Particle backscatter and relative humidity measured across cirrus clouds and comparison with microphysical cirrus modelling

Abstract

Abstract. Advanced measurement and modelling techniques are employed to estimate the partitioning of atmospheric water between the gas phase and the condensed phase in and around cirrus clouds, and thus to identify in-cloud and outof-cloud supersaturations with respect to ice. In November 2008 the newly developed balloon-borne backscatter sonde COBALD (Compact Optical Backscatter and AerosoL Detector) was flown 14 times together with a CFH (Cryogenic Frost point Hygrometer) from Lindenberg, Germany (52 • N, 14 • E). The case discussed here in detail shows two cirrus layers with in-cloud relative humidities with respect to ice between 50 % and 130 %. Global operational analysis data of ECMWF (roughly 1 • × 1 • horizontal and 1 km vertical resolution, 6-hourly stored fields) fail to represent ice water contents and relative humidities. Conversely, regional COSMO-7 forecasts (6.6 km × 6.6 km, 5-min stored fields) capture the measured humidities and cloud positions remarkably well. The main difference between ECMWF and COSMO data is the resolution of small-scale vertical features responsible for cirrus formation. Nevertheless, ice water contents in COSMO-7 are still off by factors 2-10, likely reflecting limitations in COSMO's ice phase bulk scheme. Significant improvements can be achieved by comprehensive size-resolved microphysical and optical modelling along backward trajectories based on COSMO-7 wind and temperature fields, which allow accurate computation of humidities, homogeneous ice nucleation, resulting ice particle size distributions and backscatter ratios at the COBALD wavelengths. However, only by superimposing small-scale temperature fluctuations, which remain unresolved by the numerical weather prediction models, can we obtain a satisfying agreement with the observations and reconcile the measured in-cloud nonequilibrium humidities with conventional ice cloud microphysics. Conversely, the model-data comparison provides no evidence that additional changes to ice-cloud microphysics -such as heterogeneous nucleation or changing the water vapour accommodation coefficient on ice -are required.

Balloon-borne match measurements of midlatitude cirrus clouds

“…This model has been applied previously for polar stratospheric cloud investigations and tropospheric aerosol and cirrus studies (Luo et al, 2003;Hoyle et al, 2005;Engel et al, 2013). It is also used as a column model, driven by temperature and pressure data along staggered air parcel trajectories (Brabec et al, 2012), and in the present context we will make use of this column feature. Kinetically limited uptake or release of water by ice particles or by aqueous (NH 4 ) 2 SO 4 and H 2 SO 4 solution droplets as well as the ice nucleation of liquid aerosol particles are treated in detail in a Lagrangian approach, i.e., by moving particles in the same size class after growth/evaporation within radius (r) space and avoiding the numerical diffusion that would occur in a Eulerian size scheme with prescribed size bins.…”

“…Another NWP caveat potentially contributing to the erroneous humidity representation is the one-moment bulk microphysics scheme implemented in COSMO, which is not able to capture the fine measured structure and prevents the persistence of ice supersaturation. Following a similar procedure as in Brabec et al (2012) we compare COSMO IWC to the IWC estimated from the BSR measurements. To this end we assume the ice crystal mode radius r m of in situ-formed cirrus clouds to lie in a typical range of 5 to 20 µm, yielding an overall uncertainty in the BSR of a factor of ∼ 4.…”

“…We refine a procedure that has been used for a similar purpose by Brabec et al (2012). The fluctuation frequency spectrum was adopted again from Hoyle et al (2005) based on the vertical wind and temperature measurements from the "Subsonic Aircraft: Contrail and Cloud Effects Special Study" (SUCCESS) campaign.…”

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

“…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.…”