Potential of remote sensing of cirrus optical thickness by airborne spectral radiance measurements in different viewing angles and nadir geometry

Other literature type English OPEN
Wolf, Kevin ; Ehrlich, André ; Hüneke, Tilman ; Pfeilsticker, Klaus ; Werner, Frank ; Wirth, Martin ; Wendisch, Manfred (2016)

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 &tau; 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>I</i> with respect to cirrus optical thickness &tau;, effective radius &tau;<sub>eff</sub>, viewing angle of the sensor &sigma;<sub>L</sub>, surface albedo &alpha; and ice crystal shape. From the calculations it is concluded that off-nadir measurements at wavelengths larger than &lambda; = 900 nm significantly improve the ability to measure clouds of low optical thickness. <br><br> The comparison of nadir and off-nadir retrievals of &tau; 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&plusmn;0.2 is derived by SMART and 0.49&plusmn;0.2 by mini-DOAS nadir channels, while WALES obtained a mean value of 0.32 at 532&thinsp;nm wavelength respectively. The mean of &tau; derived from the scanning mini-DOAS channels is 0.26. For the few simultaneous measurements, the scanning mini-DOAS measurements systematically underestimate (&minus;17.6&thinsp;%) the nadir observations from SMART and mini-DOAS, most likely due to the different probed scenes. The different values of &tau; 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 &tau; of thin clouds.
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