
JASMINE-1 Leg 2 CTD Data Acquisition and Processing
===================================================

November 6, 2000
Fernando Santiago-Mandujano and Roger Lukas
Dept. of Oceanography
University of Hawaii


Data Acquisition
================

The regular CTD operation consisted in deploying the package to 
10-15 dbar and waiting until the CTD pumps started operating. The 
CTD was then raised until the sensors were close to the surface 
to begin the CTD cast. (The time of each cast was corrected for 
drift in the acquisition computer clock. This correction was not 
larger than 27 seconds.) The position of each cast was obtained 
by matching the beginning time of the cast with the time in a 
navigation file obtained at 10-sec intervals.

The CTD data were acquired at a rate of 24 samples per second 
with a Sea-Bird SBE-911plus CTD package with dual temperature and 
conductivity sensors. The CTD was installed in a vertical 
position in the lower part of a 12-place Rosette, with the 
sensor's water intake located at the bottom of the Rosette. Each 
T-C sensor set had independent pump and plumbing installations. 
Between two and six 5-liter Niskin bottles were closed on each 
cast in order to provide salinity samples for calibration purposes.


Preliminary Processing
======================

After the cruise, the CTD pressure transducer was calibrated 
against a laboratory pressure standard. This pressure standard 
has been periodically recalibrated against a primary pressure 
standard at a specialized facility. The CTD pressure transducer 
showed a 1 dbar offset at 0 dbar, a 0-4500 dbar span offset of 
less than 0.5 dbar, and hysteresis of 0.05 dbar. Comparison 
against previous calibrations obtained at 6-month intervals 
indicate that the sensor has remained stable. Therefore the only 
correction applied to the CTD pressures was a constant offset 
determined at the time that the CTD first enters the water during 
each cast.

The temperature sensors used during the cruise were calibrated at 
Sea-Bird before and after the cruise to an accuracy better than 
0.5 mC. These and previous calibrations were analyzed to 
calculate the drift of each sensor. Offset corrections to the 
cruise data due to sensor drift were insignificant (about 0.1 
mC), and thus were not applied.

The 24 Hz raw CTD data were quality-controlled and screened for 
data spikes. After screening, the correct alignment of 
temperature and conductivity time-series was computed since the 
lag between temperature end conductivity depends on the placement 
of the sensors. Both T-C pairs were also aligned with each other 
by aligning the two temperature sensors. Conductivity was 
corrected for thermal inertia of the glass conductivity cell 
using the recursive filter given by Lueck (1990) and Lueck and 
Picklo (1990). The data were then averaged to half-second values.

Salinity samples were collected at selected depths from each cast 
and measured during the cruise. Samples from a large batch of 
"secondary standard" seawater were frequently measured to detect 
drift in the salinometer, and the secondary standard was 
calibrated against IAPSO standard seawater. The salinity samples 
were used to calibrate the CTD conductivities. After calibration 
the standard deviation of the difference between the CTD and 
sample salinities was less than 0.005.

The 2 Hz CTD data were further screened to eliminate salinity 
spikes which occur when the CTD samples the disturbed water of 
its wake. Samples from the downcast were rejected when the CTD 
was moving upward (due to ship roll) or when its acceleration 
exceeded 0.5 m/s^2 in magnitude.  The data were additionally 
screened by comparing the primary and secondary sensors. These 
differences permitted identification of problems in the sensors.  
Only the data from one pair of sensors, whichever was deemed most 
reliable, are included in the files. The data were then averaged 
to 2-dbar intervals. Only data from the downcast are reported, as 
upcast data are contaminated by wake effects.


Final Processing
================

Details of the final CTD data processing are reported in: "Hydrographic
observations during the Joint Air-Sea Monsoon Interaction Experiment
(JASMINE) cruises", distributed to JASMINE investigators and also
available in the world wide web:  http:\\...  This report also includes
sampling and processing procedures for salinity water samples,
thermosalinograph and ADCP data taken during the main JASMINE cruise.
Sampling and processing procedures for the pre-JASMINE CTD and ADCP
data are also included in the report.


References
==========

Lueck, R. G., 1990: Thermal inertia of conductivity cells:
Theory. Journal of Atmospheric and Oceanic Technology, 7, 741-755.

Lueck, R. G. and Picklo, J. J., 1990: Thermal inertia of conductivity 
cells: Observations with a Sea-Bird cell. Journal of Atmospheric and 
Oceanic Technology, 7, 756-768. 
