Design and characterization of specMACS, a multipurpose hyperspectral cloud and sky imager

Abstract

Abstract. The new spectrometer of the Munich Aerosol Cloud Scanner (specMACS) is a multipurpose hyperspectral cloud and sky imager designated, but is not limited to investigations of cloud-aerosol interactions in Earth's atmosphere. With its high spectral and spatial resolution, the instrument is designed to measure solar radiation in the visible and shortwave infrared region that is reflected from, or transmitted through clouds and aerosol layers. It is based on two hyperspectral cameras that measure in the solar spectral range between 400 and 2500 nm with a spectral bandwidth between 2.5 and 12.0 nm. The instrument was operated in ground-based campaigns as well as aboard the German High Altitude LOng Range (HALO) research aircraft, e.g., during the ACRIDICON-CHUVA campaign in Brazil during summer 2014.This paper describes the specMACS instrument hardware and software design and characterizes the instrument performance. During the laboratory characterization of the instrument, the radiometric response as well as the spatial and spectral resolution was assessed. Since the instrument is primarily intended for retrievals of atmospheric quantities by inversion of radiative models using measured radiances, a focus is placed on the determination of its radiometric response. Radiometric characterization was possible for both spectrometers, with an absolute accuracy of 3 % at their respective central wavelength regions. First measurements are presented which demonstrate the wide applicability of the instrument. They show that key demands are met regarding the radiometric and spectral accuracy which is required for the intended remote sensing techniques.

The North Atlantic Waveguide and Downstream Impact Experiment

“…HALO is a modif ied Gulfstream G-550 ultra-long-range business jet with a maximum flight range of about 10,000 km and a maximum endurance of 10 h (Krautstrunk and Giez 2012;Wendisch et al 2016), which allows for access to remote regions over the central North Atlantic that are not accessible by other European research aircraft. The high ceiling of almost 15 km in combination with a sophisticated remote sensing payload (see the "Active remote sensing observations for future satellite missions Aeolus and EarthCARE" sidebar and Table 2) allow HALO to f ly (Wirth et al 2009) Profiles of water vapor, backscatter coefficient lidar/color ratio, particle linear depolarization ratio, particle extinction coefficient 1, 2, 4, 5 SMART: Passive cloud spectrometer (Wendisch et al 2001;Ehrlich et al 2008) Spectral nadir radiance, spectral upward and downward irradiance (300-2,200 nm), cloud-top albedo, cloud thermodynamic phase, cloud optical thickness, effective radius, cloud cover and statistics 2, 4, 5 specMACS: Imaging cloud spectrometer plus 2D red-green-blue (RGB) camera (±35° field of view) (Ewald et al 2016) Spectral radiance (400-2,500 nm), push-broom imaging at nadir and ±17° across track, cloud thermodynamic phase, liquid and ice optical thickness, particle size, cloud cover 2, 4, 5…”

The HD(CP)<sup>2</sup> Observational Prototype Experiment (HOPE) – an overview

“…Additionally, two ground-based scanning spectral radiometers, SpecMACS from the Munich Institute for Meteorology (MIM) of the Ludwig Maximilian University (LMU) of Munich (Ewald et al, 2016) and EAGLE from Leipzig Institute of Meteorology (LIM) of the University of Leipzig (Jäkel et al, 2013), participated in the campaign. These instruments provide the solar radiation reflected at cloud sides from which vertical profiles of cloud microphysical properties shall be inferred.…”

Vertical distribution of the particle phase in tropical deep convective clouds as derived from cloud-side reflected solar radiation measurements

“…The absolute radiometric response was determined using an integrating sphere and the absolute RAdiance STAndard (RASTA; Schwarzmaier et al, 2012) traceable to absolute radiance standards of PTB (Physikalisch-Technische Bundesanstalt). The wavelength-dependent uncertainties (2σ ) of the absolute radiometric response including sensor noise and dark current drift between 3 and 14 % (in the outer region of the measured spectra) were given in Ewald et al (2016). During the ACRIDICON-CHUVA campaign, specMACS was mounted at a side view port on HALO.…”

“…Another method is based on stereographic analysis of multi-angle observations (e.g., Seiz and Davies, 2006). Differently from the scanning-point-sensor measurements presented by Martins et al (2011), this paper introduces airborne measurements of an imaging spectroradiometer called specMACS (spectrometer of the Munich Aerosol Cloud Scanner; Ewald et al, 2016). These observations were used to derive vertical profiles of the phase state of DCCs during the HALO (High Altitude and Long Range Research Aircraft) campaign ACRIDICON (Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems) -CHUVA (Cloud processes of tHe main precipitation systems in Brazil: A contribUtion to cloud resolVing modeling and to the GPM; GlobAl Precipitation Measurement) in 2014 .…”