| << | Profiling of Water Vapour and Temperature | >> |
Abstract
Atmospheric water vapor is an important driver of cloud formation, precipitation, and cloud microphysics. Water vapor profiling in the lower troposphere is im-portant for understanding complex weather and cou-pled climate processes including the aerosol direct and indirect effects. A compact, widely tunable semi-conductor based master oscillator power amplifier (MOPA) water vapor differential absorption lidar (DIAL) has been built, tested, and deployed at Mon-tana State University (MSU). The laser transmitter uses a tunable external cavity diode laser (ECDL) with a center wavelength of 830 nm to injection seed two cascaded tapered semiconductor optical amplifiers (SOA), producing up to 2 micro joules per pulse at a pulse repetition frequency and pulse width duration of 20 kHz and 1 µs respectively. The low average power and high pulse repetition frequency of the DIAL trans-mitter allows for shot to shot backscattered returns up to ~10 km and water vapor number density retrievals up to 4 km with averaging times approaching 5-10 minutes. Pulsing of the DIAL transmitter is achieved by modulating the forward current to the second SOA. The DIAL receiver utilizes a commercial 28 cm diame-ter Schmidt-Cassegrain telescope, a fiber coupled photon counting avalanche photodiode (APD) detec-tor, a 250 pm narrow band optical filter, and a multi channel scalar (MCS) to collect, discriminate, and measure the scattered light.
Water vapor number density profiles collected with the MSU water vapor DIAL instrument will be compared with co-located radiosonde measurements, demon-strating the instruments ability to measure water vapor profiles in the lower troposphere. Water vapor profiles can be collected with averaging times of less than ten minutes, providing sufficient signal to noise ratios for night time water vapor number density measurements up to ~ 4 km and day time measurements up to ~ 1.5 km. Performance characteristics as well as night time and day time water vapor number density profiles de-rived from the MSU DIAL instrument will be presented. Continuation of future work towards the development of a next generation semiconductor based compact micro pulse water vapor DIAL instrument will also be discussed.