A new water vapor and aerosol profiling lidar system for measurements in the lower stratosphere and troposphere

High vertical resolution measurements of water vapor in the Upper Troposphere and Lower Stratosphere (UT/LS) are needed for improving our understanding of how water vapor variability impacts cirrus clouds, radiative transfer, and climate. High vertical resolution measurements of water vapor are also needed in the lower troposphere and boundary layer to improve numerical weather predictions and storm development studies. Recent modeling studies suggest that an increase in lower stratospheric water vapor over large regional scales cools the stratosphere and warms the troposphere; hence water vapor in this region has been recognized as ‘a driver of decadal global climate change'. Future increases in lower stratospheric water vapor resulting from convective injections (as a result of feedback due to warming) have been postulated to further increase global warming. Airborne Differential Absorption Lidar (DIAL) systems have demonstrated the capability to provide high resolution measurements of water vapor across thin cirrus, between broken clouds, and in the presence of aerosols. DIAL systems that operate from high altitude manned or unmanned aircraft and use the strong absorption lines in the 935-nm region are ideally suited to profile water vapor in the UT/LS and are also well suited to profile the entire troposphere using other absorption lines/features in this band. A discussion of optimum absorption features near 935.6 nm for measurements of water vapor from the lower stratosphere to near surface is presented. A design concept for a new DIAL system using new laser, detector, and receiver technologies is presented in this paper. The lidar architecture is similar to that of the HSRL-2 (High Spectral Resolution Lidar-2) space instrument prototype that is planned to fly on the NASA ER-2 aircraft. This compact DIAL/HSRL system will also include the capability to provide high quality, high resolution profiles of aerosol and cloud properties at 532 and 1064 nm from the surface to the UT/LS region.

Examples of measurements from the NASA LASE system operating in the 815-nm absorption region on the NASA DC-8 aircraft during the 2010 NASA GRIP (Genesis and Rapid Intensification Processes) campaign are presented that show the capability of DIAL systems to measure water vapor over a variety of conditions across the troposphere including measurements over the tropical tropopause layer in the presence of several layers of cirrus. In order to extend the range of water vapor to the lower stratosphere DIAL measurements operating on stronger absorption lines in the 935-nm regions are required. We present a concept for a compact, high power DIAL system capable of extending the range and sensitivity achieved by LASE. This system is based on a new OPO laser under development under the Phase II NASA Small Business Innovative Research (SBIR) program at Fibertek, Inc. The objective of this program is to deliver a compact, high power, tunable, injection seeded, 1 kHz, 2-3 W output power laser that operates at 935-nm and is suitable for DIAL/HSRL operation from an aircraft/UAV. Salient features of the laser that have already been demonstrated include its efficiency, stability, single frequency operation, and tunability over strong water vapor absorption lines. This system will also include new commercially available, small area, high efficiency, low noise Si:APD detectors from First Sensors, and an optical design for background rejection needed to operate during full day background conditions. Science needs and requirements for UT/LS and lower tropospheric water vapor measurements are discussed and performance simulations of DIAL measurements under a variety of atmospheric conditions are presented.