III.2.3.1 Equipment

The Sky Arrow 650 ERA (Environmental Research Aircraft) is a commercially produced, certified small aircraft equipped with sensors to measure three dimensional wind and turbulence together with gas concentrations (H2O and CO2) and other atmospheric parameters (like temperature, surface radiometric temperature, net and UV radiation) at high frequency. The aircraft has a cruise flight speed of 45 m s^-1 with an endurance of 3.5 h, allowing it to cover flight distances of up to 500 km. Operating altitudes can range from 10 m above ground level to more than 3500 m above sea level. The aircraft position, groundspeed and attitude angles are continuously measured.

The flux calculation procedure requires the wind components to be derived from the 50 Hz raw data out of the MFP. The subsequent calculation of carbon dioxide, water vapour, sensible and latent heat and momentum fluxes are made using conventional eddy covariance technique taking into account all the necessary corrections for open-path gas analysers (Webb et al., 1980). The complete computation procedure software has been developed by IBIMET in the last years.

The main difference between airborne and ground-based eddy covariance is in the averaging technique. It has been found that there is a correlation between vertical air motion and aircraft ground speed, causing some type of turbulent structures being sampled more densely than others (Crawford et al., 1993); this can introduce bias up to 20% in the fluxes computed simply by using a time average. Hence, turbulent fluctuations (wind and associated scalars) are calculated using averages calculated over space (per meter) rather than over time (per second). This “spatial average” is defined, for instance for the vertical wind component w, by the following equation (Crawford et al., 1993):

The definition of a proper averaging length is critical to ensure that all significant flux-carrying wavelengths are taken into account. Such a length depends on the flying altitude, on the surface roughness, and on atmospheric stability, and can be determined with cospectral analysis techniques (Desjardins et al. 1989). Using such an approach, proper averaging lengths ranging from 3000 m to 4500 m have been found in the different conditions encountered in the RECAB campaigns (Gioli et al. 2004).

The ability of eddy covariance to resolve the higher frequencies carrying flux is limited by a number of factors, including the insufficient dynamic frequency response of the sensors and the length of the scalar path averaging. The dynamic response time of each sensor can be, in fact, not fast enough to adequately resolve the measurements up to the measured frequency, introducing some loss in the fluxes. Hence, proper correction factors must be applied as a function of the flying altitude, the wind speed, and the atmospheric stability.

Vertical profile measurements will also performed in the CarboEurope IP experimental campaign, providing vertical distribution data of scalar concentrations of CO2, water vapour, temperature, wind magnitude and direction, and turbulence. High frequency data will be distributed at 5 - 10 cm over the vertical.

III.2.3.2 Flight plans

Flights will be performed in the area North of the Landes, overflying instrumented sites from Bordeaux-Saucats airfield. A total of 80 measurement flight hours has been planned:

• 60 hours of surface flux measurements
• 20 hours for vertical profiles measurements and flux divergence studies.

Three different types of surface flux measurements flights will be performed, in order to cover the most important areas and land use classes of the domain:

• a transect across the forest areas of Landes (blue track)
• a transect across the vineyard areas in the northern area of the domain (green track)
• a transect across forest and agriculture areas (mainly maize, black track)

Vertical profiles will be performed in different points of the domain, to be determined on the basis of logistic and scientific requirements.

Flux divergence studies will be performed occasionally in order to characterize the flux behaviour at different heights within and above the PBL. Such flights will consist of a transect flown repeatedly at different altitudes in the PBL, from the bottom level (approx. 100 m AGL) to the entrainment zone.