Preprints
https://doi.org/10.5194/gmd-2021-300
https://doi.org/10.5194/gmd-2021-300

Submitted as: development and technical paper 02 Sep 2021

Submitted as: development and technical paper | 02 Sep 2021

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

Impacts of a Revised Surface Roughness Parameterization in the Community Land Model 5.1

Ronny Meier1, Edouard Léopold Davin1,a, Gordon B. Bonan2, David M. Lawrence2, Xiaolong Hu3, Gregory Duveiller4, Catherine Prigent5, and Sonia Isabelle Seneviratne1 Ronny Meier et al.
  • 1ETH Zurich, Institute for Atmospheric and Climate Science, Zurich, Switzerland
  • 2National Center for Atmospheric Research, Boulder, CO, 80307, USA
  • 3State Key Laboratory of Water Resources and Hydropower Engineering Sciences, Wuhan University, Wuhan, Hubei 430072, China
  • 4Max Planck Institute for Biogeochemistry, Jena, Germany
  • 5Observatoire de Paris, PSL University, Sorbonne Université, CNRS, LERMA, Paris, France
  • anow at: Wyss Academy for Nature, University of Bern, Bern, Switzerland

Abstract. The roughness of the land surface (z0) is a key property for the amount of turbulent activity above the land surface and through that for the turbulent exchange of energy, water, momentum, and chemical species between the land and the atmosphere. Variations in z0 are substantial across different types of land cover from typically less than 1 mm over fresh snow or sand deserts up to more than 1 m over urban areas or forests. In this study, we revise the parameterizations and parameter choices related to z0 in the Community Land Model 5.1 (CLM), the land component of the Community Earth System Model 2.1.2 (CESM). We propose a number modifications for z0 in CLM, which are guided by observational data. Most importantly, we increase the z0 for all types of forests, while we decrease the momentum z0 for bare soil, snow, glaciers, and crops. We then assess the effect of those modifications in land-only (CLM) and land-atmosphere coupled (CESM) simulations. Diurnal variations of the land surface temperature (LST) are dampened in regions with forests, while they are amplified over warm deserts. These changes mitigate model biases compared to MODIS remote sensing observations, which have been identified in several earlier studies. The alterations in LST are mostly stronger during the day than at night. For example, the LST at 13:30 increases by more than 4.80 K during boreal summer across the entire Sahara. The induced changes in the diurnal variability of air temperatures at the bottom of the atmosphere generally oppose changes in LST in sign and are of smaller magnitude. Further, winds close to the land surface accelerate in areas where the momentum z0 was lowered, such as the Sahara desert, the Middle East, or the Antarctica, and decelerate in regions with forests. Overall, this study highlights that the current representation of z0 in CLM is not in agreement with observational constraints for several types of land cover. The resultant model modifications are shown to considerably alter the simulated climate in terms of temperatures and wind speed at the land surface.

Ronny Meier et al.

Status: open (until 28 Oct 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Ronny Meier et al.

Data sets

MODIS-based data on the sensitivity of LST to a conversion of vegetation to bare land Gregory Duveiller https://doi.org/10.3929/ethz-b-000503165

Estimated climatology of the incoming shortwave radiation at the land surface in GSWP3 under clear–sky conditions Ronny Meier https://doi.org/10.3929/ethz-b-000503165

Model code and software

Modified CESM 2.1.2 model code Ronny Meier https://doi.org/10.3929/ethz-b-000503165

Modified CLM 5.1 model code Ronny Meier https://doi.org/10.3929/ethz-b-000503165

Ronny Meier et al.

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Short summary
We revise the roughness of the land surface in the climate model CESM. Guided by observational data, we increase the surface roughness of forests and decrease the one of bare soil, snow, ice, and crops. These modifications alter simulated temperatures and wind speeds at and above the land surface considerably, in particular over desert regions. The revised model represents the diurnal variability of the land surface temperature better compared to satellite observations over most regions.