The NASA Langley High Altitude Lidar Observatory (HALO) - Advancements in Airborne DIAL Measurements of CH4 and H2O

Atmospheric methane (CH₄) has the second largest radiative forcing of the long-lived greenhouse gasses (GHG) after carbon dioxide.  However, methane’s much shorter atmospheric lifetime and much stronger warming potential make its radiative forcing equivalent to that for CO₂ over a 20-year time horizon which makes CH₄ a particularly attractive target for mitigation strategies.  Similar to CH₄, water vapor (H₂O) is the most dominant of the short-lived GHG in the atmosphere and plays a key role in many atmospheric processes.  Atmospheric H₂O concentrations span over four orders of magnitude from the planetary boundary layer where high impact weather initiates to lower levels in the upper troposphere and lower stratosphere where water vapor has significant and long term impacts on the Earth’s radiation budget.  Active remote sensing employing the differential absorption lidar (DIAL) technique enables scientific assessments of both natural and anthropogenic sources and sinks of CH₄ with high accuracy and precision as well as and its impacts on the climate.  The DIAL technique also allows for profiling of tropospheric water vapor for weather and climate applications with unprecedented spatial and temporal resolution.  NASA Langley is developing the High Altitude Lidar Observatory (HALO) lidar system to address the observational needs of NASA’s weather, climate, carbon cycle, and atmospheric composition focus areas.  HALO is a multi-function airborne lidar being developed to measure atmospheric H₂O and CH₄ mixing ratios and aerosol and cloud optical properties using the DIAL and High Spectral Resolution Lidar (HSRL) techniques, respectively.  HALO is designed as an airborne simulator for future space based DIAL missions and will serve as test bed for risk reduction of key technologies required of future space based GHG DIAL missions.  A system level overview and up-to-date progress of the HALO lidar will be presented.  Simulations on the expected accuracy and precision of HALO CH₄ and H₂O DIAL measurements will also be presented.