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Abstract

In this work we propose a new method based on a near-IR ([900-1700]nm) full spectral retrieval to infer the optical properties of ice particles forming cirrus clouds. The quasi in situ near-IR spectroscopic measurements were collected by the mini-DOAS instrument (S/N~300) on board the NASA Global Hawk within a critical region for climate forcing, i.e. the tropical tropopause layer (TTL). The theoretical model is set up using a Monte Carlo radiative transfer model for spectral radiance simulations (1% relative error) and a Mie scattering code to calculate the optical properties (~0.0001-0.00001 relative errors). The ice particles are described as mass equivalent spheres and they are treated as a statistical ensemble by making use of two particle size distributions, both parameterized in terms of the ice water contents (IWCs). The novel method is applied to one flight segment where a cirrus cloud over the eastern Pacific TTL was probed during the convective season (NH winter). The cloud optical depths (ODs) are retrieved using the oxygen absorption band at 1270 nm, while the IWCs are inferred from the ice absorption band (1490 nm). The results are compared and discussed with the simultaneous observations of the Cloud Physics Lidar (CPL) and in situ measurements of Hawkeye, NOAA Water, UCATS and DLH. Together, they provide relevant information concerning the microphysics of cirrus within the TTL. Tropical cirrus clouds are extremely important for the global radiative net budget (greenhouse vs. albedo effects), since they exert a direct impact on the TTL and on the lower stratosphere in a changing climate. The new method proved to be a powerful tool for cirrus optical properties retrieval. The thresholds for the inferred results are 0.01 for ODs (3.2% relative error) and 0.001 [g/m^3] for the total IWCs (3.8% relative error).