Potential of remote sensing of cirrus optical thickness by airborne
spectral radiance measurements in different viewing angles and
nadir geometry
Abstract. Spectral radiance measurements from two airborne passive solar remote sensing instruments, the Spectral Modular Airborne Radiation measurement sysTem (SMART) and the Differential Optical Absorption Spectrometer (mini-DOAS), are used to compare the remote sensing of cirrus optical thickness τ in nadir and off-nadir geometry. The comparison is based on a sensitivity study using radiative transfer simulations and on measurements during the North Atlantic Rainfall VALidation (NARVAL) mission, the Mid-Latitude Cirrus Experiment (ML-CIRRUS) and the Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems (ACRIDICON) campaign. Radiative transfer simulations are used to quantify the sensitivity of measured upward radiance I with respect to cirrus optical thickness τ, effective radius τeff, viewing angle of the sensor σL, surface albedo α and ice crystal shape. From the calculations it is concluded that off-nadir measurements at wavelengths larger than λ = 900 nm significantly improve the ability to measure clouds of low optical thickness. The comparison of nadir and off-nadir retrievals of τ from mini-DOAS, SMART and independent estimates by the Water Vapour Lidar Experiment in Space (WALES) show general agreement within the range of measurement uncertainties. For the selected example case a mean optical thickness of 0.54±0.2 is derived by SMART and 0.49±0.2 by mini-DOAS nadir channels, while WALES obtained a mean value of 0.32 at 532 nm wavelength respectively. The mean of τ derived from the scanning mini-DOAS channels is 0.26. For the few simultaneous measurements, the scanning mini-DOAS measurements systematically underestimate (−17.6 %) the nadir observations from SMART and mini-DOAS, most likely due to the different probed scenes. The different values of τ derived by SMART, mini-DOAS and WALES can be potentially linked to spatial averages, ice crystal shape and the measurement strategies. The agreement of the simulations and retrievals indicate that off-nadir measurements are generally suited better to retrieve τ of thin clouds.