92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Monday, 23 January 2012: 4:15 PM
Development of An Eye-Safe Micro-Pulse Differential Absorption Lidar (DIAL) for Water Vapor Profiling in the Lower Troposphere
Room 239 (New Orleans Convention Center )
Amin R. Nehrir, NASA Langley Research Center, Hampton, VA; and K. S. Repasky, J. L. Carlsten, S. M. Spuler, R. E. Carbone, and T. M. Weckwerth

Improved measurements of water vapor are a long-standing observational challenge to the meteorological and climate research and forecasting communities. In an effort to obtain continuous, long-term, high-resolution water vapor and aerosol profiles in the lower troposphere, an eye-safe all semiconductor-based micro-pulse differential absorption lidar (DIAL) instrument has been developed. The laser transmitter utilizes two continuous wave external cavity diode lasers operating in the 830 nm absorption band as the online and offline seed laser sources. An optical switch is used to sequentially injection seed a tapered semiconductor optical amplifier (TSOA) with the two seed laser sources. The TSOA is actively current pulsed to produce up to 7 ÁJ of output energy over a 1 Ás pulse duration (150 m vertical resolution) at a 10 kHz pulse repetition frequency. The measured laser transmitter spectral linewidth is less than 500 kHz while the long term frequency stability of the stabilized on-line wavelength is ▒ 55 MHz. The laser transmitter spectral purity was measured to be greater than 0.999.

The DIAL receiver utilizes a commercially available full sky-scanning capable 35 cm Schmidt-Cassegrain telescope to collect the scattered light from the laser transmitter. Light collected by the telescope is spectrally filtered to suppress background noise using two 0.25 nm interference filters placed in series. The filtered light is then coupled into a 105 Ám core fiber optic cable which acts as the system field stop and limits the full angle field of view to 140 Árad. The narrow field of view is designed to minimize multiple scattering and background noise in the backscattered signal. The atmospheric returns are sampled with a fiber coupled avalanche photodiode detector which is operated in a Geiger mode. The returns are then summed and recorded onto a laptop computer using a 20 MHz multichannel scaler card.

The DIAL instrument is operated autonomously using the Labview programming environment where water vapor and aerosol profiles are displayed in real-time. The DIAL is capable of operating at any spectral position along the selected water vapor absorption line allowing for year round operation using a single line. The instrument is set to sequentially dwell at the online and offline DIAL wavelengths for a user defined period ranging between 1-6 seconds. This process is repeated and the signal is averaged for ten minutes at each wavelength until a sufficient signal to noise ratio is achieved at the top of the planetary boundary layer and a water vapor profile is then calculated. Aerosol profiles have been retrieved with one minute temporal averaging at the offline wavelength. A description of the current status of the water vapor DIAL instrument and future MSU-NCAR collaborative development efforts will be presented. Initial nighttime and daytime water vapor profiles from the DIAL instrument will also be presented and comparisons against collocated radiosonde, in situ, and column averaged data from SUOMINET and AERONET will also be discussed.

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