Airborne-Weather and Aerosol LIdar
The A-WALI Raman lidar is an instrument designed to sample the atmosphere horizontally and vertically from research aircraft. Developed for the European MAESTRO project, it provides simultaneous measurements of the horizontal water vapor and temperature fields, as well as aerosols and clouds. A-WALI is used to document the thermodynamic and microphysical environment associated with the mesoscale organization of tropical clouds, with the objective of improving our understanding of their role in climate and the water cycle.
The scientific requirements of the MAESTRO campaign called for a lidar system emitting at least 10 W of average power at 355 nm, while complying with eye safety constraints, and equipped with five reception channels: one elastic channel, two rotational Raman (RR) channels, and two vibrational Raman (VR) channels (N2 and H2O), in order to measure aerosols, water vapor, and relative temperature fluctuations up to 2 km across the flight path.
Operational constraints associated with airborne deployment had a major impact on the system design. The lidar had to withstand accelerations of up to 9 g along the aircraft main axis and 7 g laterally. The total optical aperture was limited to 150×210 mm2 to fit through a side window; the overall dimensions had to be compatible with a standard 19-inch rack; the total mass was restricted to 120 kg and the average power consumption to 1.2 kW. In addition, the instrument had to be robust against dust, vibrations, and temperature variations. As a result, the overall performance of the system is primarily constrained by these requirements.
The main characteristics of the A-WALI system are summarised in the following table.
| Laser | Nd:YAG, diode-pumped, Q-switched Quantel MERION–C |
| Pulse length | 6 ns |
| Energy | 60 mJ |
| Frequency | 200 Hz |
| Reception channels / Filter bandwidth | 354.70 nm / 0.25 nm 386.80 nm / 0.21 nm 407.65 nm / 0.35 nm 353.25 nm / 0.45 nm 354.10 nm / 0.21 nm |
| Telescope diameter | 150 mm |
| Field-of-view | 1.64 mrad |
| Detectors | Hamamatsu H10721P-210 photomultiplier tubes |
| Acquisition rate | 250 MHz |