DA10 is a remote sensing instrument that uses differential absorption lidar (DIAL) technology to measure water vapor profiles. Similarly to the Vaisala ceilometers, it also provides attenuated backscatter profiles, and in future releases, cloud base height, cloud depth and cloud penetration information, precipitation/fog detection, and sky condition.
DA10 uses the laser light detection and ranging (LiDAR) method where short, powerful pulses from a diode laser are sent out in vertical or near-vertical direction at 2 wavelengths:
- Online wavelength (911.0 nm) in a section of the electromagnetic spectrum that has high water vapor absorption.
- Offline wavelength (910.6 nm) in a nearby section of the spectrum with low water vapor absorption.
DA10 alternates transmission of the online and offline signals with a measurement time of approximately 30 ms for each wavelength. Part of the pulsed signals scatter back towards the instrument from atmospheric particles – liquid cloud droplets, precipitation, haze, fog, and aerosols – at each height. The rest of the light is either absorbed or scattered in directions not seen by the instrument. The portion of backscattered light is registered at the receiver using an avalanche photodiode detector. The online and offline profiles are time averaged separately for 5 s before analysis.
DA10 uses broadband DIAL approach where the spectrum of the online laser output overlaps multiple water vapor absorption lines. The estimates of the water vapor mixing ratio are based on the ratio of the zero-level corrected online and offline signals. The retrieval method minimizes a cost function between the measured and a modeled online-to-offline signal ratio with respect to an estimated water vapor concentration. The retrieval starts at ground level using surface measurements from a weather transmitter and then progresses upward in range. Most instrument and atmospheric factors are canceled out as the closely spaced measurement wavelengths are similarly affected by perturbing forces on their common path, leaving only the water vapor dependent component of the retrieved signals.
The instrument contains 2 independent lidar systems. One is optimized for the near-range measurements and the other for the far-range measurements. Each lidar system consists of a single-lens telescope used for both transmitting and receiving. The height measurement is based on the time-of-flight measurement. Time-of-flight is the time needed for a light pulse to traverse the atmosphere from the transmitter to a backscattering particle and back to the receiver.
For more information, see https://doi.org/10.1175/JTECH-D-18-0102.1.