Process-oriented large-eddy simulations of a midlatitude cirrus cloud system based on observations

Abstract

We apply a cloud-resolving model with explicit aerosol and ice microphysics and Lagrangian ice particle tracking to simulate the evolution of a cirrus cloud field observed during the US Atmospheric Radiation Measurement Program Intensive Operational Period in March 2000. This comprehensive data set includes remote sensing, radiosonde, and aircraft measurements of a midlatitude cirrus cloud system, supported by estimates of the dynamical cloud forcing. The dataset allows us to evaluate and study in great detail the process-oriented representation of the microphysical processes relevant to the formation and evolution of deep, stratiform cirrus (in particular ice crystal sedimentation and aggregation). The suite of explicitly resolved physical processes in our model enables us to better understand the sensitivity of the simulated cirrus properties on a large number ofmicrophysical and environmental parameters. The evolution of the domain-integrated cloud optical depth is largely dominated by homogeneous freezing processes.Wefind that the evolution of the observed cirrus cloud system is most dependent on updraught speed and ice supersaturation and that homogeneous freezing leads to a total, cloud-averaged ice crystal concentration of 0.1 cm−3 of air. It is not necessary to invoke heterogeneous ice nuclei to explain most of the data, but we cannot rule out that a small concentration (up to 0.002 cm−3) of such particles may have affected the cirrus cloud field in nature. Cloud-averaged ice particle size distributions are bimodal, separating two distinct growth regimes in the developed cloud. The small mode (ice particle sizes below a few 100μm) forms by homogeneous freezing of supercooled aerosol droplets and grows by deposition of water molecules from the gas phase. The large mode (sizes up to several 1000μm) forms and grows by aggregation. We demonstrate that the formation of the largest crystals by aggregation in deep cirrus is controlled in part by the nucleation of new ice crystals in dynamically active, highly supersaturated upper cloud regions. Furthermore, a pronounced increase in the number of aggregation events is predicted in sublimation zones. The combined effect of sublimation and sedimentation leads to the formation of a very thin (vertical extension ∼ 100 m) sublimation microlayer mainly composed of aggregated ice crystals, containing relatively high total ice crystal number concentrations (∼ 0.02 cm−3) comparable to those generated locally by homogeneous freezing in the upper cloud layers.