Weather System Dynamics

Moist baroclinic instability (Sebastian Schemm)

Warm Conveyor Belts are key atmospheric flow structures associated with extratropical cyclones. These coherent moist ascending airstreams are responsible for the major part of precipitation in the storm track areas. They occur most frequently during winter in the western north pacific and north Atlantic. Due to latent heating, warm conveyor belts lead to important cross isentropic mass fluxes and are characterized by the production of positive potential vorticity anomalies in the lower troposphere and for transporting low PV air close to the tropopause. Thereby, warm conveyor belts are able to generate or intensify negative PV anomalies at upper levels, which in turn can enhance the downstream propagation and amplification of Rossby waves. These effects are investigated by performing idealized moist baroclinic wave experiments utilizing the limited area weather prediction model COSMO.

The attached picture shows surface pressure (colored), isentropes (black contour lines), the isosurface of saturated air and the trajectories of the warm conveyor belt (thin grey lines) in one of my idealized simulations at day 4.5

Warm conveyor belts (Hanna Joos)

Warm conveyor belts are strongly ascending airstreams in extratropical cyclones. They originate in the warm sector and transport heat and moisture from the subtropics northwards and from the surface into the upper troposphere. During the ascent, condensation and freezing occurs and clouds form. The latent heat release leads then to an enhanced lifting. Below the regions of strongest diabatic heating, the PV increases whereas it decreases above. Thus, low PV values occur in the outflow region of a warm conveyor belt. These negative upper level PV-anomalies can stronlgy influence the large-scale dynamics. The focus of this project lies on the microphysical processes which influence the PV-development during the ascent. It is investigated how the different diabatic porcesses like condensation, evaporation, freezing or sublimation contribute to the PV change.

Orographic flows (Michael Sprenger)

Orography strongly influences the atmospheric flow on many scales. On the synoptic- and mesoscale the orography (e.g. the Alps) act as a barrier to the impinging flow and forces the air either to rise or to horizontally move around it. The resulting flow regime has a significant influence on orographic precipitation and the passage of fronts. In our reserach we study in a Lagrangian sense how the air is blocked by the Alps and horizontally deflected. Furthermore, we consider the dynamics and thermodynamics of Foehn flows in the major Alpine valleys with high-resolution numerical models. Finally, LES simulations are performed to investigate wind storms and gusts in narrow Alpine valleys.

The numerical modeling of snowfall (Claudia Frick)

Forecasting snowfall events with numerical weather prediction (NWP) models is particularly challenging. Accurate forecasts require a correct presentation of dynamical and physical processes on various scales. Uncertainties in simulated planetary-scale waves and synoptic-scale systems as well as shortcomings in the treatment of the microphysics can lead to poor forecasts of the phase and amount of precipitation. This project mainly concentrates on the fact that heavy snowfall events are typically associated with surface temperatures above 0 degC and therefore on the representation of the melting process of snow in the microphysical parameterizations of NWP models. The goal of this project is to include the prediction of "wet snow" into the COSMO model and to verify the modified model.