Microphysical modeling of orographic cirrus clouds

One of the most challenging problems in atmospheric science is the prediction of future climate change. So far, the role of clouds is very uncertain, especially, the role of cirrus clouds, which could contribute to a positive radiative forcing, i.e. a net heating of the underlying troposphere. For determining the contribution of cirrus clouds to the global warming and predicting their role in a changing climate we have to understand their formation processes, their lifetime and spatial dispersion and their breakup. Currently, only limited information about the life cycles of cirrus clouds including their specific formation regions, i.e. the ice supersaturated regions, is available. As suggested by recent studies, the dynamical impact as well as the possible impact of aerosols on the life cycle of cirrus clouds is crucial for reliable estimates of the radiative forcing. As discussed in Dean et al. (2005), exclusive consideration of synoptic-scale dynamics underestimates the frequency of occurrence of cirrus clouds. Additional effects should be taken in to account, e.g. cirrus clouds generated by mesoscale gravity waves.

Generally, orographic cirrus clouds constitute an ideal natural laboratory to observe their life cycle of cloud particles. However, there are only a few experimental studies available documenting these processes. Therefore, in our study we focus on numerically simulated orographic cirrus clouds by determining the influence of mesoscale dynamics on the generation and evolution of cirrus clouds.

For simulating cirrus clouds formed by orographic waves, we use the anelastic, non-hydrostatic model EuLag. Recently, a bulk scheme for ice microphysics was developed and implemented. We use a two-moment scheme (i.e. prognostic equations for ice number concentrations and ice water content) and include the following processes: Nucleation (homogeneous /heterogeneous), deposition growth/evaporation and sedimentation. The model contains different classes of ice (one class for homogeneous freezing, arbitrary many classes for heterogeneous freezing). Hence, the scheme can discriminate between the different nucleation processes. For each ice class there is also a background aerosol number concentration included, which can act as a limiting factor for the number of nucleated ice crystals.

The model equations are solved in 2D with a high spatial resolution (horizontal range: dx = 200 - 1000 m, vertical range: dz = 50 – 200 m). We use different kinds of mountain profiles and vary the atmospheric upstream conditions, e.g. thermal stability N(z), horizontal wind speed U(z), and relative humidity RHi(z) for studying the sensitivity of cirrus cloud properties due to the variable conditions.

Finally, we present first results of numerical simulations of orographic cirrus clouds frequently observed on the ridges of trapped lee-waves above of so-called rotors in the lee of the Sierra Nevada. Typical upstream profiles of N(z) and U(z) producing the rotor flow are combined with microphysical parameters to simulate these tropospheric cirrus clouds.