ALBERT

Description: In this study a method is introduced for retrieval of optical thickness and effective particle size of ice clouds over a wide range of optical thickness from ground-based transmitted radiance measurements. Low optical thickness of cirrus clouds and their complex microphysics present a challenge for cloud remote sensing. In transmittance, the relationship between optical depth and radiance is ambiguous. To resolve this ambiguity the retrieval utilizes the spectral slope of radiance between 485–560 nm in addition to the commonly employed combination of a visible and a shortwave infrared wavelength. An extensive test of retrieval sensitivity was conducted using synthetic test spectra in which all parameters introducing uncertainty into the retrieval were varied systematically: unknown ice crystal habit and aerosol properties, instrument noise, calibration uncertainty and the interpolation in the lookup table required by the retrieval process. The most important source of errors identified are uncertainties due to habit assumption: Averaged over all test spectra, systematic biases in the effective radius retrieval of several micrometer can be caused. The statistical uncertainties of any individual retrieval can easily exceed 10 μm. For optical thickness biases are mostly below 1, statistical uncertainties are in the range of 1 to 2.5. For demonstration and verification the retrieval is applied to observations by the LMU hyperspectral imager specMACS at the Schneefernerhaus observatory (2650 m a.s.l) during the ACRIDICON-Zugspitze campaign in September and October 2012. Results are compared to MODIS and SEVIRI satellite based cirrus retrievals. Considering the identified uncertainties for our ground based approach and for the satellite retrievals, comparison shows good agreement within the range of natural variability in the direct surrounding.