Water vapor profiling using a compact widely tunable diode laser differential absorption lidar (DIAL)

Abstract

Atmospheric water vapor is an important driver of cloud formation, precipitation, and cloud microphysical structure. Changes in the cloud microphysical structure due to the interaction of aerosols and water vapor can produce more reflective clouds, resulting in more incoming solar radiation being reflected back into space, leading to an overall negative radiative forcing. Water vapor also plays an important role in the atmospheric feedback process that acts to amplify the positive radiative forcing resulting from increasing levels of atmospheric CO2. In the troposphere, where the water vapor greenhouse effect is most important, the situation is harder to quantify. A need exists for tools that allow for high spatial resolution range resolved measurements of water vapor number density up to about 4 km. One approach to obtaining this data within the boundary layer is with the Differential Absorption Lidar (DIAL) that is being developed at Montana State University. A differential absorption lidar (DIAL) instrument for automated profiling of water vapor in the lower troposphere has been designed, tested, and is in routine operation. The laser transmitter for the DIAL instrument uses a widely tunable external cavity diode laser (ECDL) to injection seed two cascaded semiconductor optical amplifiers (SOA) to produce a laser transmitter that accesses the 824-841 nm spectral range. The DIAL receiver utilizes a 28-cm-diameter Schmidt-Cassegrain telescope, an avalanche photodiode (APD) detector, and a narrow band optical filter to collect, discriminate, and measure the scattered light. A technique of correcting for the wavelength-dependent incident angle upon the narrow band optical filter as a function of range has been developed to allow accurate water vapor profiles to be measured down to 225 m above the surface. Data comparisons using the DIAL instrument and co-located radiosonde measurements are presented demonstrating the capabilities of the DIAL instrument.