The role of clouds is crucial for our understanding of the current and the changing climate (IPCC, 2007). In this pro ject, we focus on high clouds, consisting purely of ice crystals, the so-called cirrus clouds. As low and medium-high water or mixed phase clouds, cirrus clouds modulate the Earth’s radiation budget significantly. It is assumed that (thin) cirrus clouds contribute to a net warming of the Earth-Atmosphere system (e.g. Chen et al., 2000), but the magnitude of this warming has not been quantified yet.
Cirrus clouds display a high degree of spatial and temporal variability. Sometimes, they appear as very homoge- neous regions on the sky whereas during other times a sort of “patchiness” can be observed. This inhomogeneity inside cirrus clouds influences their radiation properties. Their spatial and temporal variability point to
dy- namical processes acting on different atmospheric scales. Thus, the numerical modelling of cirrus clouds poses a multiscale problem.
In the atmosphere, microphysical processes act within a few seconds (or even milliseconds) to initiate ice crystal nucleation and growth, whereas the dynamical processes (from the turbulence on the small spatial and temporal scales to mesoscale waves and large-scale atmospheric flows) influence the subsequent evolution of the clouds.
The formation and evolution of cirrus clouds is mainly triggered by the vertical wind speed in the upper troposphere, the key parameter for the adiabatic cooling of the lifted air parcels and the associated change of the relative humidity.
However, it is very difficult to quantify which atmospheric scales are most effective for the formation and evolution of cirrus clouds in a distinct situation. In cirrus cloud measurements the impact of the different atmospheric scales is often masked and cannot easily separated. Therefore, modelling studies of the formation and evolution of cirrus clouds are needed that control the different atmospheric motion scales very precisely.
For addressing this multiscale problem, we propose to apply a new method to simulate the various temporal and spatial scales involved in the formation and evolution of cirrus clouds. Based on the anelastic non-hydrostatic equations, we derive a perturbation form in which a postulated/known environmental or ambient state is subtracted from the governing equations. For example, the environmental state itself can be a solution of the anelastic equations on a coarser mesh, representing mesoscale or large-scale dynamical processes. Moreover, the environmental state can be prescribed based on simple atmospheric balances (e.g. geostrophy). As a consequence, the cloud-resolving simulations are driven by additional forcing terms in the perturbation form of the anelastic equations. Hence, the larger scale dynamical atmospheric processes can be controlled using the calculated or prescribed solution. As a result, we will be able to investigate and quantify the impact of different atmospheric motion scales on the life cycle of cirrus clouds. Additionally, time-dependent deflections of the lower or upper model boundary can be applied to prescribe a certain spectrum of mesoscale gravity waves disturbing externally the dynamical processes in the interior model domain.
The main ob jective of the pro ject is to gain new insights on the formation and life cycle of tropospheric cirrus clouds allowing for a broad range of atmospheric motion scales. Especially, the influence of the competing large-scale and mesoscale dynamical processes on cloud formation and evolution will be investigated by a new approach of multiscale numerical modelling: Cloud- resolving numerical simulations are driven by slowly varying environmental or ambient states representing the larger scale forcing.
The results of the proposed pro ject are intended to be used to derive a more accurate representation of cirrus clouds in numerical weather prediction and general circulation models. Finally, this allows a more reliable estimate of the radiative impact of cirrus clouds on climate.