Abstract
Abstract. During the ACRIDICON-CHUVA (Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems–Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (Global Precipitation Measurement)) aircraft campaign in September 2014 over the Amazon, among other topics, aerosol effects on the development of cloud microphysical profiles during the burning season were studied. Hyperspectral remote sensing with the imaging spectrometer specMACS provided cloud microphysical information for sun-illuminated cloud sides. In order to derive profiles of phase or effective radius from cloud side observations, vertical location information is indispensable. For this purpose, spectral measurements of cloud-side-reflected radiation in the oxygen A absorption band collected by specMACS were used to determine absorption path length between cloud sides and the instrument aboard the aircraft. From these data, horizontal distance and eventually vertical height were derived. It is shown that, depending on aircraft altitude and sensor viewing direction, an unambiguous relationship of absorption and distance exists and can be used to retrieve cloud geometrical parameters. A comparison to distance and height information from stereo image analysis (using data of an independent camera) demonstrates the efficiency of the approach. Uncertainty estimates due to method, instrument and environmental factors are provided. The main sources of uncertainty are unknown in cloud absorption path contributions due to complex 3-D geometry or unknown microphysical properties, variable surface albedo and aerosol distribution. A systematic difference of 3.8 km between the stereo and spectral method is found which can be attributed to 3-D geometry effects not considered in the method's simplified cloud model. If this offset is considered, typical differences found are 1.6 km for distance and 230 m for vertical position at a typical distance around 20 km between sensor and convective cloud elements of typically 1–10 km horizontal and vertical extent.
Highlights
Information on the location and extent of clouds is central for any assessment of the role of clouds in the atmosphere
In contrast to active remote sensing techniques providing an immanent distance measurement, passive remote sensing techniques need additional information sources in order to assign a location to the observed values
A method based on high-resolution cloud reflectivity measurements in the solar spectral range is the so-called cloud side remote sensing approach, proposed by Martins et al (2011) and Zinner et al (2008) for the retrieval of cloud microphysical properties along the vertical profile of convective clouds
Summary
Information on the location and extent of clouds is central for any assessment of the role of clouds in the atmosphere. A method based on high-resolution cloud reflectivity measurements in the solar spectral range is the so-called cloud side remote sensing approach, proposed by Martins et al (2011) and Zinner et al (2008) for the retrieval of cloud microphysical properties (particle size and phase) along the vertical profile of convective clouds. For this approach, Ewald et al (2019) show that cloud surface orientation can explain most of the observable variation of cloud reflectivity due to 3D radiative transfer. Geometrical heights obtained this way are compared to stereo analysis results
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