Assessment of the calibration factors

[BaptisteVandecrux]

Coefficient factors need to be applied to the CR1000 logger files either to get back to physical units, or to account for the sensor's sensitivity. At the moment it is the same calibration factors used for all the years where CR1000 datalogger were used, assuming that there was no instrument change requiring different calibration factors.

The usual range of calibration factors are:

TCAir	CS500	WindSpeed	SWin	SWref	NetRadPos	NetRadNeg	PressureOffset
~1	~1	~1	200.0	77 or 200	80 or ~9.4	80 or ~11.6	400 or 0

Currently used calibration factors were applied by Derek. As a secondary source of information, the matlab "QCheck" processing scripts also saved the calibration factors used by Koni and Nicolas Bayou in earlier years.

Some calibration factors applied by Derek were adjusted to remove shift in the data and agreed with the calibration factors found in QCheck (see #20 and #15).

Matching calibration factors between current values and the ones found in QC were found at: Swiss Camp, CP1, NASA-U, GITS, Humboldt, Summit, Dye-2, JAR1, Saddle, South Dome, NASA-E, NASA-SE

At Tunu-N there is the following discrepancy.
Currently used Previously used by Derek:

TCAir	CS500	WindSpeed	SWin	SWref	NetRadPos	NetRadNeg	PressureOffset
1.00011	1.0	1.0	200.0	200.0	9.43	11.77	400, 0

Found in QCheck and now used :

TCAir	CS500	WindSpeed	SWin	SWref	NetRadPos	NetRadNeg	PressureOffset	LiCor	year_start	year_end
1.00012	0.998909	0.978299	200.0	200.0	9.43	11.66	400	0.934	2015	2018

At Petermann-ELA there is the following discrepancy.
Currently used:

TCAir	CS500	WindSpeed	SWin	SWref	NetRadPos	NetRadNeg	PressureOffset
1.0	1.0	1.0	200.0	200.0	9.29	11.62	400

Found in QCheck:

TCAir	CS500	WindSpeed	SWin	SWref	NetRadPos	NetRadNeg	PressureOffset	LiCor	year_start	year_end
1.0	1.0	1.0	200.0	200.0	9.51	11.88	400	1.0	2015	2016

Lastly, there is in QCheck a calibration factor called "LiCor offset" that is usually ~0.934 and that is multiplied only to the downward-looking short-wave pyranometer.

See QCDataCalibration Matlab function:

function QCDataCalibration

% -------------------------------------------------------------------------
% --- Calibrates the Dataset (creates Level B Dataset).
% --- Resets the Filters.
% ---
% --- Version 2.00: January 12th, 2011 - N. Bayou, Boulder
% ---               * Calibrates the Statistical Data
% ---               * Handles the box filtering for snow temperatures.
% ---               Modified: January 20th, 2011 - N. Bayou, Boulder
% ---                   * Reference Temperature Calibration error fixed.
% ---               Modified: February 3rd, 2011 - N. Bayou, Boulder
% ---                   * Instruments Mounting introduced
% ---                   * LiCor bias changed (CalFactors(13) -> (9))
% --- Version 2.10: February 9th, 2011 - N. Bayou, Boulder
% ---               * Reverse Mount (Wind Direction) changed
% ---               Modified: February 17th, 2011 - N. Bayou, Boulder
% ---                   * Populates the Wind Sensors Heights from the
% ---                     temperatures corrected surface heights
% --- Version 2.20: July 6th, 2011 - N. Bayou, Boulder
% ---               Corrects the Filters loading problem.
% --- Version 2.21: July 11th, 2011 - N. Bayou, Boulder
% ---               Finds the Snow Heights from the existing C File.
% --- Version 2.30: No Changes
% --- Version 2.31: September 22nd, 2011 - N. Bayou, Boulder
% ---               * Functions Names Change
% --- Version 2.40: No Changes
% -------------------------------------------------------------------------

% -------------------------------------------------------------------------
% --- Global Variables: see VLiariables_vX.xx.xlsx
% --- Dataset Channels: See DatasetChannels.xlsx
% -------------------------------------------------------------------------

% ...
QCAddMSG('Dataset Calibration.')
QCAddMSG('')
QCAddMSG('Calibration factors:')
QCAddMSG('')
QCAddMSG(['Incoming Short Wave Radiation: ',num2str(CalFactors(4)),' W/m2/Volts.'])
QCAddMSG(['Reflected Short Wave Radiation: ',num2str(CalFactors(5)),' W/m2/Volts.'])
QCAddMSG(['LiCor bias: ',num2str(CalFactors(9))])

% ...

% --- Calibrates Short Wave (SW) Radiations ---
Dataset(:,4,2) = Dataset(:,4,1)*CalFactors(4);
Dataset(:,31,2) = Dataset(:,31,1)*CalFactors(4);

Dataset(:,5,2) = Dataset(:,5,1)*CalFactors(5)*CalFactors(9);
Dataset(:,32,2) = Dataset(:,32,1)*CalFactors(5)*CalFactors(9);

Such calibration is not currently applied and the origin of this factor is unsure.

[BaptisteVandecrux]

I can confirm that the LiCor offset was being applied to OSWR in the dataset I received from Koni in 2019.
This should be applied to the data.

Note that the value of this offset is station dependent. See the list of calibration factors found in QCheck.

I can see that at some sites (Swiss Camp, CP1, JAR1) ISWR and OSWR had been exchanged. Then it means we need to first exchange variables and then to apply calibration coefficients. As for now, I apply this coefficient as an add-hoc adjustment (in the metadata/adjustment/_adj.csv), and not as a calibration factor (which should be in the header L0/L0_ini).

To points that are left unsovled:

• It is unsure if the LiCor offset applies on the entire CR1000 period. For now we assume that it does.
• For most stations, the calibration factors SWin and SWref above are the same, so the swapping of these two instrument has no impact on how the datalogger table is read and converted into real units (e.g. at CP1 and JAR1). But that is not the case of Swiss Camp. Maybe at Swiss Camp the ISWR and OSWR should be swapped before we apply SWin and SWref.