Optical Thickness Retrievals of Subtropical Cirrus and Arctic Stratus from Ground-Based and Airborne Radiance Observations Using Imaging Spectrometers

• Main Author: Schäfer, Michael
• Other Authors: Wendisch, Manfred, Trautmann, Thomas, Universität Leipzig
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2016
• Subjects: info:eu-repo/classification/ddc/530; ddc:530; Spektrale Strahlungsmessungen; Ableitung von Wolkeneigenschaften; Strahlungstransfer; Abbildendes Spektrometer; spectral radiation measurements; cloud retrieval; radiative transfer; imaging spectrometer; Arctic; albedo; Sea ice
• Online Access: https://nbn-resolving.org/urn:nbn:de:bsz:15-qucosa-207395; https://ul.qucosa.de/id/qucosa%3A14861; https://ul.qucosa.de/api/qucosa%3A14861/attachment/ATT-0/

The development and application of new cloud retrieval methods from ground–based and airborne measurements of spectral radiance fields above heteorogeneous surfaces is introduced. The potential of imaging spectrometers in remote–sensing applications is evaluated. The analyzed spectral radiance fields were measured during two international field campaigns in the visible wavelength range (400–970 nm) with high spatial (<10m) resolution. From ground–based measurements, high ice clouds were observed and from airborne measurements Arctic stratus. From the measurements, cloud optical thickness is retrieved with high spatial resolution and the horizontal cloud inhomogeneities are investigated. Depending on the measurement configuration, different uncertainties arise for the retrieval of the cloud optical thickness. A reduction of those uncertainties is derived by a specification of the ice crystal shape to improve the retrieval of the optical thickness of high ice clouds. The ice crystal shape is obtained independently from the angular information of the scattering phase function features, imprinted in the radiance fields. A performed sensitivity study reveals uncertainties of up to 90%, when neglecting this information and applying a wrong crystal shape to the retrieval. For remote-sensing of Arctic stratus, the highly variable surface albedo influences the accuracy of the cloud optical thickness retrieval. In cloudy cases the transition of reflected radiance from open water to sea ice is not instantaneous but horizontally smoothed. In general, clouds reduce the reflected radiance above bright surfaces in the vicinity of open water, while it is enhanced above open sea. This results in an overestimation of to up to 90% in retrievals of the optical thickness. This effect is investigated. Using observations and three-dimensional radiative transfer simulations, this effect is quantified to range to up to 2200 m distance to the sea-ice edge (for dark-ocean albedo of αwater = 0.042 and sea-ice albedo of αice = 0.91 at ...