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Observations of high supersaturations with respect to ice insidecirrus clouds with high ice water content (> 0.01 g kg<sup>−1</sup>)and high crystal number densities (> 1 cm<sup>−3</sup>) are challengingour understanding of cloud microphysics and of climate feedback processesin the upper troposphere. However, single measurements of acloudy air mass provide only a snapshot from which the persistence of icesupersaturation cannot be judged. We introduce here the "cirrus matchtechnique" to obtain information about the evolution of clouds and theirsaturation ratio. The aim of these coordinated balloon soundings is toanalyze the same air mass twice. To this end the standard radiosondeequipment is complemented by a frost point hygrometer, "SnowWhite", and aparticle backscatter detector, "COBALD" (Compact Optical BackscatterAerosoL Detector). Extensive trajectory calculations based on regional weather modelCOSMO (Consortium for Small-Scale Modeling) forecasts are performed for flight planning, and COSMO analyses are usedas a basis for comprehensive microphysical box modeling (with grid scale of 2and 7 km, respectively). Here we present the results of matching a cirruscloud to within 2–15 km, realized on 8 June 2010 over Payerne, Switzerland,and a location 120 km downstream close to Zurich. A thick cirrus cloud was detectedover both measurement sites. We show that in order to quantitativelyreproduce the measured particle backscatter ratios, the small-scaletemperature fluctuations not resolved by COSMO must be superimposed on thetrajectories. The stochastic nature of the fluctuations is captured byensemble calculations. Possibilities for further improvements in theagreement with the measured backscatter data are investigated by assuming avery slow mass accommodation of water on ice, the presence of heterogeneousice nuclei, or a wide span of (spheroidal) particle shapes. However, theresulting improvements from these microphysical refinements are moderate andcomparable in magnitude with changes caused by assuming different regimes oftemperature fluctuations for clear-sky or cloudy-sky conditions, highlightingthe importance of proper treatment of subscale fluctuations. The modelyields good agreement with the measured backscatter over both sites andreproduces the measured saturation ratios with respect to ice over Payerne.Conversely, the 30% in-cloud supersaturation measured in a massive 4 kmthick cloud layer over Zurich cannot be reproduced, irrespective of thechoice of meteorological or microphysical model parameters. The measuredsupersaturation can only be explained by either resorting to an unknownphysical process, which prevents the ice particles from consuming the excesshumidity, or – much more likely – by a measurement error, such as acontamination of the sensor housing of the SnowWhite hygrometer by aprecipitation drop from a mixed-phase cloud just below the cirrus layer orfrom some very slight rain in the boundary layer. This uncertainty calls forin-flight checks or calibrations of hygrometers under the special humidityconditions in the upper troposphere

Balloon-borne match measurements of midlatitude cirrus clouds