Airborne IPDA Lidar Measurements of Atmospheric Methane in Support of MERLIN

Abstract:

Space-based lidar missions targeting greenhouse gases are expected to close observational gaps, e.g., over subarctic permafrost and tropical wetlands, where in-situ and passive remote sensing techniques have difficulties. Consequently, a “Methane Remote Lidar Mission” (MERLIN) was proposed by the German and French space agencies DLR and CNES. MERLIN is now in Phase B, in which all mission components are planned in detail; launch is foreseen in 2020. An integrated path differential absorption (IPDA) lidar will measure weighted columns of atmospheric methane (XCH4) along the satellite track. Primary objective is to provide accurate global observations of methane concentration gradients for inverse numerical models in order to better quantify regional fluxes. DLR has developed an airborne demonstrator, CHARM-F, for technology demonstration and validation purposes. First successful flights on-board the German HALO research aircraft have been performed in May 2015 over Central Europe. The measurements are expected to help solve general retrieval issues for future space-borne IPDA lidars. For example, the CHARM-F flights over ocean and lakes help assess the strength and variability of backscatter from water surfaces. The IPDA weighting function, or measurement sensitivity, is dependent on atmospheric pressure and temperature, in particular close to the surface. We use ECMWF analyses interpolated in space and time to the aircraft track that provide these auxiliary data at 14 km horizontal resolution. Due to the coarse representation of orography the model’s pressure and temperature profiles have to be extrapolated down to the true lidar’s scattering surface elevation, which generates uncertainties that we assess. We also assess biases by spectroscopic uncertainties in the methane absorption lines’ parameters. Overall, the airborne results will support the development of advanced processing algorithms for future space lidar missions such as MERLIN.