The added value of hecto and kilometer scales for climate simulation

A new study by Bjorn Stevens et al. in the Journal of the Meteorological Society of Japan describes the added value of kilometer (convective storm resolving) and hectometer (large-eddy resolving) simulations for the representation of clouds and precipitation processes in climate models.

A new study by Bjorn Stevens et al. in the Journal of the Meteorological Society of Japan describes the added value of kilometer (convective storm resolving) and hectometer (large-eddy resolving) simulations for the representation of clouds and precipitation processes in climate models. The paper describes what was learned across a six year German national project to advance and explore the use of kilo and hectometer scale models to advance understanding of clouds and climate. The paper is the capstone publication of this project, called High Definition Clouds and Precipitation for Climate Prediction (HD(CP)2), and brings together input and expertise from across this project.
Increasing computing power opens new and exciting opportunities for scientists in climate modeling. Now, global storm resolving (kilometer-scale) coupled multi-decadal climate simulations are becoming feasible, and global large eddy simulations for periods of hours to days are becoming imaginable. With these computational capabilities scientists will need to rely much less on  parameterizations and be much better equipped to study the constrains of underlying physical laws on the evolution of the climate system. Stevens and his co-authors show that this offers the potential for breakthroughs.

Visualization of a frontal system with embedded convection in a South-North cross section stretching across Germany, from the Alps (left) to the Baltic Sea (right). Visualized is liquid cloud water  (greys), cloud ice (magentas), and precipitation (blues) for both a large-eddy simulation with hectometer (155 m) grid spacing (above) and for a state-of-the-art climate model (below).

Using large-eddy and storm-resolving models, the authors found a step change in the quality of the representation of precipitation using models run at kilometer and hectometer scales. Whereas the further refinement of the resolution from kilometer to hectometer scales adds details and modest quantitative differences to the representation of precipitation, hectometer scales are important for the representation of clouds. Evaluating the simulations with an unprecedented wealth of in situ, satellite and surface remote sensing the authors show that at hectometer scales it becomes possible to capture many crucial aspects of the cloud field, from the vertical distribution of cloud cover, to the distribution of cloud sizes, and to the diel (daily) cycle. Qualitative improvements particularly encompass the ability to differentiate cumulus from stratiform clouds. The study affirmatively  answers the question initially posed by the HD(CP)2 project, that being: “If we could simulate the climate system at hecto or kilometer scales would it substantially improve the representation of clouds and precipitation?”

The authors also emphasize that the possibility of performing large-eddy simulation for realistic cases over large and realistic domains, makes it possible to evaluate the approaches more critically.  This is because the simulations represent phenomena that are actually observed, rather than just their statistics, thereby bringing the models and the observations not to mention modelers and observationalists much closer together, something that bodes well for surmounting the remaining model deficiencies.

Original publication:
Stevens, B., C. Acquistapace, A. Hansen, R. Heinze, C. Klinger, D. Klocke, H. Rybka, W. Schubotz, J. Windmiller, P. Adamidis, I. Arka, V. Barlakas, J. Biercamp, M. Brueck, S. Brune, S. A. Buehler, U. Burkhardt, G. Cioni, M. Costa-Suròs, S. Crewell, T. Crüger, H. Deneke, P. Friederichs, C. C. Henken, C. Hohenegger, M. Jacob, F. Jakub, N. Kalthoff, M. Köhler, T. W. van Laar, P. Li, U. Löhnert, A. Macke, N. Madenach, B. Mayer, C. Nam, A. K. Naumann, K. Peters, S. Poll, J. Quaas, N. Röber, N. Rochetin, L. Scheck, V. Schemann, S. Schnitt, A. Seifert, F. Senf, M. Shapkalijevski, C. Simmer, S. Singh, O. Sourdeval, D. Spickermann, J. Strandgren, O. Tessiot, N. Vercauteren, J. Vial, A. Voigt, and G. Zängl, 2020: The added value of large-eddy and storm-resolving models for simulating clouds and precipitation. J. Meteor. Soc. Japan, 98, Special Edition on DYAMOND: The DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains, https://doi.org/10.2151/jmsj.2020-021

Contact:
Prof Bjorn Stevens
Max Planck Institute for Meteorology
Email: bjorn.stevens@we dont want spammpimet.mpg.de