Preprints
https://doi.org/10.5194/gmd-2020-94
https://doi.org/10.5194/gmd-2020-94

Submitted as: methods for assessment of models 23 Jul 2020

Submitted as: methods for assessment of models | 23 Jul 2020

Review status: this preprint is currently under review for the journal GMD.

Importance of radiative transfer processes in urban climate models: A study based on the PALM model system 6.0

Mohamed H. Salim1,5, Sebastian Schubert1, Jaroslav Resler2, Pavel Krč2, Björn Maronga3,4, Farah Kanani-Sühring3,6, Matthias Sühring3, and Christoph Schneider1 Mohamed H. Salim et al.
  • 1Geography Department, Humboldt-Universität zu Berlin, Berlin, Germany
  • 2Institute of Computer Science, The Czech Academy of Sciences, Prague, Czech Republic
  • 3Institute of Meteorology and Climatology, Leibniz University Hannover, Hannover, Germany
  • 4Geophysical Institute, University of Bergen, Bergen, Norway
  • 5Faculty of Energy Engineering, Aswan University, Aswan, Egypt
  • 6Harz Energie GmbH & Co. KG, Goslar, Germany

Abstract. Including radiative transfer processes within the urban canopy layer into microscale urban climate models (UCMs) is essential to obtain realistic model results. These processes include the interaction of buildings and vegetation with shortwave and longwave radiation, thermal emission, and radiation reflections. They contribute differently to the radiation budget of urban surfaces. Each process requires different computational resources and physical data for the urban elements. This study investigates how much detail modellers should include to parameterise radiative transfer in microscale building resolving UCMs. To that end, we introduce a stepwise parameterization method to the the PALM model system 6.0 to quantify individually the effects of the main radiative transfer processes on the radiation budget and on the flow field. We quantify numerical simulations of both simple and realistic urban configurations to identify the radiative transfer processes which have major effects on the radiation budget, such as surface and vegetation interaction with short wave and long wave radiation, and those which have minor effects, such as multiple reflections. The study also shows that radiative transfer processes within the canopy layer implicitly affect the incoming radiation since the radiative transfer model is coupled to the radiation model. The flow field changes considerably in response to the radiative transfer processes included in the model. The study highlights those processes which are essentially needed to assure acceptable quality of the flow field. Omitting any of these processes may lead to high uncertainties in the model results.

Mohamed H. Salim et al.

 
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Mohamed H. Salim et al.

Data sets

Data for the parameterization of radiative transfer processes in urban climate models Mohamed Hefny Salim https://doi.org/10.5281/zenodo.3934874

Mohamed H. Salim et al.

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Short summary
Radiative transfer processes are the main energy transport mechanism in urban area which influence the surface energy budget and drive local convection. We show here the importance of each process to help modellers decide on how much detail they should include in their models to parameterise radiative transfer in urban areas. We showed how the flow field may change in response to these processes and the essential processes needed to assure acceptable quality of the numerical simulations.