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Volume 11, issue 7 | Copyright

Special issue: ML-CIRRUS – the airborne experiment on natural cirrus...

Atmos. Meas. Tech., 11, 4015-4031, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Jul 2018

Research article | 11 Jul 2018

Ice particle sampling from aircraft – influence of the probing position on the ice water content

Armin Afchine1, Christian Rolf1, Anja Costa1, Nicole Spelten1, Martin Riese1, Bernhard Buchholz2, Volker Ebert2, Romy Heller3, Stefan Kaufmann3, Andreas Minikin3, Christiane Voigt3, Martin Zöger3, Jessica Smith4, Paul Lawson5, Alexey Lykov6, Sergey Khaykin7, and Martina Krämer1 Armin Afchine et al.
  • 1Forschungszentrum Jülich, Institute of Energy and Climate Research (IEK-7), Jülich, Germany
  • 2Physikalisch-Technische Bundesanstalt Braunschweig, Braunschweig, Germany
  • 3Deutsches Zentrum für Luft- und Raumfahrt, Institute for Physics of the Atmosphere, Oberpfaffenhofen, Germany
  • 4Anderson Group/CCB, Harvard University, Cambridge, MA, USA
  • 5SPEC Incorporated, Boulder, CO, USA
  • 6Department of Upper Atmospheric Layers Physics, Central Aerological Observatory (CAO), Moscow, Russia
  • 7LATMOS/IPSL, UVSQ Université Paris-Saclay, UPMC University Paris 06, CNRS, Guyancourt, France

Abstract. The ice water content (IWC) of cirrus clouds is an essential parameter determining their radiative properties and thus is important for climate simulations. Therefore, for a reliable measurement of IWC on board research aircraft, it is important to carefully design the ice crystal sampling and measuring devices. During the ML-CIRRUS field campaign in 2014 with the German Gulfstream GV HALO (High Altitude and Long Range Research Aircraft), IWC was recorded by three closed-path total water together with one gas-phase water instrument. The hygrometers were supplied by inlets mounted on the roof of the aircraft fuselage. Simultaneously, the IWC is determined by a cloud particle spectrometer attached under an aircraft wing. Two more examples of simultaneous IWC measurements by hygrometers and cloud spectrometers are presented, but the inlets of the hygrometers were mounted at the fuselage side (M-55 Geophysica, StratoClim campaign 2017) and bottom (NASA WB57, MacPex campaign 2011). This combination of instruments and inlet positions provides the opportunity to experimentally study the influence of the ice particle sampling position on the IWC with the approach of comparative measurements. As expected from theory and shown by computational fluid dynamics (CFD) calculations, we found that the IWCs provided by the roof inlets deviate from those measured under the aircraft wing. As a result of the inlet position in the shadow zone behind the aircraft cockpit, ice particle populations with mean mass sizes larger than about 25µm radius are subject to losses, which lead to strongly underestimated IWCs. On the other hand, cloud populations with mean mass sizes smaller than about 12µm are dominated by particle enrichment and thus overestimated IWCs. In the range of mean mass sizes between 12 and 25µm, both enrichment and losses of ice crystals can occur, depending on whether the ice crystal mass peak of the size distribution – in these cases bimodal – is on the smaller or larger mass mode. The resulting deviations of the IWC reach factors of up to 10 or even more for losses as well as for enrichment. Since the mean mass size of ice crystals increases with temperature, losses are more pronounced at higher temperatures, while at lower temperatures IWC is more affected by enrichment. In contrast, in the cases where the hygrometer inlets were mounted at the fuselage side or bottom, the agreement of IWCs is most frequently within a factor of 2.5 or better – due to less disturbed ice particle sampling, as expected from theory – independently of the mean ice crystal sizes. The rather large scatter between IWC measurements reflects, for example, cirrus cloud inhomogeneities and instrument uncertainties as well as slight sampling biases which might also occur on the side or bottom of the fuselage and under the wing. However, this scatter is in the range of other studies and represent the current best possible IWC recording on fast-flying aircraft.

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Short summary
The ice water content (IWC) of cirrus clouds is an essential parameter that determines their radiative properties and is thus important for climate simulations. Experimental investigations of IWCs measured on board research aircraft reveal that their accuracy is influenced by the sampling position. IWCs detected at the aircraft roof deviate significantly from wing, side or bottom IWCs. The reasons are deflections of the gas streamlines and ice particle trajectories behind the aircraft cockpit.
The ice water content (IWC) of cirrus clouds is an essential parameter that determines their...