Linda Schlemmer

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Schlemmer

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Linda

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0000-0001-8841-6681

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Publications 1 - 8 of 8

A Numerical Analysis of Six Physics-Dynamics Coupling Schemes for Atmospheric Models
Ubbiali, Stefano; Schär, Christoph; Schlemmer, Linda; et al. (2021)
Journal of Advances in Modeling Earth Systems
Six strategies to couple the dynamical core with physical parameterizations in atmospheric models are analyzed from a numerical perspective. Thanks to a suitably designed theoretical framework featuring a high level of abstraction, the truncation error analysis and the linear stability study are carried out under weak assumptions. Indeed, second-order conditions are derived which are not influenced either by the specific formulation of the governing equations, nor by the number of parameterizations, nor by the structural design and implementation details of the time-stepping methods. The theoretical findings are verified on two idealized test beds. Particularly, a hydrostatic model in isentropic coordinates is used for vertical slice simulations of a moist airflow past an isolated mountain. Self-convergence tests show that the sensitivity of the prognostic variables to the coupling scheme may vary. For those variables (e.g., momentum) whose evolution is mainly driven by the dry dynamics, the truncation error associated with the dynamical core dominates and hides the error due to the coupling. In contrast, the coupling error of moist variables (e.g., the precipitation rate) emerges gradually as the spatio-temporal resolution increases. Eventually, each coupling scheme tends toward the formal order of accuracy, upon a careful treatment of the grid cell condensation. Indeed, the well-established saturation adjustment may cap the convergence rate to first order. A prognostic formulation of the condensation and evaporation process is derived from first principles. This solution is shown effective to alleviate the convergence issues in our experiments. Potential implications for a complete forecasting system are discussed.
Intensified Cold Pool Dynamics Under Stronger Surface Heating
Haerter, Jan O.; Schlemmer, Linda (2018)
Geophysical Research Letters
The observed increase of convective extreme precipitation intensities with temperature beyond the Clausius-Clapeyron rate has recently directed attention to nonequilibrium processes that might cause the increase. While out-of-equilibrium simulations with perturbed heating conditions show clear increases in convective precipitation intensities, it has so far remained unclear, to which extent precipitation intensities can increase, when the atmosphere is in “perpetual equilibrium” (PE). We use the term PE to describe periodically forced diurnal cycles that eventually yield an approximately repetitive atmospheric response from day to day. In PE, as defined here, precipitation extremes increase at rates beyond the Clausius-Clapeyron rate. When analyzing causes for the increase, we find the variance in near-surface temperature to increase significantly as precipitation builds up throughout the day and that this temperature variance is larger when surface heating is increased. We propose that enhanced rain evaporation may drive a feedback, by which cold pool activity, and the possible collision of cold pool gust fronts, is strengthened—thereby intensifying subsequent convective updrafts and their precipitation.
A Groundwater and Runoff Formulation for Weather and Climate Models
Schlemmer, Linda; Schär, Christoph; Lüthi, Daniel; et al. (2018)
Journal of Advances in Modeling Earth Systems
Soil moisture modifies the state of the atmosphere and thus plays a major role in the climate system. Its spatial distribution is strongly modulated by the underlying orography. Yet the vertical transport of soil water and especially the generation of groundwater runoff at the bottom of the soil column are currently treated in a crude way in most atmospheric and climate models. This potentially leads to large biases in near‐surface temperatures during midlatitude summertime conditions, when the soils may dry out. Here we present a new formulation for groundwater and runoff formation. It is based on Richards equation, allows for saturated aquifers, includes a slope‐dependent groundwater discharge, and enables a subgrid‐scale treatment of the underlying orography. The proposed numerical implementation ensures a physically consistent treatment of the water fluxes in the soil column, using ideas from flux‐corrected transport methodologies. An implementation of this formulation into TERRA_ML, the land surface model of the regional climate model of the COnsortium for Small‐scale MOdeling (COSMO) in CLimate Mode (CCLM), is validated both in idealized and real‐case simulations. Idealized simulations demonstrate the important role of the lower boundary condition at the bottom of the soil column and display a physically meaningful recharge and discharge of the saturated zone. Validation against measurements at selected stations shows an improved seasonal evolution of soil water content. Finally, decade‐long climate simulations over Europe exhibit a realistic representation of the groundwater distribution across continental scales and mountainous areas, an improved annual cycle of surface latent heat fluxes, and as a consequence reductions of long‐standing biases in near‐surface temperatures in semiarid regions.
Kilometer-scale climate models: Prospects and challenges
Schär, Christoph; Fuhrer, Oliver; Arteaga, Andrea; et al. (2020)
Bulletin of the American Meteorological Society
Bulk and structural convergence at convection‐resolving scales in real‐case simulations of summertime moist convection over land
Panosetti, Davide; Schlemmer, Linda; Schär, Christoph (2019)
Quarterly Journal of the Royal Meteorological Society
Crossing multiple gray zones in the transition from mesoscale to microscale simulation over complex terrain
Katopodes Chow, Fotini; Schär, Christoph; Ban, Nikolina; et al. (2019)
Atmosphere
This review paper explores the field of mesoscale to microscale modeling over complex terrain as it traverses multiple so-called gray zones. In an attempt to bridge the gap between previous large-scale and small-scale modeling efforts, atmospheric simulations are being run at an unprecedented range of resolutions. The gray zone is the range of grid resolutions where particular features are neither subgrid nor fully resolved, but rather are partially resolved. The definition of a gray zone depends strongly on the feature being represented and its relationship to the model resolution. This paper explores three gray zones relevant to simulations over complex terrain: turbulence, convection, and topography. Taken together, these may be referred to as the gray continuum. The focus is on horizontal grid resolutions from ∼10 km to ∼10 m. In each case, the challenges are presented together with recent progress in the literature. A common theme is to address cross-scale interaction and scale-awareness in parameterization schemes. How numerical models are designed to cross these gray zones is critical to complex terrain applications in numerical weather prediction, wind resource forecasting, and regional climate modeling, among others.
Exploring a high-level programming model for the NWP domain using ECMWF microphysics schemes
Wernli, Heini; Ubbiali, Stefano; Kühnlein, Christian; et al. (2025)
Geoscientific Model Development
We explore the domain-specific Python library GT4Py (GridTools for Python) for implementing a representative physical parametrization scheme and the related tangent-linear and adjoint algorithms from the Integrated Forecasting System (IFS) of ECMWF. GT4Py encodes stencil operators in an abstract and hardware-agnostic fashion, thus enabling more concise, readable, and maintainable scientific applications. The library achieves high performance by translating the application into targeted low-level coding implementations. Here, the main goal is to study the correctness and performance portability of the Python rewrites with GT4Py against the reference Fortran code and a number of automatically and manually ported variants created by ECMWF. The present work is part of a larger cross-institutional effort to port weather and climate models to Python with GT4Py. The focus of the current work is the IFS prognostic cloud microphysics scheme, a core physical parametrization represented by a comprehensive code that takes a significant share of the total forecast model execution time. In order to verify GT4Py for numerical weather prediction (NWP) systems, we put additional emphasis on the implementation and validation of the tangent-linear and adjoint model versions which are employed in data assimilation. We benchmark all prototype codes on three European supercomputers characterized by diverse graphics processing unit (GPU) and central processing unit (CPU) hardware, node designs, software stacks, and compiler suites. Once the application is ported to Python with GT4Py, we find excellent portability, competitive GPU performance, and robust execution in all tested scenarios including with single precision.
Collective Impacts of Orography and Soil Moisture on the Soil Moisture-Precipitation Feedback
Imamovic, Adel; Schlemmer, Linda; Schär, Christoph (2017)
Geophysical Research Letters
Ensembles of convection-resolving simulations with a simplified land surface are conducted to dissect the isolated and combined impacts of soil moisture and orography on deep-convective precipitation under weak synoptic forcing. In particular, the deep-convective precipitation response to a uniform and a nonuniform soil moisture perturbation is investigated both in settings with and without orography. In the case of horizontally uniform perturbations, we find a consistently positive soil moisture-precipitation feedback, irrespective of the presence of low orography. On the other hand, a negative feedback emerges with localized perturbations: a dry soil heterogeneity substantially enhances rain amounts that scale linearly with the dryness of the soil, while a moist heterogeneity suppresses rain amounts. If the heterogeneity is located in a mountainous region, the relative importance of soil moisture heterogeneity decreases with increasing mountain height: A mountain 500 m in height is sufficient to neutralize the local soil moisture-precipitation feedback.

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Linda Schlemmer

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