RAN CTD Profile Data - HMAS LEEUWIN ProjectID: HI416LEE_H From: 2006-02-09 To: 2006-04-23

Brief description

This dataset contains quality controlled vertical profiles of pressure, temperature and salinity measured by a Conductivity, Temperature and Depth (CTD) probe. The dataset contains 8 CTD profiles (casts) obtained during RAN Hydrographic Survey cruise HI416LEE_H from HMAS LEEUWIN.

The CTD was manufactured by Falmouth Scientific and is the "2-inch Micro CTD" model. This CTD type is referred to operationally as the HS_CTD. This CTD is fitted within an inductive type conductivity cell, a platinum thermometer and silicon pressure sensor.

The CTD temperature sensor is calibrated on the ITS-90 temperature scale against a master CTD using a controlled temperature bath. The pressure sensor is calibrated using a Druck Pressure Calibrator. The conductivity sensor is calibrated in a temperature controlled bath against seawater samples of known conductivity.

The HS_CTD is lowered and raised by a hand winch sampling at a rate of 1.83 Hz. Data files were recorded in the downcast and upcast direction but most data is from downcasts due to higher quality. Raw CTD pressure data is not always monotonic due to transient up/down motion of winch/ship. Raw pressure data is subsetted to give a monotonic (increasing) sequence and then linearly interpolated to 1 decibar pressure intervals and converted to netCDF format files. Data is then flagged with quality control flags after visual inspection and comparison to average climatology and historical CTD casts.

Lineage

Statement: Initial Processing

Incoming data undergoes gross range checks and removal of any corrupted or unrealistic data. Some CTD data is subject to a pressure offset error generally less than 1 decibar. A pressure offset error occurs when the pressure sensor does not read 0 dbar in air. In cases where the pressure offset error was observed before the CTD cast the pressure readings have been corrected for the offset which is stored in the global netCDF attribute named 'CTD_pressure_offset'. In some cases there is a separate recording for the downcast and upcast. The downcast data were selected as priority for better quality data. In rarer cases only an upcast was available so it is used by default.

Salinity on the PSS-78 scale is computed from conductivity, temperature and pressure using the UNESCO (1983) algorithm with temperature corrected from the ITS-90 scale to the ITS-68 scale used by the algorithm.

The raw CTD data contains a non-monotonic sequence of pressures due to up/down motion of ship/winch. A sequence of unique monotonic pressures up to the maximum value is extracted and then linearly interpolated to 1 decibar pressure levels with the corresponding interpolated temperature and salinity. Finally the interpolated files are converted to a set of netCDF files ready for QC.

Quality Control

Quality control (QC) involves viewing location of the data on a map, visual inspection of each profile of temperature and salinity and comparison with nearest neighbours and climatology. CTD profiles are checked for any density inversions i.e. density decreasing with depth by computing sigma-t density. A density inversion may indicate unrealistic salinity and/or temperature values. QC flags are applied to indicate whether data is good, suspect, bad or not tested. QC flags may apply at the 'whole profile' level or individual pressure levels.

CTD cast positions and times undergo a land and ship speed check. Casts with position on land or unrealistic speed between casts are flagged as failed position or time. Temperature and salinity profiles are then visually compared against a 3 standard deviation envelope from the CSIRO-CARS (2009) atlas. Profiles or segments of profiles with data outside the envelope are flagged as doubtful or failed. Profiles are also checked for consistency or doubtful features by comparing with previous/next casts (buddies) and also by comparison with historical CTD casts taken in the same area and season.

Notes

Credit
Royal Australian Navy Hydrography and Metoc Branch

Purpose
Part of the long term ocean monitoring program.