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  • Author or Editor: Roland Vogt x
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Dominik Michel, Rolf Philipona, Christian Ruckstuhl, Roland Vogt, and Laurent Vuilleumier

Abstract

Net radiation flux in correlation with surface energy budget, snowmelt, glacier ice balance, and forest or agricultural flux exchange investigations is measured in numerous field experiments. Instrument costs and energy consumption versus performance and uncertainty of net radiation instruments has been widely discussed. Here the authors analyze and show performance and uncertainty of two Kipp and Zonen CNR1 net radiometers, which were compared to high standard reference radiation instruments measuring individual shortwave and longwave downward and upward flux components. The intercomparison was aimed at investigating the performance of the radiometers under different climatological conditions and was made over one year at the midlatitude Baseline Surface Radiation Network (BSRN) station in Payerne, Switzerland (490 MSL). Of the two CNR1 radiometers tested, one was installed in a ventilation and heating system, whereas the other was mounted without ventilation and heating. Uncertainties of the different flux components were found to be larger for shortwave than longwave radiation and larger for downward than upward components. Using the single sensitivity coefficient provided by the manufacturer, which for CNR1 radiometers conditions using all four sensors, rather large root-mean-square differences between 2 and 14 W m−2 were measured for the individual components for hourly averages and between 2 and 12 W m−2 for daily averages. The authors then performed a field calibration, comparing each individual sensor to the reference instrument for one particular day. With the individual field calibration the uncertainty of hourly averages was reduced significantly for all components of the ventilated and heated instrument. For the unventilated CNR1 uncertainties could not be reduced significantly for all sensors. The total net radiation uncertainty of both CNR1 is rather large with up to 26% on daily averages (∼10 W m−2) for the original sensitivity coefficients and without field calibration. Only with the field calibration and for the ventilated and heated CNR1 net radiometer is an uncertainty of 10% of the daily totals of total net radiation reached, as claimed by the manufacturer.

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Roland Schön, Martin Schnaiter, Zbigniew Ulanowski, Carl Schmitt, Stefan Benz, Ottmar Möhler, Steffen Vogt, Robert Wagner, and Ulrich Schurath

Abstract

The imaging unit of the novel cloud particle instrument Particle Habit Imaging and Polar Scattering (PHIPS) probe has been developed to image individual ice particles produced inside a large cloud chamber. The PHIPS produces images of single airborne ice crystals, illuminated with white light of an ultrafast flashlamp, which are captured at a maximum frequency of ∼5 Hz by a charge-coupled device (CCD) camera with microscope optics. The imaging properties of the instrument were characterized by means of crystalline sodium hexafluorosilicate ice analogs, which are stable at room temperature. The optical resolving power of the system is ∼2 μm. By using dedicated algorithms for image processing and analysis, the ice crystal images can be analyzed automatically in terms of size and selected shape parameters. PHIPS has been operated at the cloud simulation chamber facility Aerosol Interaction and Dynamics in the Atmosphere (AIDA) of the Karlsruhe Institute of Technology at different temperatures between −17° and −4°C in order to study the influence of the ambient conditions, that is, temperature and ice saturation ratio, on ice crystal habits. The area-equivalent size distributions deduced from the PHIPS images are compared with the retrieval results from Fourier transform infrared (FTIR) extinction spectroscopy in case of small (<20 μm) and with single particle data from the cloud particle imager in case of larger (>20 μm) ice particles. Good agreement is found for both particle size regimes.

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