Shake and Bake!
Posted by sogasex on April 4, 2008
By Ludovic Bariteau, CIRES/NOAA PSD
Among the numerous measurements made on the ship are the flux measurements. I have learned and simplified this flux recipe from Chris Fairall, my spiritual Guru.
Yield Unlimited servings
Time About 45 days
Ingredients required for determination of air-sea fluxes:
- Wind speed and direction
- Air temperature and humidity
- Atmospheric pressure
- Downward shortwave and longwave radiations
- Sea surface temperature
- CO2, DMS and Ozone
Utensils and Personnel used for the GasEx recipe
- A ship, the RHB.
- 4 cooks. Persons used on this project are: Ale, Dr Zap, Byron and I.
- On the foremast: 3 sonic anemometers, 3 motion packs, 5 Licors 7500 (fast CO2/hygrometer), 3 mean RH/T (Relative Humidity/Temperatures) sensors and an optical rain gauge.
- 4 Eppley radiometers setup on a wood pole
- 3 Licors 6262
- 1 fast ozone instrument and 1 fast DMS instrument with the sampling inlets located on the jackstaff
- A bunch of data loggers, computers, cables, tie wraps…
The previous sensors used for fluxes have been adapted for observations over the ocean. They are designed for marine applications and thus are protected from the corrosive effect of sea salt and spray. These instruments are also used because of their accuracy and frequency response. Our sampling is typically done at 10 or 20 Hz in order to get the turbulent fluctuations of the atmospheric variables (wind, temperature, humidity, gas …).
We most certainly make sure that all sensors are freshly calibrated.
- Get the sonic anemometers and motion packs. These instruments are the center pieces of the flux system, so taking good care of them is very important.
- Put these sensors together forward on the ship. The jackstaff is a perfect location for that as it is ahead of the engine exhausts, and it’s as far away as possible from any obstacles. Nevertheless the ship’s central superstructure will always create some flow distortion. The wind is deflected upward, and the wind speed is modified. Some modest flow distortion corrections are done later in the recipe.
- Add the other sensors to the mast: Licors, RH/T, sampling inlets…
- Secure everything with tie wraps, clamps, bolts…
- Install the rest of the equipment on the ship (ozone, DMS, radiometers, Licors 6262); forward on the ship is an excellent spot.
- Put the RHB in the Southern Ocean for ~45 days and let everything shake gently… or vigorously.
- Meanwhile, log all sources of data to a central data acquisition system, commonly called “DAS”.
- Put the cooks at work (they were already working on previous steps). They will get the data out and bake them. The baking process is very straightforward. Take the three components of the wind vector and mix them with the motion data. Rotate them to fixed earth coordinates and you get the corrected wind velocities (it’s a bit more complicated than that!).
- Add your favorite variables to the corrected wind components: more sonic for momentum flux, some moisture for latent heat flux, some CO2, DMS or O3, for gas fluxes… it’s your choice!
Finally, take everything out of the RHB and bring the data home for meticulous analysis.
- Bulk meteorological variables and eddy-correlation fluxes based on preliminary analysis during the cruise taste good fresh and hot. But quality controlled fluxes produced later during post-processing are even better. Scientists love them as it brings them tons of information!
What you need to make good fluxes! From back to front: Sonics and motion packs standing up high with the sampling inlets of various sensors, Licors, RH/T sensor. Other sensors are down below on the mast.
Flux kitchen and the 4 cooks. All recipes are prepared with tradition. Left to right: Ale, Ludovic, Byron and Chris.
A good baking process removes the motion peak in the power spectra of the wind components. Measured (black broken line) and corrected (blue solid line) vertical velocity power spectra. The green straight line represent the -2/3 inertial subrange slope.
Freshly baked fluxes! Covariance spectra for the longitudinal component of the momentum flux (blue) and for the sensible heat flux (red).