Ice Particle Sampling from Aircraft—Influence of the Probing Position on the Ice Water Content

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 of 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 GV HALO (High Altitude LOng range) 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 (Geophysica, StratoClim campaign 2017) and bottom (NASA WB-57, 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. 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. Caused by 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 25μm, both enrichment and losses of ice crystal can occur, depending on whether the ice crystal mass peak of the - in these cases bimodal - size distribution 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 - due to less disturbed ice particle sampling, as expected from theory - most frequently within a factor of 2.5 or better, independently of the mean ice crystal sizes. The rather large scatter between IWC measurements might reflect instrument uncertainties, cirrus cloud inhomogeneities as well as slight sampling biases which might occur also at 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 aircrafts.