Constant volume balloons (CVBs) are cylindrical and overpressurized. This excess of pressure ensures that their volume remains quite constant, whatever their vertical displacement in the atmosphere. Given that their mass is also constant, it results that CVBs fly at a constant density level. Strictly speaking, they are therefore not Lagrangian: when the balloon departs from its density-equilibrium level (as occurring when it enters an updraft or a downdraft), its buoyancy brings it back to its density level. They can thus be considered as horizontally-Lagrangian vectors.

The CVBs are operated by the Space French Agency (Centre National d’Etudes Spatiales – CNES). Their ballast is computed in order they fly at the chosen density level: for CarboEurope experiment, this will correspond to an altitude of 800 to 1000m. CVBs embark a radiosonde, from which their position, as well as temperature and moisture along the trajectory, are recorded via telemetry by a ground station.

CVBs will be launched and tracked from the site “La Cape Sud” (CS), at the same location as for the radiosonde launches. It is expected to track the balloons for horizontal distances larger than 50km.

CVBs will be used in conjunction to airplanes Lagrangian missions, in order to help to the monitoring of the airmass trajectories during the flights. About 4 days with CVBs could be planned during the campaign.

figure III.3-a : Example of CVB trajectory observed during the ESCOMPTE campaign. The balloon was launched from a site close to the shoreline in south-east of France, and tracked over the Mediterranean sea during about five hours