Ice particle sampling from aircraft – influence of the probing position on the ice water content
Authors: Afchine Armin , Rolf Christian , Costa Anja , Spelten Nicole , Riese Martin , Buchholz Bernhard , Ebert Volker , Heller Romy , Kaufmann Stefan , Minikin Andreas , Voigt Christiane , Zöger Martin , Smith Jessica , Lawson Paul , Lykov Alexey , Khaykin Sergey , Krämer Martina 
Published: July 2018

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 25 µm radius are subject to
losses, which lead to strongly underestimated IWCs. On the other
hand, cloud populations with mean mass sizes smaller than about 12 µm are dominated by particle enrichment and thus
overestimated IWCs. In the range of mean mass sizes between 12 and
25 µm, 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.