Balloon-borne match measurements of mid-latitude cirrus clouds

Abstract

Observations of persistent high supersaturations with respect to ice inside cirrus clouds are challenging our understanding of cloud microphysics and of climate feedback processes in the upper troposphere. Single measurements of a cloudy air mass provide only a snapshot from which the persistence of ice supersaturation cannot be judged. We introduce here the "cirrus match technique" to obtain information of the evolution of clouds and their saturation ratio. The aim of these coordinated balloon soundings is to analyze the same air mass twice. To this end the standard radiosonde equipment is complemented by a frost point hygrometer "SnowWhite" and a particle backscatter detector "COBALD" (Compact Optical Backscatter Aerosol Detector). Extensive trajectory calculations based on regional weather model COSMO forecasts are performed for flight planning and COSMO analyses are used as basis for comprehensive microphysical box modeling (with grid scale 2 km and 7 km, respectively). Here we present the results of matching a cirrus cloud 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 was detected over both measurement sites. We show that in order to quantitatively reproduce the measured particle backscatter ratios, the small-scale temperature fluctuations not resolved by COSMO must be superimposed on the trajectories. The stochastic nature of the fluctuations is captured by ensemble calculations. Possibilities for further improvements in the agreement with the measured backscatter data are investigated by assuming a very slow mass accommodation of water on ice, the presence of heterogeneous ice nuclei, or a wide span of (spheroidal) particle shapes. However, the resulting improvements from microphysical refinements are moderate and comparable in magnitude with changes caused by assuming different regimes of temperature fluctuations for clear sky or cloudy sky conditions, highlighting the importance of a proper treatment of subscale fluctuations. The model yields good agreement with the measured backscatter over both sites and reproduces the measured saturation ratios with respect to ice over Payerne. Conversely, the 30% in-cloud supersaturation measured in a massive, 4-km thick cloud layer over Zurich cannot be reproduced, irrespective of the choice of meteorological or microphysical model parameters. The measured supersaturation can only be explained by either resorting to an unknown physical process, which prevents the ice particles from consuming the excess humidity, or – much more likely – by a measurement error, such as a contamination of the sensor housing of the SnowWhite hygrometer by a precipitation drop from a mixed phase cloud just below the cirrus layer or from some very slight rain in the boundary layer. This uncertainty calls for in-flight checks or calibrations of hygrometers under the extreme humidity conditions in the upper troposphere

The AquaVIT-1 intercomparison of atmospheric water vapor measurement techniques

“…While loss of frost condensate on the mirror, feedback loop instability, and inlet contamination are SnowWhitespecific problems that remain to be solved (Cirisan et al, 2013), the above analysis confirms the manufacturer's erroneous estimate of approximately ±5 % in mixing ratio for water vapor amounts above 10 ppm when instrumental operation was reliable and comparable to balloon flight conditions.…”

Arctic stratospheric dehydration – Part 2: Microphysical modeling

“…Therefore, ensemble calculations applying different sets of small-scale temperature fluctuations need to be performed in order to retrieve the dependence of PSC properties on the stochastic effects caused by the fluctuations. Cirisan et al (2013) have performed similar calculations for cirrus clouds. Figure 6 presents a comparison of profiles of BSR and H 2 O for various scenarios with and without heterogeneous nucleation and with 10 different sets of small-scale temperature fluctuations.…”

“…Gary (2006) found that a significant component of the short-term vertical displacements of isentropic surfaces remains unresolved also by current numerical weather prediction models. Recent microphysical modeling studies confirmed the importance of an adequate representation of cooling rates for cirrus (Brabec et al, 2012;Cirisan et al, 2013) and polar stratospheric clouds . We follow the same approach chosen in these studies and make use of the vertical velocity and temperature time series obtained from the SUC-CESS (Subsonic Aircraft: Contrail and Cloud Effects Special Study) data analyzed by Hoyle et al (2005) fluctuations.…”

“…Accordingly, the aerosol backscatter ratio is equal to BSR − 1. COBALD has already been applied in several different field studies, for example, measuring the volcanic aerosol plume after the eruption of the Eyjafjallajökull (Bukowiecki et al, 2011) and cirrus clouds (Brabec et al, 2012;Cirisan et al, 2013).…”

“…2003a; Brabec et al, 2012;Cirisan et al, 2013). Sedimentation of ice and NAT particles is realized by allowing particles to sediment within the advected column from one box to the next lower one.…”