Dr. Fabian Hoffmann is a junior research group leader (funded by the Emmy Noether program of the German Research Foundation) at the Meteorological Institute of the Ludwig-Maximilians-Universität München, Germany. He received his Ph.D. from the Leibniz University Hannover, Germany, and was a Visiting Fellow of CIRES at the NOAA Earth System Research Laboratory, Boulder, Colorado, USA.
Fabian's research focuses on clouds, covering microphysics (rain initiation, aerosol-cloud interactions, mixed-phase processes) and cloud dynamics (entrainment, mixing, turbulence). For his studies, Fabian develops and applies idealized theoretical models as well as high-resolution large-eddy simulations with Lagrangian cloud microphysics, a very detailed representation of cloud microphysics developed by him. Recently, Fabian began to use machine learning approaches to explore the physics of entire cloud systems.
Boundary Layer Meteorology (lecture, winter term 2022/2023)
The following topics are suitable for Bachelor's and Master's theses. Interested students should contact Dr. Fabian Hoffmann.
Stratocumulus Liquid Water Steady States
Stratocumulus clouds play an essential role in the global radiation budget. Due to their high albedo, stratocumulus reflect large amounts of incident shortwave radiation back to space. This ability is predominantly determined by the stratocumulus liquid water path, the vertically integrated liquid water content.
While it is accepted that the stratocumulus liquid water path is a result of longwave radiative cooling and entrainment warming/drying, it is disputed whether this liquid water path is in a steady state, i.e., is constant in time. Based on a theoretical mixed-layer approach, we were able to determine analytical solutions for the steady-state liquid water path and the entrainment velocity in stratocumulus (Hoffmann et al. 2020). These analytical solutions have been compared successfully to a wide range of (idealized) large-eddy simulations.
In this thesis, we want to go a step further and compare the solutions to reanalysis data of the ECMWF or satellite measurements, covering a much wider range of realistic stratocumulus clouds, including their transition to cumulus convection. This thesis is of interest to students interested in clouds, climate, and dynamical systems theory.
Entrainment in Stratocumulus
Stratocumulus clouds play an essential role in the global radiation budget. Due to their high albedo, stratocumulus reflect large amounts of incident shortwave radiation back to space. However, entrainment, i.e., the flux of warm and dry air from the free troposphere above, tends to dissipate this cloud type, with commensurate implications for the climate system.
The entrainment in stratocumulus is linked classically to the so-called cloud-top entrainment instability (CTEI). CTEI describes a positive feedback, which postulates ever-increasing entrainment rates in stratocumulus until the cloud is evaporated completely. However, this theory is primarily built upon thermodynamic arguments and neglects the impact of cloud microphysics, i.e., the number and size of droplets. Accordingly, in this thesis, we want to analyze the impact of cloud microphysics on CTEI, especially how CTEI changes with regard to the number concentration of cloud droplets.
This thesis requires highly detailed modeling of the involved processes and will begin with idealized parcel simulations and end with realistic large-eddy simulations. This thesis is of interest to students interested in clouds, dynamics, microphysics, and numerical modeling.
Collisional Breakup in Lagrangian Cloud Models
To initiate precipitation in warm clouds, droplets need to grow large enough by diffusion, enabling them sediment and collide and coalesce with smaller droplets. Creating these so-called precipitation embryos is a well-known bottleneck in cloud microphysics. However, once a small number of very large drops are created by collision and coalescence, they might break up into several smaller ones. If these newly created droplets are large enough to collide and coalesce with other droplets, they may start a positive feedback that dissipates a cloud quickly by precipitation.
In this thesis, we want to implement a representation of collisional breakup in a Lagrangian microphysical model and evaluate its effects on the development of a cumulus cloud. This thesis can be split up into a numerical part, in which the implementation of collisional breakup will be developed and evaluated, as well as a part that will investigate the role of collisional breakup in clouds. Accordingly, this thesis is of interest to students interested in clouds, microphysics, and numerical modeling.
Dr. Fabian Hoffmann
— Fabian Hoffmann 2020/11/12 00:26