Balloon-borne match measurements of midlatitude cirrus clouds

Abstract

Abstract.Observations of high supersaturations with respect to ice inside cirrus clouds with high ice water content (> 0.01 g kg −1 ) and high crystal number densities (> 1 cm −3 ) are challenging our understanding of cloud microphysics and of climate feedback processes in the upper troposphere. However, 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 about 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 (Consortium for Small-Scale Modeling) forecasts are performed for flight planning, and COSMO analyses are used as a basis for comprehensive microphysical box modeling (with grid scale of 2 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 cloud was detected over both measurement sites. We show that in order to quantitatively reproduce the measured particle backscatter ratios, the smallscale 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 these microphysical refinements are moderate and comparable in magnitude with changes caused by assuming different regimes of temperature fluctuations for clear-sky or cloudysky conditions, highlighting the importance of 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 hygrom...

Sensitivities of Lagrangian modelling of mid-latitude cirrus clouds to trajectory data quality

“…For further details on ZOMM we refer the reader to Sect. 3.4 in Cirisan et al (2014). We treat ice nucleation as pure homogeneous nucleation of metastable solution droplets based on Koop et al (2000) or, alternatively, with additional heterogeneous nucleation on solid particles such as dust or ash as suggested by Marcolli et al (2007) for Arizona Test Dust.…”

“…Luo et al, 2003b;Engel et al, 2013) and cirrus clouds (e.g. Luo et al, 2003a;Hoyle et al, 2005;Brabec et al, 2012;Cirisan et al, 2014). ZOMM is forced with thermodynamic data (T , p) from the trajectories and is initialized with the COSMO-2 model humidity at the trajectory starting point.…”

“…Several studies have addressed these uncertainties in individual case studies in Lagrangian and Eulerian modelling frameworks (e.g. Jensen et al, 1994bJensen et al, , 2013Hoyle et al, 2005;Cirisan et al, 2014;Muhlbauer et al, 2014a, b). Most studies have focused on the role of different representations of ice nucleation and small-scale temperature fluctuations, while the impact of the temporal resolution of the trajectory data (not relevant for Eulerian studies) and forecast errors in the initial moisture content has so far received less attention.…”

“…While temperature variations at spatial scales of several tens of kilometres can be captured by regional NWP models, vertical velocity and temperature fluctuations at smaller spatial scales remain unresolved due to limited spatial resolution. Several studies have resorted to including the unresolved vertical motions in cirrus cloud modelling by superimposing artificial fluctuations on the trajectory data (Hoyle et al, 2005;Brabec et al, 2012;Rolf et al, 2012;Spichtinger and Krämer, 2013;Cirisan et al, 2014;Murphy, 2014;Dinh et al, 2015). Amplitudes and frequency distributions of the unresolved motions are typically taken from field measurements, which are unrelated to the investigation at hand.…”

“…The box model simulations are usually conducted along backward trajectories, which provide the required temperature and pressure history of the air parcels (e.g. Jensen et al, 1994a, b;Haag and Kärcher, 2004;Hoyle et al, 2005;Comstock et al, 2008;Brabec et al, 2012;Rolf et al, 2012;Jensen et al, 2013;Cirisan et al, 2014). The Zurich Optical and Microphysical Model (ZOMM; Luo et al, 2003a, b), the Model for Aerosol and Ice Dynamics (MAID; Bunz et al, 2008) and the Advanced Particle Simulation Code (APSC; are some of the models used.…”