SOFIA Methodology

As part of ASTEX experiment, the oceanic ship Le Suroit (from IFREMER) was equipped to make the following measurements:

• Local turbulence measurements using correlation and inertial-dissipative methods. P.K. Taylor, Wormley, U.K.
• Local turbulence and SL gradients measurements measured by a tethered balloon (away from the ship wake ).
• A "minisodar" on a stabilized platform developed at CRPE (3 couples of bistatic antennas, frequency 6000 Hz) to compute the vertical profile of wind and momentum up to 100 m .
• A vertical Doppler sodar (monostatic, frequency 2000 Hz, on a stabilized platform) to measure the vertical velocity, the vertical velocity variance, and if possible, the inversion height.
• A microwave radiometer similar to ATSRM radiometer on board ERS-1 (frequencies 26 and 37 GHz) to measure the precipitable water and liquid water content.
• A pyranometer, pyrgeometer, and a radiometer (for net radiation measurements).
• An automatic radiosounding system (the SARE system from METEO FRANCE/SETIM) for profiles of temperature, humidity, pressure, and wind.
• A hydrophone system behind the ship.

Surface stations included:

• Five drifting buoys developed by CMM at Brest with bathythermic chains (100 m long), all with surface measurements of SST, pressure, and wind.
• A wave buoy (from IFREMER) to obtain the wave spectrum (not directional measurement). This buoy was drogued to have a slow speed displacement.
• Another ship, from SHOM, participated in the experiment to launch and to survey drifting buoys. This ship performed an XBT transect at the beginning of the experiment to locate the Azores current. The drifting buoys were launched across the current at distances of 50 meters from each other.

Above surface measurements included:

• Constant level balloons (systems "BABA" of CNES-LA) were launched in the ABL from Santa Maria. They provided the mesoscale characteristics of pressure, temperature, and humidity fields. They typically gave estimates of streamlines, as well as streamline thermodynamic characteristics.

The Fokker F-27 (ARAT) with flux measurement package and the airborne lidar Leandre was used during ASTEX. The Fokker F-27 capabilities were as follows:

• Turbulence measurements of wind, temperature and moisture. Fast response sensors were located on a nose boom 5 m long, on which were measured:
  • attack and sideslip angles by mobile vanes and by a five hole probe (Rosemount 858)
  • true airspeed by a Pitot probe
  • temperature by a fast response "INSU " probe
  • humidity by a Lyman-alpha humidity meter
  • Mean state sensors
    • Rosemount temperature probe
    • Reverse-flow temperature probe
    • General Eastern dew point sensor
  • Aerosols and cloud microphysics
    • l-D drop size measurements from 0-6000 microns by four Knollenberg sensors
    • 2-D sensor OAP 2DC for drop sizes between 25 and 800 microns
  • Liquid water content
    • Johnson-Williams sensors
  • Radiative measurements, upward and downward
    • Longwave (14-40 microns) Eppley radiometers
    • Shortwave (0.2 to 2.8 microns) Eppley radiometers
    • Radiances (7.8 to 14 microns) Barnes PRT5 radiometers
  • Chemical measurements (isokinetic veins )
  • Pointing backscatter lidar (Leandre)
  • Directional reflectances measurements (POLDER, Polarized Direct Reflectance)

A second aircraft, the Merlin IV from METEO FRANCE/CAM/LIMA, was used for fluxes. The Merlin IV was equipped for dynamic and thermodynamic flights. It had:

• Radiative measurements, up and down (visible, IR, SST (Barnes)).
• Knollenberg (1D FFSP)

The strategy was to fly three types of airborne missions devoted to:

1. Surface flux measurement and boundary layer study in the ship zone. The ship was located about 300 km south east of Santa Maria (Zone of Priority Investigation (ZIP) of ASTEX). The ARAT with the lidar system Leandre performed a high altitude flight while the Merlin measureed surface fluxes* and boundary layer fluxes.
2. Boundary layer structure investigation close to Santa Maria island. This procedure brought into operation both planes simultaneously and the constant volume balloons. The observation zone were located in a 150 km range of operation from the airport. The Fokker F-27 performed high level flights (FL120) to describe boundary layer structure with the airborne lidar and surface fluxes at very low level. During the same time, the Merlin described horizontal planes inside the boundary layer.
3. Study of boundary layer structures at long distance. This procedure required the use of two instrumented aircraft. The analysis on a large domain was conducted using the Fokker F-27 (with Leandre) at high levels and the Merlin IV in the boundary layer.
4. Cloud structure analysis with POLDER and Leandre combined with Landsat and SPOT observations.

*For surface fluxes measurements the flight height was between 100 and 150', and if possible, close to the surface.

The one month experiment was carried out from June 1-28, 1992. Some measurements were systematically and routinely performed (sodar measurements, daily radiosounding, and wave spectra, etc.), but several Intensive Observations Periods (IOP) were conducted, during which additional radiosounding, fluxes measurements, and aircraft measurements (with and without the airborne lidar system Leandre) were performed.

Considering the climatology of the Azores-Madeira region, about 20 IOP's were performed for different meteorological situations during the one month of the experiment:

• One quiet situation with "no wind" to study heat and momentum flux parameterizations
• One situation with meteorological perturbations (statistically possible during this period)
• Several situations with low cloud cover and with different conditions of wind and waves.

Satellite observations in real time were fundamental to initiate the experimental procedures.

During the whole campaign SOFIA used about 70 Fokker F-27 flight hours, 10 Merlin IV flight hours, and 70 meteorological soundings.