Abstract
Abstract A second-generation diode-laser-based master oscillator power amplifier (MOPA) configured micropulse differential absorption lidar (DIAL) instrument for profiling of lower-tropospheric water vapor is presented. The DIAL transmitter is based on a continuous wave (cw) external cavity diode laser (ECDL) master oscillator that is used to injection seed two cascaded tapered semiconductor optical power amplifiers, which deliver up to 2-μJ pulse energies over a 1-μs pulse duration at 830 nm with an average power of ∼40 mW at a pulse repetition frequency of 20 kHz. The DIAL receiver utilizes a commercial 28-cm-diameter Schmidt–Cassegrain telescope, a 250-pm narrowband optical filter, and a fiber-coupled single-photon-counting Avalanche photodiode (APD) detector, yielding a far-field full-angle field of view of 170 μrad. A detailed description of the second-generation Montana State University (MSU) DIAL instrument is presented. Water vapor number density profiles and time–height cross sections collected with the water vapor DIAL instrument are also presented and compared with collocated radiosonde measurements, demonstrating the instruments ability to measure night- and daytime water vapor profiles in the lower troposphere.
Highlights
Water vapor is the most dominant greenhouse gas in the atmosphere (Trenberth et al 2007)
Micropulse differential absorption lidar (DIAL) instruments such as the water vapor DIAL that is deployed at Montana State University (MSU) exploit high pulse repetition frequencies, and spatial and temporal averaging to obtain reasonable signal-to-noise ratio (SNR) such that number density or mixing ratio measurements of the particulates of interest can accurately be calculated
Initial testing of the second-generation ground-based MSU DIAL instrument was performed during a small nighttime campaign in order to maximize the instrument performance, which would otherwise be limited by the daytime background solar radiation
Summary
Water vapor is the most dominant greenhouse gas in the atmosphere (Trenberth et al 2007). The radiative forcing for a clear sky due to water vapor is 75 W m22, while for carbon dioxide (CO2) it is a factor of 2 weaker at 32 W m22 (Kiehl and Trenberth 1997). The sensitivity of radiative forcing due to a change in water vapor–CO2 concentrations in the equatorial regions is small due to the already large greenhouse effect and has a small direct impact on the reemitted downward infrared radiation. In the cold dry polar regions, the effects of a small increase in water vapor–CO2 caused by equatorial convective circulations are much greater (Trenberth et al 2007). Water vapor is the dominant positive feedback mechanism in our climate system and a major reason why Aerosols play an important role in the earth’s complex climate system.
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