Investigation of cirrus clouds properties in the Tropical Tropopause Layer using high-altitude limb scanning near-IR spectroscopy during the NASA-ATTREX Experiment

Abstract:

Quantification of thin and sub-visible cirrus cloud properties is important for a better assessment of their impact on the Earth’s radiation budget. Remote sensing of ice and oxygen absorption in the near-infrared (NIR) offers a unique opportunity to provide spatially-averaged information on ice particle concentration. Limb viewing measurements from a high-altitude aircraft or satellite are particularly useful since they provide longer light paths through cirrus clouds. We demonstrate the potential of this approach using NIR limb observations of scattered solar radiation during the 2014 NASA ATTREX aircraft deployment at altitudes 14.5–17 km over the tropical Pacific. Ice slant paths and oxygen differential slant column densities were determined using an iterative least-squares retrieval scheme. A new radiative transfer model, VLIDORT-QS, which is based on the widely used VLIDORT model and capable of efficient calculations in spherical atmospheres, was used to simulate the ATTREX observations and to perform sensitivity tests. Our analyses have enabled a fundamental physical understanding of this observation strategy, including the relative contributions of single and multiple scattering, as well as their dependence on particle size distribution, surface albedo, and altitude. Spatially-averaged ice slant paths retrieved from the ATTREX data will be presented.