Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-970
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
04 Nov 2016
Review status
A revision of this discussion paper was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Potential of remote sensing of cirrus optical thickness by airborne spectral radiance measurements in different viewing angles and nadir geometry
Kevin Wolf1, André Ehrlich1, Tilman Hüneke2, Klaus Pfeilsticker2, Frank Werner1,a, Martin Wirth3, and Manfred Wendisch1 1Leipzig Institute for Meteorology, University of Leipzig, Leipzig, Germany
2Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany
3Institute of Atmospheric Physics, German Aerospace Center, Oberpfaffenhofen, Germany
anow at: Joint Center for Earth Systems Technology, University of Maryland, Baltimore, MD, USA
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.


Citation: Wolf, K., Ehrlich, A., Hüneke, T., Pfeilsticker, K., Werner, F., Wirth, M., and Wendisch, M.: Potential of remote sensing of cirrus optical thickness by airborne spectral radiance measurements in different viewing angles and nadir geometry, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-970, in review, 2016.
Kevin Wolf et al.
Kevin Wolf et al.

Viewed

Total article views: 195 (including HTML, PDF, and XML)

HTML PDF XML Total BibTeX EndNote
150 39 6 195 5 5

Views and downloads (calculated since 04 Nov 2016)

Cumulative views and downloads (calculated since 04 Nov 2016)

Viewed (geographical distribution)

Total article views: 195 (including HTML, PDF, and XML)

Thereof 195 with geography defined and 0 with unknown origin.

Country # Views %
  • 1

Saved

Discussed

Latest update: 26 Mar 2017
Publications Copernicus
Download
Short summary
The potential of airborne radiance measurements in off-nadir and nadir direction for cirrus remote sensing is investigated. Therefore radiative transfer simulations were used and the sensitivity of upward radiance with respect to optical thickness, effective radius, surface albedo, wavelength and viewing angle was studied. It was shown that off-nadir observations provide better results.
The potential of airborne radiance measurements in off-nadir and nadir direction for cirrus...
Share