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

Abstract

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.

POLSTRACC: Airborne Experiment for Studying the Polar Stratosphere in a Changing Climate with the High Altitude and Long Range Research Aircraft (HALO)

“…The coordinated flight with orbits of the CALIPSO satellite provided an unprecedented opportunity to validate the spaceborne CALIOP lidar with independent high spectral resolution observations by the airborne WALES lidar for an improved PSC classification. Nitric acid containing particles were observed at flight altitude-however, a quantification of the particle sizes and the amount of HNO 3 contained is challenging because of uncertainties due to the enrichments in the inlets (Afchine et al 2018). Extensive denitrification in the lower stratosphere and a nitrification at lower altitudes were observed throughout the winter (Khosrawi et al 2017;Braun et al 2019).…”

Ice crystal number concentration estimates from lidar–radar satellite remote sensing – Part 1: Method and evaluation

“…m L16 (D) is based on a m − D relation by Mitchell et al (2010), which has been slightly modified to improve the representativeness of the mass concentration for small ice crystals. The validity of this type of approach, and of m L16 (D) in particular, was recently consolidated by Erfani and Mitchell (2016) and Afchine et al (2018), who demonstrated their accuracy and generalizability for all types of ice clouds from T c < −20 • C. Afchine et al (2018) in particular has shown that this relation should be applicable to tropical clouds and that the influence of different m − D relations on IWC is small in the temperature range of cirrus clouds. Considering this, and because Mitchell et al (2010) and Erfani and Mitchell (2016) developed and tested their m − D relation using 2D-S measurements from tropical and midlatitude campaigns (including SPARTICUS), m L16 (D) should also be applicable to SPARTICUS and ATTREX2014.…”

“…Indeed, provided that a PSD is properly estimated, the corresponding number concentration (the zeroth moment of the PSD, or M 0 ) can be extracted. Important developments regarding applying these methods to satellite observations can be attributed to Austin and Stephens (2001) who, through an elaborate variational scheme, used the sensitivity of radar reflectivity (Z e ) and τ c to other moments, namely M 6 and M 2 , respectively, to constrain PSD shape parameters. This method, initially dedicated to liquid clouds, allowed for the retrieval of profiles of droplet geometric mean radius and a vertically homogeneous N d .…”

Coupling aerosols to (cirrus) clouds in the global EMAC-MADE3 aerosol–climate model

“…2 could be responsible for this high sensitivity, due to an overestimated cloud lifetime effect. This is a common problem in coarse-resolution global models, which are not able to resolve the enhanced entrainment of dry air in clouds with higher CDNC, which would lead to droplet evaporation and thus partly offset the CDNC-induced increase in cloud lifetime (Ackerman et al, 2004;Xue and Feingold, 2006;Jiang et al, 2006;Altaratz et al, 2008;Mülmenstädt et al, 2019). Other studies show that the choice of the tuning parameters can also determine the resulting aerosol ERF: Hoose et al (2009) and Neubauer et al (2019) found a smaller (i.e., less negative) aerosol ERF when increasing CDNC min from 10 cm −3 (−1.7 W m −2 ) to 40 cm −3 (−1.0 W m −2 ) in the ECHAM6 model, while Golaz et al (2011) reported a linear dependency of the aerosol radiative flux perturbation on the autoconversion threshold radius in the GFDL AM3 model.…”