Potential of remote sensing of cirrus optical thickness by airborne spectral radiance measurements at different sideward viewing angles

Spectral radiance measurements collected in nadir and sideward viewing directions by 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 results of cirrus optical thickness tau. The comparison is based on a sensitivity study using radiative transfer simulations (RTS) and on data obtained during three airborne field campaigns: the North Atlantic Rainfall VALidation (NARVAL) mission, the MidLatitude 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 tau, ice crystal effective radius r(eff), viewing angle of the sensor theta(V), spectral surface albedo alpha, and ice crystal shape. From the calculations it is concluded that sideward viewing measurements are generally better suited than radiance data from the nadir direction to retrieve tau of optically thin cirrus, especially at wavelengths larger than lambda = 900 nm. Using sideward instead of nadir-directed spectral radiance measurements significantly improves the sensitivity and accuracy in retrieving tau, in particular for optically thin cirrus of tau <= 2. The comparison of retrievals of tau based on nadir and sideward viewing radiance measurements from SMART, miniDOAS and independent estimates of tau from an additional active remote sensing instrument, theWater Vapor Lidar Experiment in Space (WALES), shows general agreement within the range of measurement uncertainties. For the selected example a mean tau of 0.54 +/- 0.2 is derived from SMART, and 0.49 +/- 0.2 by mini-DOAS nadir channels, whileWALES obtained a mean value of tau = 0.32 +/- 0.02 at 532 nm wavelength, respectively. The mean of tau derived from the sideward viewing mini-DOAS channels is 0.26 +/- 0.2. For the few simultaneous measurements, the mini-DOAS sideward channel measurements systematically underestimate (-17.6 %) the nadir observations from SMART and mini-DOAS. The agreement between mini-DOAS sideward viewing channels and WALES is better, showing the advantage of using sideward viewing measurements for cloud remote sensing for tau <= 1. Therefore, we suggest sideward viewing measurements for retrievals of tau of thin cirrus because of the significantly enhanced capability of sideward viewing compared to nadir measurements.