arXiv subjects

Cathy Hohenegger

Publications and source records attributed to Cathy Hohenegger.

5 recordsLinked to original sources

arXiv2026

Spontaneous Zonal Symmetry Breaking of Tropical Rain Belt

The intertropical convergence zone (ITCZ) is a central component of tropical climate, but the conditions under which a tropical rain belt remains zonally extended or becomes unstable to zonal organization are not well understood. We investigate this problem using idealized nonrotating kilometer-scale simulations forced by a prescribed sea surface temperature (SST) distribution that varies only in the meridional direction. This setup produces an ITCZ-like rain belt while allowing spontaneous zonal convective self-aggregation (ZCSA) to emerge. A parameter sweep shows that ZCSA occurs preferentially when both the peak SST and the meridional SST amplitude are large. ZCSA cases exhibit a temporary weakening of the meridional near-surface convergence. Boundary-layer momentum and thermodynamic analyses link this weakening to enhanced lower-tropospheric stability over the cool subsiding region, a shallower boundary layer, and stronger effective frictional damping of the meridional inflow. However, weak convergence alone is not sufficient for ZCSA. Aggregating cases also have a large meridional contrast in moist static energy forcing, implying a strong demand for meridional energy transport. Consistently, ZCSA reorganizes meridional moist static energy transport, including enhanced stationary eddy export from the warm region, and is accompanied by growing zonal moisture variability and weakening meridional moisture contrast. These results suggest that zonal symmetry breaking of an ITCZ-like rain belt is favored when weakened meridional inflow coincides with a large imposed meridional MSE-forcing contrast.

physics.ao-ph↗

arXiv2026

Evaluating local climate in global storm-resolving models with the K\"oppen-Geiger classification

Global storm-resolving models aspire to become digital twins of the Earth, delivering information at the local scale at which humans experience climate. We evaluated how well two such models, ICON and IFS-FESOM, reproduce the climate as classified by the K\"oppen-Geiger system, using 30-year (2020-2049) simulations from the nextGEMS project at 9~km global resolution under SSP3-7.0 scenario. Both models capture the global distribution of the five main climate categories, encouraging given the infancy of storm-resolving climate modelling. Substantial regional biases nonetheless remain. Both underestimate tropical rainforest (Af) extent due to insufficient dry-month precipitation in Amazonia and equatorial Africa. ICON almost eliminates hot arid desert (BWh) across Australia through excessive precipitation, while IFS-FESOM reproduces it well. The two models show opposing biases along the temperate--continental boundary: IFS-FESOM winters are too cold in western Europe, ICON winters too warm. Substituting observed temperature or precipitation into the model fields reveals that precipitation errors dominate misclassification, while temperature biases play a secondary role confined to mid-latitude climate zone boundaries. Under climate change, the two models and CMIP6 projections agree on the direction of climate zone shifts: expansion of tropical savanna and hot desert at the expense of subarctic, tundra, and ice cap zones. However, inter-model differences in present-day climate exceed the 30-year climate change signal for many zones, calling for caution in regional projections and adaptation planning. Our results expose where local-scale climate representation still falls short of the digital twin ambition, while confirming that storm-resolving models already perform well across many regions. We propose K\"oppen-Geiger classification as a standard diagnostic to help track further progress.

physics.ao-ph↗

arXiv2025

Computing the Full Earth System at 1 km Resolution

We present the first-ever global simulation of the full Earth system at 1.25 km grid spacing, achieving highest time compression with an unseen number of degrees of freedom. Our model captures the flow of energy, water, and carbon through key components of the Earth system: atmosphere, ocean, and land. To achieve this landmark simulation, we harness the power of 8192 GPUs on Alps and 20480 GPUs on JUPITER, two of the world's largest GH200 superchip installations. We use both the Grace CPUs and Hopper GPUs by carefully balancing Earth's components in a heterogeneous setup and optimizing acceleration techniques available in ICON's codebase. We show how separation of concerns can reduce the code complexity by half while increasing performance and portability. Our achieved time compression of 145.7 simulated days per day enables long studies including full interactions in the Earth system and even outperforms earlier atmosphere-only simulations at a similar resolution.

physics.ao-ph↗

arXiv2017

What determines the distribution of shallow convective mass flux through cloud base?

The distribution of cloud-base mass flux is studied using large-eddy simulations (LES) of two reference cases, one representing conditions over the tropical ocean, and another one representing mid-latitude conditions over land. To examine what sets the difference between the two distributions, nine additional LES cases are set up as variations of the two reference cases. We find that the total surface heat flux and its changes over the diurnal cycle do not influence the distribution shape. The latter is also not determined by the level of organization in the cloud field. It is instead determined by the ratio of the surface sensible heat flux to the latent heat flux, the Bowen ratio $B$. $B$ sets the thermodynamic efficiency of the moist convective heat cycle, which determines the portion of the total surface heat flux that can be transformed into mechanical work of convection against mechanical dissipation. The thermodynamic moist heat cycle sets the average mass flux per cloud $\langle m \rangle$, and through $\langle m \rangle$ it also controls the shape of the distribution. An expression for $\langle m \rangle$ is derived based on the moist convective heat cycle and is evaluated against LES. This expression can be used in shallow cumulus parameterizations as a physical constraint on the mass flux distribution. The similarity between the mass flux and the cloud area distributions indicate that $B$ also has a role in shaping the cloud area distribution, which could explain its different shapes and slopes observed in previous studies.

physics.ao-ph↗

arXiv2015

Intensification of convective extremes driven by cloud-cloud interaction

In a changing climate, a key role may be played by the response of convective-type cloud and precipitation to temperature changes. Yet, it is unclear if precipitation intensities will increase mainly due to modified thermodynamic forcing or due to stronger convective dynamics. In gradual self-organization, convective events produce highest intensities late in the day. Tracking rain cells throughout their life cycles, we find that interacting events respond strongly to changes in boundary conditions. Conversely, events without interaction remain unaffected. Increased surface temperature indeed leads to more interaction and higher precipitation extremes. However, a similar intensification occurs when leaving temperature unchanged but simply granting more time for self-organization.Our study implies that the convective field as a whole acquires a memory of past precipitation and inter-cloud dynamics, driving extremes. Our results implicate that the dynamical interaction between convective clouds must be incorporated in global climate models to describe convective extremes and the diurnal cycle more realistically.

physics.ao-ph↗