Retrieval of Cirrus Clouds Properties Using Limb-Scanning Near-IR Spectroscopy in the Tropical Tropopause Layer During the NASA Attrex Mission

Tropical Tropopause Layer (TTL) cirrus clouds and their radiative effects represent a major uncertainty in the evaluation of Earth's energy budget. It is thus crucial to study cirrus cloud optical properties and composition, especially in the TTL. High altitude aircraft offer an opportunity to provide observations at cirrus cloud altitudes, most commonly using in-situ measurements of ice particle properties. However, remote sensing of scattering properties and near-IR ice water absorption in the limb can provide unique insights into sub-visible cirrus clouds. Here we present novel spectroscopic observations of path-averaged ice water absorptions on-board NASA's Global Hawk aircraft, during the Airborne Tropical TRopopause Experiment (ATTREX) missions in 2011, 2013, and 2014. The UCLA/U. Heidelberg mini-DOAS instrument provided multi-angle limb-scanning observations of scattered solar radiation in the near-IR (900-1726 nm), allowing the identification of ice and liquid water, O2, CO2 and H2O. Ice water path retrieval in limb geometry requires a full spherical radiative transfer (RT) calculation of both single and multiple scattering components of radiance fluxes. We developed and validated the FOMS (First Order Multiple Scattering) routine, based on the VLIDORT RT code, to simulate high altitude limb observations for varied cloud scenarios. We applied the spectral retrievals for a particularly interesting case during Science Flight 2 over Guam in February 2014, during which the aircraft flew in circles in the same general area for an extended period of time. The measurements of ice particle scattering and absorption at different azimuths relative to the sun and at different altitudes represents a unique opportunity to test our approach and to infer properties of the ice particles, together with information on cirrus cloud radiative transfer.