Crommelin, D.T. (Daan)

Jansson, F.R. (Fredrik)

Oord, G. (Gijs) van den

Siebesma, A.P. (Pier)

English

CWI's Institutional Repository

Resolving clouds in a global atmosphere model - amultiscale approach with nested modelsFredrik JanssonCentrum Wiskunde & InformaticaAmsterdam, the Netherlandsjansson@cwi.nlGijs van den OordNetherlands eScience CenterAmsterdam, the NetherlandsPier SiebesmaKNMI and Delft University of TechnologyDelft, the NetherlandsDaan CrommelinCentrum Wiskunde & Informaticaand University of AmsterdamAmsterdam, the NetherlandsIndex Terms—Atmospheric modeling, superparametrization,multi-scale modelClouds are challenging to incorporate in global weatherand climate models, since they involve processes on lengthscales much smaller than the typical simulation grid resolution.Therefore clouds are usually not resolved, but representedin a simplified way using so-called parametrization schemes.Accurate cloud modeling is important, since one of the largestremaining uncertainties in climate simulations is related toclouds and the strong effects of cloud feedback on the climate.We present our project on replacing the cloud parametriza-tion scheme in a global weather model by another small-scale model which can explicitly resolve the cloud processes.This can be seen as a two-way nesting of a high-resolutionmodel inside selected grid columns of the global model. Theglobal weather model in our case is OpenIFS, a version ofthe ECMWF operational weather forecasting model. The high-resolution model is DALES [1], a 3D large-eddy simulationcode.This nesting, also called superparametrization [2], [3], canpotentially lead to more accurate simulations, at the cost ofrequiring more computational power. It also opens the possi-bility to compare the prediction of the cloud parametrizationscheme with the results from the cloud-resolving model andcan thus provide a way to validate or improve the cloudparametrizations.New in our approach is the use of a fully 3D high resolutionmodel for superparametrization, and the possibility of runningthe high-resolution model in selected grid columns of theglobal model.For coupling the two models, we use the OMUSE frame-work [4]. Both OpenIFS and DALES are written in Fortran.We have created a Python interface to each model usingOMUSE. Using these interfaces, both models are controlledfrom a master script written in Python, which also performsthe calculations required for the coupling.We present results from our superparametrized simulations(see Fig. 1), and discuss the role of small-scale variabilityfor clouds in the superparametrization approach. We alsodiscuss performance measurements for the coupled system,some optimizations, and our experiences with using OMUSEfor model coupling.Fig. 1. 72 Large Eddy Simulations (blue boxes) over the Netherlands coupledto OpenIFS (purple background). Cloudiness (liquid water path) is shown inshades of white.REFERENCES[1] T. Heus, C. C. van Heerwaarden, H. J. J. Jonker, A. P. Siebesma,S. Axelsen, K. van den Dries, O. Geoffroy, A. F. Moene, D. Pino,S. R. de Roode, and J. Vila`-Guerau de Arellano, “Formulation of theDutch Atmospheric Large-Eddy Simulation (DALES) and overview ofits applications,” GMD, vol. 3, no. 2, pp. 415–444, 2010.[2] W. W. Grabowski, “An improved framework for superparameterization,”J. Atmos. Sci., vol. 61, no. 15, pp. 1940–1952, 2004.[3] M. Khairoutdinov, D. Randall, and C. DeMott, “Simulations of theatmospheric general circulation using a cloud-resolving model as asuperparameterization of physical processes,” J. Atmos. Sci., vol. 62, no. 7,pp. 2136–2154, 2005.[4] I. Pelupessy, B. van Werkhoven, A. van Elteren, J. Viebahn, A. Candy,S. Portegies Zwart, and H. Dijkstra, “The oceanographic multipurposesoftware environment (OMUSE v1.0),” GMD, vol. 10, no. 8, pp. 3167–3187, 2017.

Resolving clouds in a global atmosphere model - a multiscale approach with nested models

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