Abbildendes Gitterspektrometer
Zusammenfassung der Projektergebnisse
First results using the hyperspectral system were gained from a field experiment in Longyearbyen on Svalbard (Norwegian Arctic) within the frame of the SoRPIC (Radiation and Phase Discrimination of Arctic Clouds) project. Cloud optical thickness and droplet effective radius of Arctic boundarylayer clouds were retrieved from airborne measurements using combined data of the SMART-Albedometer (measuring nadir radiances between 0.3 – 2.0 µm wavelength) and the imaging spectroradiometer system (0.4 – 1.0 µm). Both systems were installed on the Polar 5 aircraft owned by the AWI. The field of view (FOV) of the device includes that of the SMART-Albedometer, whereas the spatial resolution is an order of a magnitude higher than for the SMART-Albedometer (4 m vs. 100 m). To retrieve fields of cloud optical thickness with high spatial resolution from the imaging spectroradiometer, assumptions concerning the effective radius were needed. Because of the limited wavelength range of the system, the effective radius was needed to be constrained externally in this retrieval by using results of the SMART-Albedometer. Such a highly-resolved spatial distribution of the cloud optical thickness can be used as a realistic cloud field for a three-dimensional radiative transfer model. For ground-based applications the system has shown high potential to retrieve optical properties of thin cirrus as observed during the CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over Barbados) campaign on Barbados. Fields of cirrus optical thickness were retrieved by comparison with foreward radiative transfer modeling with a detection limit in the range of subvisible cirrus. The measured radiance at each spatial pixel was converted with help of the scattering angle into a directional distribution of radiance. Comparison with radiative transfer simulations were used to estimate the ice crystal shape. Furthermore, the system was applied for cloud side observations during the ACRIDICON-Zugspitze (Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems) campaign in 2012 and two HD(CP)²-Hope experiments (High Definition Clouds and Precipitation for advancing climate prediction) in 2013. Since convective clouds are complex in spatial and temporal dimensions single profile measurements using a non-imaging spectroradiometer in scanning mode can give only a limited picture of the observed cloud. This picture is improved by additional information about macroscopic 2D information from the available system. Since cloud shadows cannot be used for microphysical retrievals, the high spatial and temporal resolution of the system allows the identification of cloud shadows. Analyzing spectral signatures in the visible spectral range the cloud scenes could be statistically evaluated. In most of the measurement cases we found three modes in the histograms of the normalized spectral slopes between 451 nm and 550 nm wavelength. Each of the modes indicates either cloud free or shadowed cloud or illuminated cloud areas. These modes were used to identify the illuminated cloud sides of a measured cloud field by defining thresholds.
Projektbezogene Publikationen (Auswahl)
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Optical thickness and effective radius of Arctic boundary-layer clouds retrieved from airborne nadir and imaging spectrometry, Atmos. Meas. Tech. 6, 1189- 1200, 2013
Bierwirth, E., Ehrlich, A., Wendisch, M., Gayet, J.-F., Gourbeyre, C., Dupuy, R., Herber, A., Neuber, R., and Lampert, A.
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Retrieval of Cirrus Optical Thickness and Assessment of Ice Crystal Shape from Ground-Based Imaging Spectrometry, Atmos. Meas. Tech., 6, 1855-1868, 2013
Schäfer, M., Bierwirth, E., Ehrlich, A., Heyner, F., and Wendisch, M.
