Compact, Automated Differential Absorption Lidar for Tropospheric Profiling of Water Vapor

Knowledge of the distribution of water vapor in the lower troposphere is important for understanding long term climate change, atmospheric chemistry and transport, high impact weather systems, and accurate weather forecasting. Currently, balloon-borne radiosondes are used to measure water vapor fields but provide information at the time of the launch, offering limited spatial and temporal coverage. New observational instruments for real time monitoring of water vapor in the lower troposphere are needed to provide a better understanding of the thermodynamic state of the atmosphere and to improve mesoscale weather forecasting. These new observational instruments need to operate autonomously in a cost effective manner.

Researchers at Montana State University (MSU) are working to develop a diode laser based DIfferential Absorption Lidar (DIAL) for water vapor profiling in the lower troposphere. This DIAL instrument utilizes two external cavity diode lasers (ECDL's) operating near 820 nm. The on-line laser's frequency is locked to the center (or side-line) frequency of a water vapor absorption line and the off-line laser's frequency is chosen to be near the first but removed from any absorption feature. These ECDL's are used to injection seed a pulsed, tapered semiconductor optical amplifier (TSOA) overdriven by a pulsed drive current. The laser transmitter currently produces up to 13 μJ in a 1 μs pulse at a 10 kHz pulse repetition rate. A 35.6 cm diameter Schmidt-Cassegrain telescope collects backscattered light and an avalanche photodiode (APD) is used in conjunction with a multichannel scalar (MSC) to monitor the return signal. The transmitter laser beam divergence and receiver field-of-view fully overlap at a range of 1.5 km. This instrument has demonstrated water vapor retrievals up to 6 km at night and 4 km during the day with retrievals validated through comparisons with radiosondes. Work is in progress demonstrating long term operation of this instrument.

This presentation will review ongoing work to improve system performance to meet the needs of the research community. Improvements include increased laser transmitter output power of up to 20 μJ based on pulsing the drive current to a 4 mm long TSOA, enhanced mechanical stability based on an improved mechanical design for the outgoing laser transmitter beam, improved lower altitude performance resulting from lowering the full overlap height so that the stable nocturnal boundary layer can be observed, and robust packaging for long term deployment. Future field demonstration plans for the water vapor DIAL will be discussed including deployment of the next generation instrument at the Department of Energy (DoE) Southern Great Plains (SGP) Site for inter-comparison with the DoE Raman lidar.