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.
Highlights
The land surface interacts in numerous ways with the atmosphere
70 of surface roughness for each land cover type in Community Land Model 5.1 (CLM) to observational data and parameterizations that were proposed in the literature. Based on this comparison we introduce five modifications to CLM: (1) A new parameterization of the vegetation surface roughness based on Raupach (1992) with optimized parameters to match the data collected in Hu et al (2020) for different types of vegetation; (2) new globally constant z0m for bare soil, snow, and glaciers based on field measurements collected in the literature; (3) the parameterization of Yang et al (2008) for z0h and z0q over bare soil, snow, and glaciers; (4) a 75 spatially explicit z0m input field for bare soil based on the data of Prigent et al (2005); and (5) the parameterization of z0m for snow based on accumulated snow melt as proposed in Brock et al (2006)
565 In this study, we have compared the representation of z0 in CLM to observations and parameterizations that exist in the literature, conducted revisions of CLM when clearly supported by this comparison, and assessed the impact of these revisions on simulated temperatures at the land surface and wind speed
Summary
Based on this comparison we introduce five modifications to CLM: (1) A new parameterization of the vegetation surface roughness based on Raupach (1992) with optimized parameters to match the data collected in Hu et al (2020) for different types of vegetation; (2) new globally constant z0m for bare soil, snow, and glaciers based on field measurements collected in the literature; (3) the parameterization of Yang et al (2008) for z0h and z0q over bare soil, snow, and glaciers; (4) a 75 spatially explicit z0m input field for bare soil based on the data of Prigent et al (2005); and (5) the parameterization of z0m for snow based on accumulated snow melt as proposed in Brock et al (2006) The latter two modifications replace the respective globally constant z0m for bare soil and snow and may be activated individually through switches that were added to the model. We confront the 80 default and modified model configuration with MODIS remote sensing observations of diurnal variations in the land surface temperature (LST) and the sensitivity of LST to a conversion of vegetation to bare land, based on the approach of Duveiller et al (2018)
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