A2 - Structure formation on cloud scale and impact on larger scales
Cloud particles are determined by microscopic processes, such as nucleation/condensation, growth, aggregation and sedimentation. These processes can feedback on dynamics or organize themselves and form macroscopic cloud structures on the order of tens of kilometers. At particles scales (order of micrometers) only little energy is transferred in the system. However through forming structures on cloud scales, diabatic heat sources are confined and concentrated on this scale and can interact with atmospheric flows.
Former studies show effects on synoptic-scale flows driven by these diabatic cloud effects. For instance, diabatic heat sources might change potential vorticity distributions in the middle and upper troposphere, changing the conditions for large-scale flows. However, the structure formation of clouds and inside clouds is not well understood, but is the key issue for the propagating effect; cloud structures lead to spatial and temporal concentration of diabatic heat effects. Structured heat sources will actually change the environmental flow in a different way than homogeneous heat sources. The influence of diabatic heat sources will propagate to larger scales, since flow characteristics might have changed. Thus, it is expected that heat sources assigned to structured clouds lead to forecast errors for atmospheric flows in an upward propagating way.
In this project the formation of cloud structures and structures in clouds will be investigated. We will identify and determine possible structures in clouds containing ice crystals, i.e., mixed-phase clouds and pure ice clouds. In addition, we will identify the governing processes leading to structure formation and investigate the impact of cloud structures on processes on larger scales than cloud scale.
Our approach is two-fold, using high-resolution modeling of clouds and mathematical analysis of cloud physics equations. For consistency, we start with a common analytical cloud model, which will be used in both parts of the project. In the modeling part of the project we will carry out high-resolution numerical simulation of clouds, represented by the cloud model coupled to equations of atmospheric motion (sound-proof models of compressible viscous flows). We will concentrate on convective situations, starting with moist Rayleigh-Benard convection, extended to multiphase systems, but proceed to more realistic convective scenarios. The output of the simulations will be evaluated in terms of temporal and/or spatial structures of clouds. Complementary, we will investigate the underlying equation of cloud physics combined with atmospheric dynamics using mathematical analysis. We will use different methods in order to identify possible structure formation. For direct analysis we will use techniques from dynamical system theory in order to analyze the equations in terms of equilibrium states, limit cycles, Lyapunov exponents, bifurcations due to parameters and attractors, respectively. On the other hand, we will use reduction techniques (e.g., as used for Landau-Ginzburg equations or reduced order methods) in order to simplify the underlying equations towards the governing processes determining structure formation.
In a synthesis of these methods (structures in numerical modeling vs. mathematical analysis) we will finally derive some minimal models describing structure formation on cloud scale. These models will allow us to determine the impact of cloud scale structures on larger scales.
Finally, we will carry out first numerical investigations on the impact of structured heat sources on atmospheric flows. Here, minimal models as derived during the project will be used for describing the structured heat sources, embedded into an atmospheric flow for certain idealized flow conditions (e.g., large-scale flow phenomena, as warm conveyor belts).