Ship-borne rotating shadowband radiometer for determining of aerosol optical properties and cloud radiative effects

The Martime Aerosol Network (MAN) has been established as a sub-project of AERONET and a long-term programme to collect ship-borne aerosol optical depth measurements over ocean. Its purpose is to serve as reliable reference dataset for the evaluation of models and satellite products. Data are currently collected by handheld MICROTOPS II radiometers, as the automated acquisition of data from sun photometers on stabilized platforms is so far too expensive for wide-spread use. A promising alternative to the sun photometer is the rotating shadowband radiometer, whose principle of operation allows the determination of the direct-beam component of solar radiation without stabilizing the instrument, if the orientation of the detector horizontal is known. OCEANET, a project to investigate the exchange fluxes of energy and matter between the atmosphere and ocean, has contributed aerosol observations to MAN on several of its cruises on RV Polarstern during the transit between the hemispheres. On the recent Polarstern cruise (PS 83) from Cape Town to Bremerhaven, TROPOS has operated for the first time a 19 channel rotating shadowband radiometer built by Biospherical Instruments, as a possible means to provide automated aerosol optical depth measurements. Calibration and processing of the raw data will be described, and an initial evaluation of the instrumental performance will be given. Aerosol optical depths derived from MICROTOPS measurements and the rotating shadowband radiometer will be compared. The suitability of the instrument to automate MAN observations is discussed, and an outlook to to the use of the instrument to also derive cloud properties is given.