Application of Water Vapor Lidar for Improved Understanding of Boundary Layer Processes

Water vapor is the most dominant short-lived greenhouse gases in the atmosphere and plays a key role in many atmospheric processes critical to driving Earth’s weather and climate systems. Understanding Earth’s energy cycle, which is driven primarily through latent heat transfer of water, is fundamental to gaining a better understanding of Earth’s weather and climate systems. Water’s radiative properties determine the magnitude of the greenhouse effect, the planetary albedo, and hence Earth’s surface temperature. Additionally, a deeper understanding of how clouds will respond to a warming climate is one of the outstanding challenges in climate sciences. Uncertainties in cloud response, particularly shallow clouds, have been identified as a dominant source of discrepancy within model estimates of equilibrium climate sensitivity. Recently, the National Research Council identified accurate and high resolution measurements of water vapor profiles in the planetary boundary layer as a target observable in the 2017 Earth Science Decadal Survey. Water vapor profiles were also identified as synergetic observations with almost all of the prioritized list of target observables within the survey. To that end, high accuracy and high spatial resolution measurements of water vapor profiles in the lower troposphere using the differential absorption lidar (DIAL) technique were explicitly called out in the decadal survey.

DIAL is an active remote sensing technique that allows for self-calibrated measurements of water vapor profiles from airborne and space based platforms during day and night, throughout all seasons. The DIAL approach has significant advantages over existing passive measurements. These existing measurements rely predominantly on spectrally-resolved microwave and infrared radiances, which are weighted toward the upper troposphere and are less sensitive to lower tropospheric water vapor, where the largest variations are present and produce the most prominent impacts on weather and cloud microphysics. In this presentation we will discuss NASA’s extensive history in developing and fielding water vapor DIAL instruments in support of weather and climate focused process studies. We will also discuss the development of a new DIAL system, the High Altitude Lidar Observatory (HALO), which serves as an airborne prototype and technology testbed for future space-based DIAL missions explicitly called out by the decadal survey. Synergetic observations of atmospheric state parameters ranging from winds to in-cloud water vapor measurements will also be discussed for future airborne campaigns and satellite missions.