTD temperature and found the difference

– For each pair:• Minimized the sum of the squares of the temperature differences scaled by the

number of differences for each pressure shift • Found the corresponding pressure offset based on the results of minimizing the

sum of the squares– Included the pressure offset only if the minimum was greater than 5

dbar from the end of the shift to try to include cases where a true minimum was found (609 cases where within 5 db of end)

Location of Argo – CTD matches

Conclusions on CTD data

• Recent CTD data is available both through CCHDO (public and private) and WOD/NODC to be added to the QC reference database

• Ideally, an update for the database would be available each year with the cruises from both centers in the past year.

• Desires for the yearly update include:– One format for all data that is easily incorporated into

current database – List of the cruises included in the database

• How best to accomplish this goal?

Conclusions on comparing profiles

• Decent number of matched Argo-CTD pairs, especially in 2004-2007.

• Coverage in time (2008) and space could still be improved

• 2007 mean is a bit larger in magnitude than others years. Need to wait and see if this continues for 2008 as more CTD data becomes available

• Could do same investigation by float type to look for differences

• Method used to find pressure offset does have the potential concern that the thermocline could be dominating the calculation