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Ground-based Cloud Side Observations by Spectral Radiometry

funded by: DFG (JA 2023/2-2)

Project - Description

The ground-based remote sensing instrument package LIRAS (LIdar and RAdiation System for cloud profiling) is used to retrieve vertical profiles of microphysical parameters of convective clouds to study the vertical evolution from cloud base to anvil. Profiles of the thermodynamic phase can be retrieved by spectral information of the radiation measurements and from depolarization data of the Lidar system. For the determination of the particle sizes, three-dimensional (3D) radiative transfer simulations are required which consider the 3D structures of convective clouds. The results of this project serves as a proof-of-concept for applications on airborne (on HALO in particular) and future satellite missions.



Instrumental Setup

The setup of LIRAS is presented in Fig.1. The entrance optics of the Zeiss-spectrometers is mounted together with the imaging hyperspectral spectrometer Aisa-EAGLE on a scanning device. The Lidar with lower temporal resolution of 10 - 30 s is scanning with a separate device. Aisa-EAGLE gives spatial information on radiances in the spectral range between 400 and 1000 nm, while the Zeiss-spectrometers also cover the NIR which is needed for the determination of the thermodynamic phase and the effective radius. Because of the high temporal resolution of the Aisa-EAGLE and the Zeiss-spectrometers a full scan of a cloud can be performed in the order of a few seconds.



Identification of illuminated cloud parts

The spectral signature of the reflected signal from illuminated, non-shadowed cloud elements depends mainly on the spectral signature of the downward solar radiation and its spectral extinction by the observed cloud element. In contrast, for shadowed cloud elements the incident radiation is mostly determined by diffuse radiation of unknown directions. This diffuse radiation is strongly affected by the spectral extinction of the shadowing cloud element but may also be affected by the spectral surface albedo. To identify the illuminated cloud portions all measured spectra needs to be classified with respect to possible contaminations. For that purpose normalized radiances can be used which show typical spectral features depending on illumination condition and surface albedo (Fig. 2).


From these normalized spectra using the data set of the Aisa-EAGLE histograms with three modes indicating a threshold for the illuminated cloud sides is derived (Fig. 3).



Identification of Thermodynamic Phase

The spectral slope of the radiance differs for ice and liquid water in the wavelength range of 1.5 -1.7 µm. A phase index is defined, which is positive for ice clouds and negative for liquid water clouds as shown in Fig. 4.


Application

Measurements using the LIRAS-system were performed during the HD(CP)2 Observational Prototype Experiment (HOPE) in Julich 2013. Fig. 5 presents one example of cloud masking and phase index retrieval.


Publications


Jäkel et al., Thermodynamic phase retrieval of convective clouds: impact of sensor viewing geometry and vertical distribution of cloud properties and fields of cirrus optical thickness., Atmos. Meas. Tech., 6, 539-547, 2013


Last modification on 2018/7/24 by Evi Jäkel